llvm dev - Jul 2016 - [PM] I think that the new PM needs to learn about inter-analysis dependencies...

I'm not quite sure what post to respond to for a status update, but I guess
this one will do (and you can check my log for more info of course).

My current working branch for the analysis manager stuff is at
https://github.com/chisophugis/llvm/commits/analysis-manager
(4ecf6115890bd01caa52c0b99424974e3469291e)

I described in my log a bit more my thought process and how to do this
without having to churn every single new PM pass (search for "Okay, so now
that I think about it, adding the dependency management is a second step
after making the analysis manager work for all IRUnit’s."). Essentially,
the first step is to still have AnalysisManager<Function>,
AnalysisManager<Module>, etc. but the template argument is just a dummy.
The real templating is on the methods, which can accept any IRUnit. The
advantage of this is that it is source compatible with all the existing
code using multiple manager, proxies, etc.

The code on my branch at least passes check-llvm. But of course the
existing code doesn't test any of the situations where being able to handle
multiple IRUnitT's in a single analysis manager would matter.
Tomorrow, I get to churn all the passes that have been ported so far,
removing the proxies etc.





Another thing that just came to me: the current way things work with
adaptors means that if a function transformation invalidates a module
analysis, the module analysis won't be invalidated until after the
ModuleToFunctionPassAdapter returns.

So we can end up running an arbitrary number of function transformations
that query that stale module analysis. This is (I think?) mostly benign
given our current set of module analyses (more info in the log; search for
"We may be getting by simply because we seem to not have many module
analyses"). But if we invalidate more "correctly" (after every
transformation on any contained IRUnit), there's a potential quadratic
compile time issue lurking here once have the unified analysis manager: we
could easily end up invalidating/recomputing a module analysis once per
function visitation.

For a pipeline that just does a lot of churn (e.g. function simplification
passes we run as part of the regular O3 pipeline; that can delete most code
in a function, change the cfg, etc.) it's not clear what useful properties
we can guarantee will be preserved which would prevent us from invalidating
pretty much all non-immutable outer analyses (module or
(theoretically)CGSCC). So for any really nontrivial module analysis, we may
end up having to change the interface of module passes to have some way to
incrementally recompute themselves as function passes mutate the IR?

In some sense, Chandler's CGSCC patch https://reviews.llvm.org/D21464 is
trying to do this for a specific module analysis (lazy call graph) and even
for just that one (and lots of special handling in the adaptors etc.) it
still only gets incrementally updated it after a potentially large set of
function passes have run. And it's already quite tricky.

Broadly speaking, I can think of two major kinds of "solutions" to
this
problem:
- severely restrict the interaction of function transformations and module
analyses in some way. Essentially, we could define a restricted class of
module analyses that "are conservatively correct in the face of any
transformation that can be made by a function transformation" (this
includes immutable module analyses and IIRC things like GlobalsModRef). But
the issue is a bit deeper, because it is really about transformations at
any smaller IRUnitT. This includes CGSCC. And CGSCC can do substantial
interprocedural transformation (e.g. delete a wholefunction IIRC?); we
would then need to have another class of module analyses that are
"conservatively correct in the face of CGSCC transformations" which
seems
quite restrictive.
-- alternatively or in addition, this can involve restricting what kind of
information a function pass can get out of a module analysis
- provide some sort of update callback for the outer IRUnit analysis to
update itself if the inner one is modified (not sure how practical this
will be). E.g. `AM.invalidate<FooModuleAnalysis>(F);` doesn't actually
invalidate FooModuleAnalysis, but rather invokes a callback for it to
update itself (possibly as an opt-in for the module analysis).
-- The current analysis manager does have a `invalidate` hook that it will
call, but the argument is always inherently the IRUnit that the analysis
itself is cached on so it isn't useful for partial recomputation. We would
need to extend that, which requires making the AnalysisResultConcept more
complicated (and potentially having to have centralized awareness of all
the different IRUnitT's, which until now are fairly decoupled in the new
PM).


I'm curious what others' thoughts are on this issue.

-- Sean Silva

On Sun, Jul 24, 2016 at 2:58 AM, Sean Silva <chisophugis at gmail.com>
wrote:
> I've started looking specifically at the existing code and how it needs
to
> be changed. It seems like the concept-based polymorphism stuff in
> PassManagerInternal.h actually don't need to be changed that much.
> AnalysisPassConcept and AnalysisResultConcept need to be changed to take a
> type-erased IRUnit (e.g. a void* or something). AnalysisPassModel and
> AnalysisResultModel (which inherit from their respective abstract base
> classes I mentioned above) are already templated on the IRUnitT and so they
> can just cast the void* back to the right type.
>
> Adding the dependency tracking seems like it will be mostly a data
> structure change with some isolated "algorithmic" changes to
> track/invalidate dependencies.
>
> Most of the methods that need to know the specific IRUnitT type already
> take the IRUnit as an argument (e.g. getResult, getCachedResult,
> invalidate) and so it's actually somewhat natural for the analysis
manager
> not be templated on IRUnitT (but rather to have just those methods be
> templated).
>
> Also, up until now I hadn't noticed this weird "registerPass"
thing on the
> analysis manager (AnalysisManagerBase to be specific). Effectively, it
> allows analyses to hold state (this is separate from analysis *results*
> which of course can hold state). But the state is effectively global to the
> AnalysisManager (and hence the pass pipeline, and hence (essentially) the
> context (since you can currently only run a single pass pipeline
> concurrently on a single LLVMContext)). The net result is that you can
> specify a context-global "configuration" for each analysis type
(not to be
> confused with the analysis *result* type!). Right now, AAManager is the
> only thing that uses it though (the "configuration" is the AA
pipeline).
>
>
>
> btw, I've started to keep a log for this like I do for my projects at
> home:
>
https://docs.google.com/document/d/1-nrq2y_hTiZhrsJDmH8TzFjFEeApfccs6wwGyaXjSkg/edit?usp=sharing
> It's fairly stream-of-consciousness. My log style is mostly append-only
(I
> rarely edit stuff from a previous day or from too long ago on the same day;
> if I do I usually don't overwrite and instead insert something like
"(edit:
> I was actually totally wrong about this, see below)"). So if you want
to
> follow this across multiple days just go to the end and scroll up until you
> see something that you've already looked at. (right now there is just
one
> day though so there is not very much).
>
> (some interesting things to search for are "interesting",
"okay", "oh",
> and "?"; I tend to use these a lot)
>
> I've gone back to google docs for this log since it is easier to share
on
> the mailing list. Unfortunately google docs does not have an explicit
> notion of "cell" like Mathematica does, so I've tried to just
insert lots
> of line breaks for things where there would have just been a cell boundary
> in Mathematica. (I use Mathematica for my logs at home).
>
>
> -- Sean Silva
>
> On Fri, Jul 22, 2016 at 1:55 AM, Sean Silva <chisophugis at
gmail.com> wrote:
>
>> The more closely I look at this, the more it seems like there may be a
>> useful incremental step in the transition to the new PM: use the new PM
>> analysis machinery in the old PM. If this is possible, it will simplify
the
>> old PM and (hopefully) allow an incremental transition to the new PM
>> instead of a flag day transition for the switch.
>>
>> I.e., AFAICT, the new PM transition is essentially about 2 mostly
>> orthogonal aspects of running optimization pipelines:
>> 1. Analysis computation and analysis result lifetime management
>> (including avoiding analysis recomputation)
>> 2. Running transformation passes over their respective IRUnit's in
some
>> order
>>
>> These are conflated in the old PM. In reality, the only interaction
>> between them (with the new PM machinery for 1.) is a small number of
places
>> within 2. which need to call 'invalidate'.
>>
>> I'm pretty sure that 2. is fairly similar in the new PM and old PM
(the
>> main difference is that the notion of "adapters" is split out
in the new
>> PM). The analysis handling seems to be what makes the old PM so
difficult
>> to understand (e.g. it is the cause of the multiple inheritance in the
>> implementation). Trying to unify analyses and transformations (and some
>> questionable (in hindsight) implementation decisions) seems to be the
main
>> "problem" with the design of the old PM AFAICT (there are
other issues, but
>> they are more "nice to have").
>>
>> IMO it is an anti-pattern to think of analyses as "passes".
There are
>> just "analyses" and "transformations" and they are
two separate things. In
>> fact, the `run` method on analyses should probably be called
>> `computeResult` or something like that to avoid confusion. And the
`run`
>> method on transformations could just as easily be called
>> `performTransformation`.
>>
>>
>> I remember asking and getting and answer from Chandler (
>>
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150907/299083.html)
>> about how to coexist the old and new PM compatibly so individual passes
>> would be able to work for both and we wouldn't need to have a flag
day. I
>> wasn't able to find the discussions that Chandler cited, but I
suspect that
>> the answer is that we didn't have enough analyses ported at that
point to
>> consider sharing the analysis management between the old and new PM.
>>
>>
>> If this works out it may be the evolutionary path we have all been
>> wanting for this pass manager transition. Fingers crossed. Hopefully
I'm
>> not overlooking some major issue.
>>
>> Anyway... back to working on the analysis manager dependency tracking.
>>
>> -- Sean Silva
>>
>> On Thu, Jul 21, 2016 at 1:59 AM, Sean Silva <chisophugis at
gmail.com>
>> wrote:
>>
>>> We did some basic sanity checking that memory usage didn't go
out of
>>> control (it doesn't; at least with with a simple
>>> preserves-all/preserves-none invalidation scheme and the current
LTO
>>> pipeline). Also, I did some basic sanity checking for compile time.
The
>>> simple preserves-all/preserves-none invalidation scheme seems
marginally
>>> slower, but no real conclusions (besides a simple sanity check) can
be
>>> drawn without the real analysis preservation semantics in place.
>>>
>>> I'll start working on fixing the analysis managers. There seem
to
>>> basically be two parts (although they may need to be done
simultaneously to
>>> make sure all the pieces fit together):
>>> - unify all the analysis managers into a single analysis manager
for all
>>> IRUnitT's (requires type-erasing the IRUnit)
>>> - introduce the dependency tracking machinery
>>>
>>> I think I gave a reasonable outline in the two posts above:
>>> - the one starting with "To clarify, it seems like the current
new PM
>>> is essentially trying to solve the problem of maintaining/updating
a
>>> mapping"
>>> - the one starting with "Yeah, the mechanics of maintaining
this fully
>>> general mapping are straightforward in the abstract"
>>>
>>> I'm happy to do a centralized writeup if anybody wants. Just
let me know.
>>>
>>> As far as changes to the code, the updates to the new PM passes
should
>>> hopefully be mechanical (despite there being many of them).
However, from
>>> what I can tell, fixing this problem will require touching most
lines of
>>> the analysis manager machinery so the diff will probably be a bit
scary,
>>> even though I think we can keep the same basic structure (i.e. a
per-IRUnit
>>> std::list holding one analysis result (at a stable address) per
element,
>>> combined with a DenseMap from (Analysis, IRUnit) to an element of
the
>>> per-IRUnit storage list (see AnalysisResultListMapT and
AnalysisResultMapT
>>> in include/llvm/IR/PassManager.h)).
>>> The main changes to the analysis manager will be:
>>> - type-erasing the IRUnit
>>> - the elements of the AnalysisResultListMapT will need to keep
track of
>>> any dependents
>>> - the analysis manager will need to update those dependencies as it
is
>>> re-entered by analyses getting results of other analyses
>>> - the analysis manager will need to walk the dependencies to do
>>> transitive invalidation
>>>
>>> I think the most robust approach is for analysis dependencies to be
>>> implicitly constructed by the analysis manager via tracking
entry/exit from
>>> get{,Cached}Result.
>>> One alternative is for analyses to explicitly pass in their ID to
>>> getResult to indicate the dependency, but that seems error-prone
(and also
>>> verbose). The issue is that we will need a getResult API that does
not
>>> track dependencies for use by transformation passes (since there is
no
>>> dependency to track in that case); so an innocuous copy-paste from
a
>>> transform pass to an analysis can result in a failure to track
dependencies
>>> and risk of use-after-free (we could fight this with the type
system, but I
>>> think that would get a bit verbose (I'm willing to try it
though if people
>>> would prefer))
>>> One restriction of the implicit tracking approach is that it
requires
>>> all calls into the analysis manager to occur in the `run` method of
the
>>> analysis (so that the dependencies are implicitly tracked via
re-entrance
>>> to get{,Cached}Result); is this a reasonable restriction?
>>>
>>>
>>> One annoying problem is that I think that the dependency links will
need
>>> to be bidirectional. To use the example analysis cache from my
other post:
>>> (AssumptionAnalysis, function @bar) -> (AssumptionCache for
@bar,
>>> [(SomeModuleAnalysis, module TheModule)])
>>> (AssumptionAnalysis, function @baz) -> (AssumptionCache for
@baz,
>>> [(SomeModuleAnalysis, module TheModule)])
>>> (SomeModuleAnalysis, module TheModule) ->
(SomeModuleAnalysisResult for
>>> TheModule, [(SomeFunctionAnalysis, function @baz)])
>>> (SomeFunctionAnalysis, function @baz) ->
(SomeFunctionAnalysisResult for
>>> @baz, [])
>>>
>>> if we delete function @baz, then the dependent list 
[(SomeFunctionAnalysis,
>>> function @baz)] for `(SomeModuleAnalysis, module TheModule)` will
now
>>> have a stale pointer to function @baz. I think that in practice we
can
>>> check to see if `(SomeFunctionAnalysis, function @baz)` is in our
hash
>>> table and if it isn't then just ignore the dependency as
"this dependent
>>> analysis result has already been freed". In the worst case
(memory
>>> allocator reuses the memory for another function) we may spuriously
free an
>>> analysis result for a different function. However it is still
unsettling
>>> (and may actually be UB in C++?).
>>> Ideally we would track bidirectional links; that way when we remove
an
>>> analysis result we also have it remove itself from dependence lists
of all
>>> of its dependencies.
>>>
>>> -- Sean Silva
>>>
>>> On Fri, Jul 15, 2016 at 8:40 PM, Sean Silva <chisophugis at
gmail.com>
>>> wrote:
>>>
>>>>
>>>>
>>>> On Fri, Jul 15, 2016 at 8:39 PM, Sean Silva <chisophugis at
gmail.com>
>>>> wrote:
>>>>
>>>>> It looks like there is really no sane fix within the
current
>>>>> infrastructure. I've had to essentially trigger
invalidation (except in the
>>>>> PreservedAnalyses::all() case) in the function pass manager
and function to
>>>>> loop adapters.
>>>>>
>>>>
>>>> invalidation of *everything* I mean.
>>>>
>>>> -- Sean Silva
>>>>
>>>>
>>>>>
>>>>> So we basically need to get the analysis manager dependency
tracking
>>>>> fixed.
>>>>>
>>>>> Davide and I will get measurements on the resident set
impact of all
>>>>> this caching (even with the overconservative invalidation
for now) to see
>>>>> the impact. If there is a big rss impact then we probably
want to consider
>>>>> that problem at the same time as the rewrite of the
analysis manager.
>>>>>
>>>>> -- Sean Silva
>>>>>
>>>>> On Thu, Jul 14, 2016 at 12:51 AM, Sean Silva
<chisophugis at gmail.com>
>>>>> wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On Wed, Jul 13, 2016 at 1:48 AM, Sean Silva
<chisophugis at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Wed, Jul 13, 2016 at 12:34 AM, Chandler Carruth
<
>>>>>>> chandlerc at gmail.com> wrote:
>>>>>>>
>>>>>>>> On Wed, Jul 13, 2016 at 12:25 AM Sean Silva
<chisophugis at gmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> On Tue, Jul 12, 2016 at 11:39 PM, Chandler
Carruth <
>>>>>>>>> chandlerc at gmail.com> wrote:
>>>>>>>>>
>>>>>>>>>> On Tue, Jul 12, 2016 at 11:34 PM Sean
Silva <
>>>>>>>>>> chisophugis at gmail.com> wrote:
>>>>>>>>>>
>>>>>>>>>>> On Tue, Jul 12, 2016 at 11:32 PM,
Xinliang David Li <
>>>>>>>>>>> davidxl at google.com> wrote:
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> On Tue, Jul 12, 2016 at 10:57
PM, Chandler Carruth <
>>>>>>>>>>>> chandlerc at gmail.com>
wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> Yea, this is a nasty
problem.
>>>>>>>>>>>>>
>>>>>>>>>>>>> One important thing to
understand is that this is specific to
>>>>>>>>>>>>> analyses which hold
references to other analyses. While this isn't unheard
>>>>>>>>>>>>> of, it isn't as common
as it could be. Still, definitely something we need
>>>>>>>>>>>>> to address.
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> We can call this type of
dependencies (holding references)
>>>>>>>>>>>> hard-dependency. The soft
dependency refers to the case where analysis 'A'
>>>>>>>>>>>> depends on 'B' during
computation, but does not need 'B' once it is
>>>>>>>>>>>> computed.
>>>>>>>>>>>>
>>>>>>>>>>>> There are actually quite a few
examples of hard-dependency
>>>>>>>>>>>> case. For instance
LoopAccessInfo, LazyValueInfo etc which hold references
>>>>>>>>>>>> to other analyses.
>>>>>>>>>>>>
>>>>>>>>>>>> Problem involving
hard-dependency is actually easier to detect,
>>>>>>>>>>>> as it is usually a compile time
problem. Issues involving soft dependencies
>>>>>>>>>>>> are more subtle and can lead to
wrong code gen.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Did you mean to say that
soft-dependency problems are easier to
>>>>>>>>>>> detect? At least my intuition is
that soft-dependency is easier because
>>>>>>>>>>> there is no risk of dangling
pointers to other analyses.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> The issue is that the fact that there
is *any* dependency isn't
>>>>>>>>>> clear.
>>>>>>>>>>
>>>>>>>>>> However, I think the only real problem
here are these "hard
>>>>>>>>>> dependencies" (I don't really
like that term though). For others, only an
>>>>>>>>>> analysis that is *explicitly* preserved
survives. So I'm not worried about
>>>>>>>>>> the fact that people have to remember
this.
>>>>>>>>>>
>>>>>>>>>> The question is how often there are
cross-data-structure
>>>>>>>>>> references. David mentions a few
examples, and I'm sure there are more, but
>>>>>>>>>> it isn't clear to me yet whether
this is pervasive or occasional.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I just did a quick run-through of
PassRegistry.def and this is
>>>>>>>>> what I found:
>>>>>>>>>
>>>>>>>>> Module analyses: 0/5 hold pointers to other
analyses
>>>>>>>>> CallGraph: No pointers to other analyses.
>>>>>>>>> LazyCallGraph: No pointers to other
analyses.
>>>>>>>>> ProfileSummaryAnalysis: No pointers to
other analyses.
>>>>>>>>> TargetLibraryAnalysis: No pointers to other
analyses.
>>>>>>>>> VerifierAnalysis: No pointers to other
analyses.
>>>>>>>>>
>>>>>>>>> Module alias analyses: 1/1 keeps pointer to
other analysis.
>>>>>>>>> GlobalsAA: Result keeps pointer to TLI
(this is a function
>>>>>>>>> analysis).
>>>>>>>>>
>>>>>>>>> Function analyses: 9/17 keep pointers to
other analysis
>>>>>>>>> AAManager: Its Result holds TLI pointer and
pointers to individual
>>>>>>>>> AA result objects.
>>>>>>>>> AssumptionAnalysis: No pointers to other
analyses.
>>>>>>>>> BlockFrequencyAnalysis: Its Result holds
pointers to LoopInfo and
>>>>>>>>> BPI.
>>>>>>>>> BranchProbabilityAnalysis: Stores no
pointers to other analyses.
>>>>>>>>> (uses LoopInfo to "recalculate"
though)
>>>>>>>>> DominatorTreeAnalysis: Stores no pointers
to other analyses.
>>>>>>>>> PostDominatorTreeAnalysis: Stores no
pointers to other analyses.
>>>>>>>>> DemandedBitsAnalysis: Stores pointers to
AssumptionCache
>>>>>>>>> and DominatorTree
>>>>>>>>> DominanceFrontierAnalysis: Stores no
pointers to other analyses.
>>>>>>>>> (uses DominatorTreeAnalysis for
"recalculate" though).
>>>>>>>>> LoopInfo: Uses DominatorTreeAnalysis for
"recalculate" but stores
>>>>>>>>> no pointers.
>>>>>>>>> LazyValueAnalysis: Stores pointers to
AssumptionCache,
>>>>>>>>> TargetLibraryInfo, DominatorTree.
>>>>>>>>> DependenceAnalysis: Stores pointers to
AliasAnalysis,
>>>>>>>>> ScalarEvolution, LoopInfo
>>>>>>>>> MemoryDependenceAnalysis: Stores pointers
to AliasAnalysis,
>>>>>>>>> AssumptionCache, TargetLibraryInfo,
DominatorTree
>>>>>>>>> MemorySSAAnalysis: Stores pointers to
AliasAnalysis, DominatorTree
>>>>>>>>> RegionInfoAnalysis: Stores pointers to
DomTree, PostDomTree,
>>>>>>>>> DomFrontier
>>>>>>>>> ScalarEvolutionAnalysis: Stores pointers to
TargetLibraryInfo,
>>>>>>>>> AssumptionCache, DominatorTree, LoopInfo
>>>>>>>>> TargetLibraryAnalysis: Has no dependencies
>>>>>>>>> TargetIRAnalysis: Has no dependencies.
>>>>>>>>>
>>>>>>>>> Function alias analyses: 3/5 keep pointers
to other analyses
>>>>>>>>> BasicAA: Keeps pointers to
TargetLibraryInfo, AssumptionCache,
>>>>>>>>> DominatorTree, LoopInfo
>>>>>>>>> CFLAA: Keeps pointer to TargetLibraryInfo
>>>>>>>>> SCEVAA: Keeps pointer to ScalarEvolution
>>>>>>>>> ScopedNoAliasAA: No dependencies
>>>>>>>>> TypeBasedAA: No dependencies
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Total: 13/28 analyses (~50%) hold pointers
to other analyses.
>>>>>>>>> Of the 15/28 analyses that don't hold
pointers, 12/15 simply have
>>>>>>>>> no dependencies. Only 3/15 (BPI, LoopInfo,
DominanceFrontier) have
>>>>>>>>> dependencies that are used just for a
"recalculate" step that retains no
>>>>>>>>> pointers.
>>>>>>>>> So I think it is fair to say that analyses
which hold pointers to
>>>>>>>>> other analyses is not an exceptional case.
In fact, analyses that use other
>>>>>>>>> analyses just for a "recalculate"
step seems to be the exceptional case
>>>>>>>>> (only 3/28 or about 10%)
>>>>>>>>>
>>>>>>>>
>>>>>>>> Interesting!
>>>>>>>>
>>>>>>>> Most of these look like they hold a pointer to
the root analysis as
>>>>>>>> opposed to detailed objects *inside* the
analysis?
>>>>>>>>
>>>>>>>> It might make sense to try to handle this very
specific pattern in
>>>>>>>> a special way of overriding the invalidate
routines is too error prone.... We
>>>>>>>> could try to make this work
"automatically" but I'm worried this would be
>>>>>>>> challenging to get right. Open to suggestions
of course.
>>>>>>>>
>>>>>>>> Any other ideas about what would make sense to
handle this?
>>>>>>>>
>>>>>>>> Does it make sense to override the invalidate
routines now and
>>>>>>>> iterate from there? I feel like you've done
a lot of the research necessary
>>>>>>>> for this already...
>>>>>>>>
>>>>>>>
>>>>>>> I'll keep pushing forward tomorrow with
building test-suite
>>>>>>> successfully using the new PM for the LTO pipeline
(I was doing some
>>>>>>> unrelated LLD stuff for most of today). It will be
interesting to see how
>>>>>>> many `invalidate` overrides will be needed to avoid
these issues for just
>>>>>>> the LTO pipeline on test-suite.
>>>>>>>
>>>>>>
>>>>>> I spent the better part of today working on this and
will continue
>>>>>> tomorrow; this problem seems nastier than I thought.
For some reason the
>>>>>> LTO pipeline (or something about LTO) seems to hit on
these issues much
>>>>>> more (I'm talking like 40k lines of ASan error
reports from building
>>>>>> test-suite with the LTO pipeline in the new PM; per-TU
steps still using
>>>>>> the old PM). Some notes:
>>>>>>
>>>>>> - BasicAA's dependence on domtree and loopinfo in
the new PM seems to
>>>>>> account for quite a few of the problems.
>>>>>> - BasicAA and other stuff are marked (by overriding
`invalidate` to
>>>>>> return false) to never be invalidated because they are
"stateless". However
>>>>>> they still hold pointers and so they do need to be
invalidated.
>>>>>> - CallGraph uses AssertingVH (PR28400) and so I needed
a workaround
>>>>>> similar to r274656 in various passes.
>>>>>> - D21921 is holding up -- I haven't hit any issues
with the core
>>>>>> logic of that patch.
>>>>>> - AAResults holds handles to various AA result objects.
This means it
>>>>>> pretty much always needs to be invalidated unless you
are sure that none of
>>>>>> the AA's will get invalidated.
>>>>>>
>>>>>>
>>>>>> The existing `invalidate` method doesn't have the
right semantics for
>>>>>> even an error-prone solution :( We are going to need to
make some
>>>>>> significant changes to even get basic sanity I think.
Perhaps each analysis
>>>>>> can expose a "preserve" static function. E.g.
instead of
>>>>>> `PA.preserve<Foo>();` you have to do
`Foo::setPreserved(PA);`.
>>>>>> I'm actually not quite sure that that will even
work. Once I have
>>>>>> test-suite fully building successfully with the LTO
pipeline in the new PM
>>>>>> I'll be able to give a more confident answer (esp.
w.r.t. the manager for
>>>>>> different IRUnitT's).
>>>>>> But at this point I'm not confident running *any*
pass pipeline in
>>>>>> the new PM without at least assertions+ASan.
>>>>>>
>>>>>> We may want to have a proper design discussion around
this problem
>>>>>> though.
>>>>>>
>>>>>> Also I'd like to have test-suite working (by hook
or by crook) with
>>>>>> LTO in the new PM so we can get some numbers on the
resident set impact of
>>>>>> all this caching; if it is really problematic then we
may need to start
>>>>>> talking front-and-center about different invalidation
policies for keeping
>>>>>> this in check instead of leaving it as something that
we will be able to
>>>>>> patch later.
>>>>>>
>>>>>>
>>>>>>
>>>>>> The more I think about it, the more I'm convinced
that the real
>>>>>> "hard" problem that the new PM is exposing us
to is having the ability for
>>>>>> any pass to ask for any analysis on any IRUnitT (and
any specific IRUnit of
>>>>>> that IRUnitT) and have the result stored somewhere and
then invalidated.
>>>>>> This means that "getAnalysisUsage" is not
just a list of passes, but much
>>>>>> more complicated and is essentially a set of arbitrary
pairs "(analysis,
>>>>>> IRUnit)" (and the associated potential tangle of
dependencies between the
>>>>>> state cached on these tuples). With the old PM, you
essentially are looking
>>>>>> at a problem of scheduling the lifetime of analyses of
the same IRUnit
>>>>>> intermingled with transformation passes on that same
IRUnit, so you only
>>>>>> have the "analysis" part of the tuple above,
making things much simpler
>>>>>> (and handling dependencies is much simpler too).
We've obviously outgrown
>>>>>> this model with examples like LAA,
AssumptionCacheTracker, etc. that hack
>>>>>> around this in the old PM. We may want to have a fresh
re-examination of
>>>>>> what problems we are exactly trying to solve.
>>>>>>
>>>>>> For me, my main concern now is what changes need to be
made in order
>>>>>> to feel confident running a pipeline in the new PM
without assertions+ASan.
>>>>>>
>>>>>>
>>>>>> Sorry for the long post, just brain-dumping before
heading home.
>>>>>>
>>>>>> -- Sean Silva
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> -- Sean Silva
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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----- Original Message -----
> From: "Sean Silva" <chisophugis at gmail.com>
> To: "Chandler Carruth" <chandlerc at gmail.com>
> Cc: "Xinliang David Li" <davidxl at google.com>,
"llvm-dev"
> <llvm-dev at lists.llvm.org>, "Davide Italiano"
> <dccitaliano at gmail.com>, "Tim Amini Golling"
> <mehdi.amini at apple.com>, "Hal Finkel" <hfinkel at
anl.gov>, "Sanjoy
> Das" <sanjoy at playingwithpointers.com>, "Pete
Cooper"
> <peter_cooper at apple.com>
> Sent: Tuesday, July 26, 2016 3:32:55 AM
> Subject: Re: [PM] I think that the new PM needs to learn about
> inter-analysis dependencies...
> I'm not quite sure what post to respond to for a status update, but I
> guess this one will do (and you can check my log for more info of
> course).
> My current working branch for the analysis manager stuff is at
> https://github.com/chisophugis/llvm/commits/analysis-manager
> (4ecf6115890bd01caa52c0b99424974e3469291e)
> I described in my log a bit more my thought process and how to do
> this without having to churn every single new PM pass (search for
> "Okay, so now that I think about it, adding the dependency
> management is a second step after making the analysis manager work
> for all IRUnit’s."). Essentially, the first step is to still have
> AnalysisManager<Function>, AnalysisManager<Module>, etc. but
the
> template argument is just a dummy. The real templating is on the
> methods, which can accept any IRUnit. The advantage of this is that
> it is source compatible with all the existing code using multiple
> manager, proxies, etc.
> The code on my branch at least passes check-llvm. But of course the
> existing code doesn't test any of the situations where being able to
> handle multiple IRUnitT's in a single analysis manager would matter.
> Tomorrow, I get to churn all the passes that have been ported so far,
> removing the proxies etc.
> Another thing that just came to me: the current way things work with
> adaptors means that if a function transformation invalidates a
> module analysis, the module analysis won't be invalidated until
> after the ModuleToFunctionPassAdapter returns.
> So we can end up running an arbitrary number of function
> transformations that query that stale module analysis. This is (I
> think?) mostly benign given our current set of module analyses (more
> info in the log; search for "We may be getting by simply because we
> seem to not have many module analyses"). But if we invalidate more
> "correctly" (after every transformation on any contained IRUnit),
> there's a potential quadratic compile time issue lurking here once
> have the unified analysis manager: we could easily end up
> invalidating/recomputing a module analysis once per function
> visitation.
> For a pipeline that just does a lot of churn (e.g. function
> simplification passes we run as part of the regular O3 pipeline;
> that can delete most code in a function, change the cfg, etc.) it's
> not clear what useful properties we can guarantee will be preserved
> which would prevent us from invalidating pretty much all
> non-immutable outer analyses (module or (theoretically)CGSCC). So
> for any really nontrivial module analysis, we may end up having to
> change the interface of module passes to have some way to
> incrementally recompute themselves as function passes mutate the IR?
> In some sense, Chandler's CGSCC patch https://reviews.llvm.org/D21464
> is trying to do this for a specific module analysis (lazy call
> graph) and even for just that one (and lots of special handling in
> the adaptors etc.) it still only gets incrementally updated it after
> a potentially large set of function passes have run. And it's
> already quite tricky.
> Broadly speaking, I can think of two major kinds of "solutions"
to
> this problem:
> - severely restrict the interaction of function transformations and
> module analyses in some way. Essentially, we could define a
> restricted class of module analyses that "are conservatively correct
> in the face of any transformation that can be made by a function
> transformation" (this includes immutable module analyses and IIRC
> things like GlobalsModRef). But the issue is a bit deeper, because
> it is really about transformations at any smaller IRUnitT. This
> includes CGSCC. And CGSCC can do substantial interprocedural
> transformation (e.g. delete a wholefunction IIRC?); we would then
> need to have another class of module analyses that are
> "conservatively correct in the face of CGSCC transformations"
which
> seems quite restrictive.
> -- alternatively or in addition, this can involve restricting what
> kind of information a function pass can get out of a module analysis
> - provide some sort of update callback for the outer IRUnit analysis
> to update itself if the inner one is modified (not sure how
> practical this will be). E.g. `AM.invalidate<FooModuleAnalysis>(F);`
> doesn't actually invalidate FooModuleAnalysis, but rather invokes a
> callback for it to update itself (possibly as an opt-in for the
> module analysis).
> -- The current analysis manager does have a `invalidate` hook that it
> will call, but the argument is always inherently the IRUnit that the
> analysis itself is cached on so it isn't useful for partial
> recomputation. We would need to extend that, which requires making
> the AnalysisResultConcept more complicated (and potentially having
> to have centralized awareness of all the different IRUnitT's, which
> until now are fairly decoupled in the new PM).
I think that one of these last two options seems best. We should support the
module-level analysis incrementally updating itself.

However, I think that we should invalidate eagerly by default, even though that
leads to a potential quadratic compile-time problem, and then make sure we fix
passes in the pipeline not to invalidate the module-level passes we use. This
particular problem is, as it were, by design.

Thanks again, 
Hal 
> I'm curious what others' thoughts are on this issue.
> -- Sean Silva
> On Sun, Jul 24, 2016 at 2:58 AM, Sean Silva < chisophugis at gmail.com
>
> wrote:
> > I've started looking specifically at the existing code and how it
> > needs to be changed. It seems like the concept-based polymorphism
> > stuff in PassManagerInternal.h actually don't need to be changed
> > that much. AnalysisPassConcept and AnalysisResultConcept need to be
> > changed to take a type-erased IRUnit (e.g. a void* or something).
> > AnalysisPassModel and AnalysisResultModel (which inherit from their
> > respective abstract base classes I mentioned above) are already
> > templated on the IRUnitT and so they can just cast the void* back
> > to
> > the right type.
> 
> > Adding the dependency tracking seems like it will be mostly a data
> > structure change with some isolated "algorithmic" changes to
> > track/invalidate dependencies.
> 
> > Most of the methods that need to know the specific IRUnitT type
> > already take the IRUnit as an argument (e.g. getResult,
> > getCachedResult, invalidate) and so it's actually somewhat natural
> > for the analysis manager not be templated on IRUnitT (but rather to
> > have just those methods be templated).
> 
> > Also, up until now I hadn't noticed this weird
"registerPass" thing
> > on the analysis manager (AnalysisManagerBase to be specific).
> > Effectively, it allows analyses to hold state (this is separate
> > from
> > analysis *results* which of course can hold state). But the state
> > is
> > effectively global to the AnalysisManager (and hence the pass
> > pipeline, and hence (essentially) the context (since you can
> > currently only run a single pass pipeline concurrently on a single
> > LLVMContext)). The net result is that you can specify a
> > context-global "configuration" for each analysis type (not
to be
> > confused with the analysis *result* type!). Right now, AAManager is
> > the only thing that uses it though (the "configuration" is
the AA
> > pipeline).
> 
> > btw, I've started to keep a log for this like I do for my projects
> > at
> > home:
> >
https://docs.google.com/document/d/1-nrq2y_hTiZhrsJDmH8TzFjFEeApfccs6wwGyaXjSkg/edit?usp=sharing
> 
> > It's fairly stream-of-consciousness. My log style is mostly
> > append-only (I rarely edit stuff from a previous day or from too
> > long ago on the same day; if I do I usually don't overwrite and
> > instead insert something like "(edit: I was actually totally
wrong
> > about this, see below)"). So if you want to follow this across
> > multiple days just go to the end and scroll up until you see
> > something that you've already looked at. (right now there is just
> > one day though so there is not very much).
> 
> > (some interesting things to search for are "interesting",
"okay",
> > "oh", and "?"; I tend to use these a lot)
> 
> > I've gone back to google docs for this log since it is easier to
> > share on the mailing list. Unfortunately google docs does not have
> > an explicit notion of "cell" like Mathematica does, so
I've tried
> > to
> > just insert lots of line breaks for things where there would have
> > just been a cell boundary in Mathematica. (I use Mathematica for my
> > logs at home).
> 
> > -- Sean Silva
> 
> > On Fri, Jul 22, 2016 at 1:55 AM, Sean Silva < chisophugis at
gmail.com
> > >
> > wrote:
> 
> > > The more closely I look at this, the more it seems like there may
> > > be
> > > a useful incremental step in the transition to the new PM: use
> > > the
> > > new PM analysis machinery in the old PM. If this is possible, it
> > > will simplify the old PM and (hopefully) allow an incremental
> > > transition to the new PM instead of a flag day transition for the
> > > switch.
> > 
> 
> > > I.e., AFAICT, the new PM transition is essentially about 2 mostly
> > > orthogonal aspects of running optimization pipelines:
> > 
> 
> > > 1. Analysis computation and analysis result lifetime management
> > > (including avoiding analysis recomputation)
> > 
> 
> > > 2. Running transformation passes over their respective
IRUnit's
> > > in
> > > some order
> > 
> 
> > > These are conflated in the old PM. In reality, the only
> > > interaction
> > > between them (with the new PM machinery for 1.) is a small number
> > > of
> > > places within 2. which need to call 'invalidate'.
> > 
> 
> > > I'm pretty sure that 2. is fairly similar in the new PM and
old
> > > PM
> > > (the main difference is that the notion of "adapters"
is split
> > > out
> > > in the new PM). The analysis handling seems to be what makes the
> > > old
> > > PM so difficult to understand (e.g. it is the cause of the
> > > multiple
> > > inheritance in the implementation). Trying to unify analyses and
> > > transformations (and some questionable (in hindsight)
> > > implementation
> > > decisions) seems to be the main "problem" with the
design of the
> > > old
> > > PM AFAICT (there are other issues, but they are more "nice
to
> > > have").
> > 
> 
> > > IMO it is an anti-pattern to think of analyses as
"passes". There
> > > are
> > > just "analyses" and "transformations" and
they are two separate
> > > things. In fact, the `run` method on analyses should probably be
> > > called `computeResult` or something like that to avoid confusion.
> > > And the `run` method on transformations could just as easily be
> > > called `performTransformation`.
> > 
> 
> > > I remember asking and getting and answer from Chandler (
> > >
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150907/299083.html
> > > ) about how to coexist the old and new PM compatibly so
> > > individual
> > > passes would be able to work for both and we wouldn't need to
> > > have
> > > a
> > > flag day. I wasn't able to find the discussions that Chandler
> > > cited,
> > > but I suspect that the answer is that we didn't have enough
> > > analyses
> > > ported at that point to consider sharing the analysis management
> > > between the old and new PM.
> > 
> 
> > > If this works out it may be the evolutionary path we have all
> > > been
> > > wanting for this pass manager transition. Fingers crossed.
> > > Hopefully
> > > I'm not overlooking some major issue.
> > 
> 
> > > Anyway... back to working on the analysis manager dependency
> > > tracking.
> > 
> 
> > > -- Sean Silva
> > 
> 
> > > On Thu, Jul 21, 2016 at 1:59 AM, Sean Silva <
> > > chisophugis at gmail.com
> > > >
> > > wrote:
> > 
> 
> > > > We did some basic sanity checking that memory usage
didn't go
> > > > out
> > > > of
> > > > control (it doesn't; at least with with a simple
> > > > preserves-all/preserves-none invalidation scheme and the
> > > > current
> > > > LTO
> > > > pipeline). Also, I did some basic sanity checking for
compile
> > > > time.
> > > > The simple preserves-all/preserves-none invalidation scheme
> > > > seems
> > > > marginally slower, but no real conclusions (besides a simple
> > > > sanity
> > > > check) can be drawn without the real analysis preservation
> > > > semantics
> > > > in place.
> > > 
> > 
> 
> > > > I'll start working on fixing the analysis managers.
There seem
> > > > to
> > > > basically be two parts (although they may need to be done
> > > > simultaneously to make sure all the pieces fit together):
> > > 
> > 
> 
> > > > - unify all the analysis managers into a single analysis
> > > > manager
> > > > for
> > > > all IRUnitT's (requires type-erasing the IRUnit)
> > > 
> > 
> 
> > > > - introduce the dependency tracking machinery
> > > 
> > 
> 
> > > > I think I gave a reasonable outline in the two posts above:
> > > 
> > 
> 
> > > > - the one starting with " To clarify, it seems like the
current
> > > > new
> > > > PM is essentially trying to solve the problem of
> > > > maintaining/updating a mapping"
> > > 
> > 
> 
> > > > - the one starting with " Yeah, the mechanics of
maintaining
> > > > this
> > > > fully general mapping are straightforward in the
abstract"
> > > 
> > 
> 
> > > > I'm happy to do a centralized writeup if anybody wants.
Just
> > > > let
> > > > me
> > > > know.
> > > 
> > 
> 
> > > > As far as changes to the code, t he updates to the new PM
> > > > passes
> > > > should hopefully be mechanical (despite there being many of
> > > > them).
> > > > However, from what I can tell, fixing this problem will
require
> > > > touching most lines of the analysis manager machinery so the
> > > > diff
> > > > will probably be a bit scary, even though I think we can
keep
> > > > the
> > > > same basic structure (i.e. a per-IRUnit std::list holding
one
> > > > analysis result (at a stable address) per element, combined
> > > > with
> > > > a
> > > > DenseMap from (Analysis, IRUnit) to an element of the
> > > > per-IRUnit
> > > > storage list (see AnalysisResultListMapT and
AnalysisResultMapT
> > > > in
> > > > include/llvm/IR/PassManager.h)).
> > > 
> > 
> 
> > > > The main changes to the analysis manager will be:
> > > 
> > 
> 
> > > > - type-erasing the IRUnit
> > > 
> > 
> 
> > > > - the elements of the AnalysisResultListMapT will need to
keep
> > > > track
> > > > of any dependents
> > > 
> > 
> 
> > > > - the analysis manager will need to update those
dependencies
> > > > as
> > > > it
> > > > is re-entered by analyses getting results of other analyses
> > > 
> > 
> 
> > > > - the analysis manager will need to walk the dependencies to
do
> > > > transitive invalidation
> > > 
> > 
> 
> > > > I think the most robust approach is for analysis
dependencies
> > > > to
> > > > be
> > > > implicitly constructed by the analysis manager via tracking
> > > > entry/exit from get{,Cached}Result.
> > > 
> > 
> 
> > > > One alternative is for analyses to explicitly pass in their
ID
> > > > to
> > > > getResult to indicate the dependency, but that seems
> > > > error-prone
> > > > (and also verbose). The issue is that we will need a
getResult
> > > > API
> > > > that does not track dependencies for use by transformation
> > > > passes
> > > > (since there is no dependency to track in that case); so an
> > > > innocuous copy-paste from a transform pass to an analysis
can
> > > > result
> > > > in a failure to track dependencies and risk of
use-after-free
> > > > (we
> > > > could fight this with the type system, but I think that
would
> > > > get
> > > > a
> > > > bit verbose (I'm willing to try it though if people
would
> > > > prefer))
> > > 
> > 
> 
> > > > One restriction of the implicit tracking approach is that it
> > > > requires
> > > > all calls into the analysis manager to occur in the `run`
> > > > method
> > > > of
> > > > the analysis (so that the dependencies are implicitly
tracked
> > > > via
> > > > re-entrance to get{,Cached}Result); is this a reasonable
> > > > restriction?
> > > 
> > 
> 
> > > > One annoying problem is that I think that the dependency
links
> > > > will
> > > > need to be bidirectional. To use the example analysis cache
> > > > from
> > > > my
> > > > other post:
> > > 
> > 
> 
> > > > (AssumptionAnalysis, function @bar) -> (AssumptionCache
for
> > > > @bar,
> > > > [(SomeModuleAnalysis, module TheModule)])
> > > 
> > 
> 
> > > > (AssumptionAnalysis, function @baz) -> (AssumptionCache
for
> > > > @baz,
> > > > [(SomeModuleAnalysis, module TheModule)])
> > > 
> > 
> 
> > > > (SomeModuleAnalysis, module TheModule) ->
> > > > (SomeModuleAnalysisResult
> > > > for TheModule, [(SomeFunctionAnalysis, function @baz)])
> > > 
> > 
> 
> > > > (SomeFunctionAnalysis, function @baz) ->
> > > > (SomeFunctionAnalysisResult
> > > > for @baz, [])
> > > 
> > 
> 
> > > > if we delete function @baz, then the dependent list
> > > > [(SomeFunctionAnalysis, function @baz)] for `
> > > > (SomeModuleAnalysis,
> > > > module TheModule)` will now have a stale pointer to function
> > > > @baz.
> > > > I
> > > > think that in practice we can check to see if `
> > > > (SomeFunctionAnalysis, function @baz)` is in our hash table
and
> > > > if
> > > > it isn't then just ignore the dependency as "this
dependent
> > > > analysis
> > > > result has already been freed". In the worst case
(memory
> > > > allocator
> > > > reuses the memory for another function) we may spuriously
free
> > > > an
> > > > analysis result for a different function. However it is
still
> > > > unsettling (and may actually be UB in C++?).
> > > 
> > 
> 
> > > > Ideally we would track bidirectional links; that way when we
> > > > remove
> > > > an analysis result we also have it remove itself from
> > > > dependence
> > > > lists of all of its dependencies.
> > > 
> > 
> 
> > > > -- Sean Silva
> > > 
> > 
> 
> > > > On Fri, Jul 15, 2016 at 8:40 PM, Sean Silva <
> > > > chisophugis at gmail.com
> > > > >
> > > > wrote:
> > > 
> > 
> 
> > > > > On Fri, Jul 15, 2016 at 8:39 PM, Sean Silva <
> > > > > chisophugis at gmail.com
> > > > > >
> > > > > wrote:
> > > > 
> > > 
> > 
> 
> > > > > > It looks like there is really no sane fix within
the
> > > > > > current
> > > > > > infrastructure. I've had to essentially
trigger
> > > > > > invalidation
> > > > > > (except
> > > > > > in the PreservedAnalyses::all() case) in the
function pass
> > > > > > manager
> > > > > > and function to loop adapters.
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > invalidation of *everything* I mean.
> > > > 
> > > 
> > 
> 
> > > > > -- Sean Silva
> > > > 
> > > 
> > 
> 
> > > > > > So we basically need to get the analysis manager
dependency
> > > > > > tracking
> > > > > > fixed.
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > Davide and I will get measurements on the resident
set
> > > > > > impact
> > > > > > of
> > > > > > all
> > > > > > this caching (even with the overconservative
invalidation
> > > > > > for
> > > > > > now)
> > > > > > to see the impact. If there is a big rss impact
then we
> > > > > > probably
> > > > > > want to consider that problem at the same time as
the
> > > > > > rewrite
> > > > > > of
> > > > > > the
> > > > > > analysis manager.
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > -- Sean Silva
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > On Thu, Jul 14, 2016 at 12:51 AM, Sean Silva <
> > > > > > chisophugis at gmail.com
> > > > > > > wrote:
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > On Wed, Jul 13, 2016 at 1:48 AM, Sean Silva
<
> > > > > > > chisophugis at gmail.com
> > > > > > > >
> > > > > > > wrote:
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > On Wed, Jul 13, 2016 at 12:34 AM,
Chandler Carruth <
> > > > > > > > chandlerc at gmail.com > wrote:
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > On Wed, Jul 13, 2016 at 12:25 AM
Sean Silva <
> > > > > > > > > chisophugis at gmail.com
> > > > > > > > > >
> > > > > > > > > wrote:
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > On Tue, Jul 12, 2016 at 11:39
PM, Chandler Carruth
> > > > > > > > > > <
> > > > > > > > > > chandlerc at gmail.com >
wrote:
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > On Tue, Jul 12, 2016 at
11:34 PM Sean Silva <
> > > > > > > > > > > chisophugis at gmail.com
> > > > > > > > > > > >
> > > > > > > > > > > wrote:
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > > On Tue, Jul 12, 2016
at 11:32 PM, Xinliang
> > > > > > > > > > > > David
> > > > > > > > > > > > Li
> > > > > > > > > > > > <
> > > > > > > > > > > > davidxl at
google.com > wrote:
> > > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > > > On Tue, Jul 12,
2016 at 10:57 PM, Chandler
> > > > > > > > > > > > > Carruth
> > > > > > > > > > > > > <
> > > > > > > > > > > > > chandlerc at
gmail.com > wrote:
> > > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > > > > Yea, this
is a nasty problem.
> > > > > > > > > > > > > 
> > > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > > > > One
important thing to understand is that
> > > > > > > > > > > > > > this
> > > > > > > > > > > > > > is
> > > > > > > > > > > > > > specific
> > > > > > > > > > > > > > to
> > > > > > > > > > > > > > analyses
which hold references to other
> > > > > > > > > > > > > > analyses.
> > > > > > > > > > > > > > While
> > > > > > > > > > > > > > this
> > > > > > > > > > > > > > isn't
> > > > > > > > > > > > > > unheard
of, it isn't as common as it could
> > > > > > > > > > > > > > be.
> > > > > > > > > > > > > > Still,
> > > > > > > > > > > > > > definitely
> > > > > > > > > > > > > > something
we need to address.
> > > > > > > > > > > > > 
> > > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > > > We can call
this type of dependencies
> > > > > > > > > > > > > (holding
> > > > > > > > > > > > > references)
> > > > > > > > > > > > >
hard-dependency. The soft dependency refers
> > > > > > > > > > > > > to
> > > > > > > > > > > > > the
> > > > > > > > > > > > > case
> > > > > > > > > > > > > where
> > > > > > > > > > > > > analysis
'A' depends on 'B' during
> > > > > > > > > > > > > computation,
> > > > > > > > > > > > > but
> > > > > > > > > > > > > does
> > > > > > > > > > > > > not
> > > > > > > > > > > > > need
> > > > > > > > > > > > > 'B'
once it is computed.
> > > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > > > There are
actually quite a few examples of
> > > > > > > > > > > > > hard-dependency
> > > > > > > > > > > > > case.
> > > > > > > > > > > > > For
> > > > > > > > > > > > > instance
LoopAccessInfo, LazyValueInfo etc
> > > > > > > > > > > > > which
> > > > > > > > > > > > > hold
> > > > > > > > > > > > > references
> > > > > > > > > > > > > to
> > > > > > > > > > > > > other analyses.
> > > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > > > Problem
involving hard-dependency is actually
> > > > > > > > > > > > > easier
> > > > > > > > > > > > > to
> > > > > > > > > > > > > detect,
> > > > > > > > > > > > > as
> > > > > > > > > > > > > it
> > > > > > > > > > > > > is usually a
compile time problem. Issues
> > > > > > > > > > > > > involving
> > > > > > > > > > > > > soft
> > > > > > > > > > > > > dependencies
are more subtle and can lead to
> > > > > > > > > > > > > wrong
> > > > > > > > > > > > > code
> > > > > > > > > > > > > gen.
> > > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > > Did you mean to say
that soft-dependency
> > > > > > > > > > > > problems
> > > > > > > > > > > > are
> > > > > > > > > > > > easier
> > > > > > > > > > > > to
> > > > > > > > > > > > detect? At least my
intuition is that
> > > > > > > > > > > > soft-dependency
> > > > > > > > > > > > is
> > > > > > > > > > > > easier
> > > > > > > > > > > > because there is no
risk of dangling pointers
> > > > > > > > > > > > to
> > > > > > > > > > > > other
> > > > > > > > > > > > analyses.
> > > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > The issue is that the
fact that there is *any*
> > > > > > > > > > > dependency
> > > > > > > > > > > isn't
> > > > > > > > > > > clear.
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > However, I think the only
real problem here are
> > > > > > > > > > > these
> > > > > > > > > > > "hard
> > > > > > > > > > > dependencies" (I
don't really like that term
> > > > > > > > > > > though).
> > > > > > > > > > > For
> > > > > > > > > > > others,
> > > > > > > > > > > only an analysis that is
*explicitly* preserved
> > > > > > > > > > > survives.
> > > > > > > > > > > So
> > > > > > > > > > > I'm
> > > > > > > > > > > not
> > > > > > > > > > > worried about the fact
that people have to
> > > > > > > > > > > remember
> > > > > > > > > > > this.
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > > The question is how often
there are
> > > > > > > > > > > cross-data-structure
> > > > > > > > > > > references.
> > > > > > > > > > > David mentions a few
examples, and I'm sure there
> > > > > > > > > > > are
> > > > > > > > > > > more,
> > > > > > > > > > > but
> > > > > > > > > > > it
> > > > > > > > > > > isn't clear to me yet
whether this is pervasive
> > > > > > > > > > > or
> > > > > > > > > > > occasional.
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > I just did a quick run-through
of PassRegistry.def
> > > > > > > > > > and
> > > > > > > > > > this
> > > > > > > > > > is
> > > > > > > > > > what
> > > > > > > > > > I
> > > > > > > > > > found:
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > Module analyses: 0/5 hold
pointers to other
> > > > > > > > > > analyses
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > CallGraph: No pointers to
other analyses.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > LazyCallGraph: No pointers to
other analyses.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > ProfileSummaryAnalysis: No
pointers to other
> > > > > > > > > > analyses.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > TargetLibraryAnalysis: No
pointers to other
> > > > > > > > > > analyses.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > VerifierAnalysis: No pointers
to other analyses.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > Module alias analyses: 1/1
keeps pointer to other
> > > > > > > > > > analysis.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > GlobalsAA: Result keeps
pointer to TLI (this is a
> > > > > > > > > > function
> > > > > > > > > > analysis).
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > Function analyses: 9/17 keep
pointers to other
> > > > > > > > > > analysis
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > AAManager: Its Result holds
TLI pointer and
> > > > > > > > > > pointers
> > > > > > > > > > to
> > > > > > > > > > individual
> > > > > > > > > > AA
> > > > > > > > > > result objects.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > AssumptionAnalysis: No
pointers to other analyses.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > BlockFrequencyAnalysis: Its
Result holds pointers
> > > > > > > > > > to
> > > > > > > > > > LoopInfo
> > > > > > > > > > and
> > > > > > > > > > BPI.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > BranchProbabilityAnalysis:
Stores no pointers to
> > > > > > > > > > other
> > > > > > > > > > analyses.
> > > > > > > > > > (uses LoopInfo to
"recalculate" though)
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > DominatorTreeAnalysis: Stores
no pointers to other
> > > > > > > > > > analyses.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > PostDominatorTreeAnalysis:
Stores no pointers to
> > > > > > > > > > other
> > > > > > > > > > analyses.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > DemandedBitsAnalysis: Stores
pointers to
> > > > > > > > > > AssumptionCache
> > > > > > > > > > and
> > > > > > > > > > DominatorTree
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > DominanceFrontierAnalysis:
Stores no pointers to
> > > > > > > > > > other
> > > > > > > > > > analyses.
> > > > > > > > > > (uses DominatorTreeAnalysis
for "recalculate"
> > > > > > > > > > though).
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > LoopInfo: Uses
DominatorTreeAnalysis for
> > > > > > > > > > "recalculate"
> > > > > > > > > > but
> > > > > > > > > > stores
> > > > > > > > > > no
> > > > > > > > > > pointers.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > LazyValueAnalysis: Stores
pointers to
> > > > > > > > > > AssumptionCache,
> > > > > > > > > > TargetLibraryInfo,
DominatorTree.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > DependenceAnalysis: Stores
pointers to
> > > > > > > > > > AliasAnalysis,
> > > > > > > > > > ScalarEvolution, LoopInfo
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > MemoryDependenceAnalysis:
Stores pointers to
> > > > > > > > > > AliasAnalysis,
> > > > > > > > > > AssumptionCache,
TargetLibraryInfo, DominatorTree
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > MemorySSAAnalysis: Stores
pointers to
> > > > > > > > > > AliasAnalysis,
> > > > > > > > > > DominatorTree
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > RegionInfoAnalysis: Stores
pointers to DomTree,
> > > > > > > > > > PostDomTree,
> > > > > > > > > > DomFrontier
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > ScalarEvolutionAnalysis:
Stores pointers to
> > > > > > > > > > TargetLibraryInfo,
> > > > > > > > > > AssumptionCache,
DominatorTree, LoopInfo
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > TargetLibraryAnalysis: Has no
dependencies
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > TargetIRAnalysis: Has no
dependencies.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > Function alias analyses: 3/5
keep pointers to other
> > > > > > > > > > analyses
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > BasicAA: Keeps pointers to
TargetLibraryInfo,
> > > > > > > > > > AssumptionCache,
> > > > > > > > > > DominatorTree, LoopInfo
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > CFLAA: Keeps pointer to
TargetLibraryInfo
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > SCEVAA: Keeps pointer to
ScalarEvolution
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > ScopedNoAliasAA: No
dependencies
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > TypeBasedAA: No dependencies
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > Total: 13/28 analyses (~50%)
hold pointers to other
> > > > > > > > > > analyses.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > Of the 15/28 analyses that
don't hold pointers,
> > > > > > > > > > 12/15
> > > > > > > > > > simply
> > > > > > > > > > have
> > > > > > > > > > no
> > > > > > > > > > dependencies. Only 3/15 (BPI,
LoopInfo,
> > > > > > > > > > DominanceFrontier)
> > > > > > > > > > have
> > > > > > > > > > dependencies that are used
just for a "recalculate"
> > > > > > > > > > step
> > > > > > > > > > that
> > > > > > > > > > retains no pointers.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > So I think it is fair to say
that analyses which
> > > > > > > > > > hold
> > > > > > > > > > pointers
> > > > > > > > > > to
> > > > > > > > > > other analyses is not an
exceptional case. In fact,
> > > > > > > > > > analyses
> > > > > > > > > > that
> > > > > > > > > > use other analyses just for a
"recalculate" step
> > > > > > > > > > seems
> > > > > > > > > > to
> > > > > > > > > > be
> > > > > > > > > > the
> > > > > > > > > > exceptional case (only 3/28 or
about 10%)
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > Interesting!
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > Most of these look like they hold a
pointer to the
> > > > > > > > > root
> > > > > > > > > analysis
> > > > > > > > > as
> > > > > > > > > opposed to detailed objects
*inside* the analysis?
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > It might make sense to try to
handle this very
> > > > > > > > > specific
> > > > > > > > > pattern
> > > > > > > > > in
> > > > > > > > > a
> > > > > > > > > special way of overriding the
invalidate routines is
> > > > > > > > > too
> > > > > > > > > error
> > > > > > > > > prone.... We could try to make this
work
> > > > > > > > > "automatically"
> > > > > > > > > but
> > > > > > > > > I'm
> > > > > > > > > worried this would be challenging
to get right. Open
> > > > > > > > > to
> > > > > > > > > suggestions
> > > > > > > > > of course.
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > Any other ideas about what would
make sense to handle
> > > > > > > > > this?
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > Does it make sense to override the
invalidate
> > > > > > > > > routines
> > > > > > > > > now
> > > > > > > > > and
> > > > > > > > > iterate from there? I feel like
you've done a lot of
> > > > > > > > > the
> > > > > > > > > research
> > > > > > > > > necessary for this already...
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > I'll keep pushing forward tomorrow
with building
> > > > > > > > test-suite
> > > > > > > > successfully using the new PM for the
LTO pipeline (I
> > > > > > > > was
> > > > > > > > doing
> > > > > > > > some
> > > > > > > > unrelated LLD stuff for most of today).
It will be
> > > > > > > > interesting
> > > > > > > > to
> > > > > > > > see how many `invalidate` overrides will
be needed to
> > > > > > > > avoid
> > > > > > > > these
> > > > > > > > issues for just the LTO pipeline on
test-suite.
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > I spent the better part of today working on
this and will
> > > > > > > continue
> > > > > > > tomorrow; this problem seems nastier than I
thought. For
> > > > > > > some
> > > > > > > reason
> > > > > > > the LTO pipeline (or something about LTO)
seems to hit on
> > > > > > > these
> > > > > > > issues much more (I'm talking like 40k
lines of ASan
> > > > > > > error
> > > > > > > reports
> > > > > > > from building test-suite with the LTO
pipeline in the new
> > > > > > > PM;
> > > > > > > per-TU
> > > > > > > steps still using the old PM). Some notes:
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > - BasicAA's dependence on domtree and
loopinfo in the new
> > > > > > > PM
> > > > > > > seems
> > > > > > > to
> > > > > > > account for quite a few of the problems.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > - BasicAA and other stuff are marked (by
overriding
> > > > > > > `invalidate`
> > > > > > > to
> > > > > > > return false) to never be invalidated because
they are
> > > > > > > "stateless".
> > > > > > > However they still hold pointers and so they
do need to
> > > > > > > be
> > > > > > > invalidated.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > - CallGraph uses AssertingVH (PR28400) and so
I needed a
> > > > > > > workaround
> > > > > > > similar to r274656 in various passes.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > - D21921 is holding up -- I haven't hit
any issues with
> > > > > > > the
> > > > > > > core
> > > > > > > logic of that patch.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > - AAResults holds handles to various AA
result objects.
> > > > > > > This
> > > > > > > means
> > > > > > > it
> > > > > > > pretty much always needs to be invalidated
unless you are
> > > > > > > sure
> > > > > > > that
> > > > > > > none of the AA's will get invalidated.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > The existing `invalidate` method doesn't
have the right
> > > > > > > semantics
> > > > > > > for
> > > > > > > even an error-prone solution :( We are going
to need to
> > > > > > > make
> > > > > > > some
> > > > > > > significant changes to even get basic sanity
I think.
> > > > > > > Perhaps
> > > > > > > each
> > > > > > > analysis can expose a "preserve"
static function. E.g.
> > > > > > > instead
> > > > > > > of
> > > > > > > `PA.preserve<Foo>();` you have to do
> > > > > > > `Foo::setPreserved(PA);`.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > I'm actually not quite sure that that
will even work.
> > > > > > > Once
> > > > > > > I
> > > > > > > have
> > > > > > > test-suite fully building successfully with
the LTO
> > > > > > > pipeline
> > > > > > > in
> > > > > > > the
> > > > > > > new PM I'll be able to give a more
confident answer (esp.
> > > > > > > w.r.t.
> > > > > > > the
> > > > > > > manager for different IRUnitT's).
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > But at this point I'm not confident
running *any* pass
> > > > > > > pipeline
> > > > > > > in
> > > > > > > the new PM without at least assertions+ASan.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > We may want to have a proper design
discussion around
> > > > > > > this
> > > > > > > problem
> > > > > > > though.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > Also I'd like to have test-suite working
(by hook or by
> > > > > > > crook)
> > > > > > > with
> > > > > > > LTO in the new PM so we can get some numbers
on the
> > > > > > > resident
> > > > > > > set
> > > > > > > impact of all this caching; if it is really
problematic
> > > > > > > then
> > > > > > > we
> > > > > > > may
> > > > > > > need to start talking front-and-center about
different
> > > > > > > invalidation
> > > > > > > policies for keeping this in check instead of
leaving it
> > > > > > > as
> > > > > > > something that we will be able to patch
later.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > The more I think about it, the more I'm
convinced that
> > > > > > > the
> > > > > > > real
> > > > > > > "hard" problem that the new PM is
exposing us to is
> > > > > > > having
> > > > > > > the
> > > > > > > ability for any pass to ask for any analysis
on any
> > > > > > > IRUnitT
> > > > > > > (and
> > > > > > > any
> > > > > > > specific IRUnit of that IRUnitT) and have the
result
> > > > > > > stored
> > > > > > > somewhere and then invalidated. This means
that
> > > > > > > "getAnalysisUsage"
> > > > > > > is not just a list of passes, but much more
complicated
> > > > > > > and
> > > > > > > is
> > > > > > > essentially a set of arbitrary pairs
"(analysis, IRUnit)"
> > > > > > > (and
> > > > > > > the
> > > > > > > associated potential tangle of dependencies
between the
> > > > > > > state
> > > > > > > cached
> > > > > > > on these tuples). With the old PM, you
essentially are
> > > > > > > looking
> > > > > > > at
> > > > > > > a
> > > > > > > problem of scheduling the lifetime of
analyses of the
> > > > > > > same
> > > > > > > IRUnit
> > > > > > > intermingled with transformation passes on
that same
> > > > > > > IRUnit,
> > > > > > > so
> > > > > > > you
> > > > > > > only have the "analysis" part of
the tuple above, making
> > > > > > > things
> > > > > > > much
> > > > > > > simpler (and handling dependencies is much
simpler too).
> > > > > > > We've
> > > > > > > obviously outgrown this model with examples
like LAA,
> > > > > > > AssumptionCacheTracker, etc. that hack around
this in the
> > > > > > > old
> > > > > > > PM.
> > > > > > > We
> > > > > > > may want to have a fresh re-examination of
what problems
> > > > > > > we
> > > > > > > are
> > > > > > > exactly trying to solve.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > For me, my main concern now is what changes
need to be
> > > > > > > made
> > > > > > > in
> > > > > > > order
> > > > > > > to feel confident running a pipeline in the
new PM
> > > > > > > without
> > > > > > > assertions+ASan.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > Sorry for the long post, just brain-dumping
before
> > > > > > > heading
> > > > > > > home.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > -- Sean Silva
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > -- Sean Silva
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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Okay, did the big renaming and de-templating today (and removing the
proxies).

check-llvm worked fine, except that this test caught a bug (yay!):
unittests/IR/PassManagerTest.cpp:326

After removing the proxies, the module pass is not invalidating the
function passes (since the invalidation is on the module IRUnit). we now
don't have a method for delegating the invalidation from outer IRUnit's
to
inner IRUnit's.
I'll try to add a mechanism for the analysis manager to do this itself if
there's a straightforward way to do so. Not sure the best way. We can't
just iterate over all the functions calling `invalidate`; a module pass may
have e.g. deleted a function, so we would never call `invalidate` on the
deleted function. A simple solution is to add another member to the
analysis manager which tracks, for each IRUnitT, all IRUnit's that we have
state cached for (so we can easily walk them).

Interestingly, it seems like the current state in trunk is that a module
analysis can't preserve a specific function analysis. The only way that
analysis invalidation can propagate downwards is by clearing the entire
inner analysis manager.
So I think this means that the current state in trunk is that if a function
transformation returns PreservedAnalyses::none(), it will invalidate all
loop analyses for all functions.
(I think Chandler mentioned this issue in passing here:
https://reviews.llvm.org/D21921#inline-187608)

(slightly more info in my log)

Anyway, for now, I'll try to do something that is as NFC as possible.
Hopefully just tracking all the IRUnit's per IRUnitT will allow us to
emulate the proxy-based invalidation fairly well.


It is probably relevant to discuss how we want to "properly" deal with
this
with the unified analysis manager for all IRUnitT's.

One solution is to have the pass managers (actually, the adaptors),
push/pop a stack of the current IRUnit nest (e.g. the stack might contain
at a given point in time something like `[Module foo, Function @bar]` or
`[Module foo, SCC bar, Function @baz, Loop %qux]`).
The key information that is needed is for the analysis manager to be able
to reconstruct a parent-child relationship of IRUnit's (which might contain
parent-child links that are no longer obtainable from the parent IRUnit;
e.g. when you go to invalidate analyses after running a module pass that
deleted a function; the analysis manager must still track that relationship
even if the module has already forgotten about the function).

I don't see any reasonable way to do this without cooperation from the
adaptors, because (like the example "IRUnit stacks" I showed above)
the
analysis manager would need a way to know whether to map back to a
particular SCC or to a particular Module (or potentially both).

-- Sean Silva



On Tue, Jul 26, 2016 at 1:32 AM, Sean Silva <chisophugis at gmail.com>
wrote:
> I'm not quite sure what post to respond to for a status update, but I
> guess this one will do (and you can check my log for more info of course).
>
> My current working branch for the analysis manager stuff is at
> https://github.com/chisophugis/llvm/commits/analysis-manager
> (4ecf6115890bd01caa52c0b99424974e3469291e)
>
> I described in my log a bit more my thought process and how to do this
> without having to churn every single new PM pass (search for "Okay, so
now
> that I think about it, adding the dependency management is a second step
> after making the analysis manager work for all IRUnit’s.").
Essentially,
> the first step is to still have AnalysisManager<Function>,
> AnalysisManager<Module>, etc. but the template argument is just a
dummy.
> The real templating is on the methods, which can accept any IRUnit. The
> advantage of this is that it is source compatible with all the existing
> code using multiple manager, proxies, etc.
>
> The code on my branch at least passes check-llvm. But of course the
> existing code doesn't test any of the situations where being able to
handle
> multiple IRUnitT's in a single analysis manager would matter.
> Tomorrow, I get to churn all the passes that have been ported so far,
> removing the proxies etc.
>
>
>
>
>
> Another thing that just came to me: the current way things work with
> adaptors means that if a function transformation invalidates a module
> analysis, the module analysis won't be invalidated until after the
> ModuleToFunctionPassAdapter returns.
>
> So we can end up running an arbitrary number of function transformations
> that query that stale module analysis. This is (I think?) mostly benign
> given our current set of module analyses (more info in the log; search for
> "We may be getting by simply because we seem to not have many module
> analyses"). But if we invalidate more "correctly" (after
every
> transformation on any contained IRUnit), there's a potential quadratic
> compile time issue lurking here once have the unified analysis manager: we
> could easily end up invalidating/recomputing a module analysis once per
> function visitation.
>
> For a pipeline that just does a lot of churn (e.g. function simplification
> passes we run as part of the regular O3 pipeline; that can delete most code
> in a function, change the cfg, etc.) it's not clear what useful
properties
> we can guarantee will be preserved which would prevent us from invalidating
> pretty much all non-immutable outer analyses (module or
> (theoretically)CGSCC). So for any really nontrivial module analysis, we may
> end up having to change the interface of module passes to have some way to
> incrementally recompute themselves as function passes mutate the IR?
>
> In some sense, Chandler's CGSCC patch https://reviews.llvm.org/D21464
is
> trying to do this for a specific module analysis (lazy call graph) and even
> for just that one (and lots of special handling in the adaptors etc.) it
> still only gets incrementally updated it after a potentially large set of
> function passes have run. And it's already quite tricky.
>
> Broadly speaking, I can think of two major kinds of "solutions"
to this
> problem:
> - severely restrict the interaction of function transformations and module
> analyses in some way. Essentially, we could define a restricted class of
> module analyses that "are conservatively correct in the face of any
> transformation that can be made by a function transformation" (this
> includes immutable module analyses and IIRC things like GlobalsModRef). But
> the issue is a bit deeper, because it is really about transformations at
> any smaller IRUnitT. This includes CGSCC. And CGSCC can do substantial
> interprocedural transformation (e.g. delete a wholefunction IIRC?); we
> would then need to have another class of module analyses that are
> "conservatively correct in the face of CGSCC transformations"
which seems
> quite restrictive.
> -- alternatively or in addition, this can involve restricting what kind of
> information a function pass can get out of a module analysis
> - provide some sort of update callback for the outer IRUnit analysis to
> update itself if the inner one is modified (not sure how practical this
> will be). E.g. `AM.invalidate<FooModuleAnalysis>(F);` doesn't
actually
> invalidate FooModuleAnalysis, but rather invokes a callback for it to
> update itself (possibly as an opt-in for the module analysis).
> -- The current analysis manager does have a `invalidate` hook that it will
> call, but the argument is always inherently the IRUnit that the analysis
> itself is cached on so it isn't useful for partial recomputation. We
would
> need to extend that, which requires making the AnalysisResultConcept more
> complicated (and potentially having to have centralized awareness of all
> the different IRUnitT's, which until now are fairly decoupled in the
new
> PM).
>
>
> I'm curious what others' thoughts are on this issue.
>
> -- Sean Silva
>
> On Sun, Jul 24, 2016 at 2:58 AM, Sean Silva <chisophugis at
gmail.com> wrote:
>
>> I've started looking specifically at the existing code and how it
needs
>> to be changed. It seems like the concept-based polymorphism stuff in
>> PassManagerInternal.h actually don't need to be changed that much.
>> AnalysisPassConcept and AnalysisResultConcept need to be changed to
take a
>> type-erased IRUnit (e.g. a void* or something). AnalysisPassModel and
>> AnalysisResultModel (which inherit from their respective abstract base
>> classes I mentioned above) are already templated on the IRUnitT and so
they
>> can just cast the void* back to the right type.
>>
>> Adding the dependency tracking seems like it will be mostly a data
>> structure change with some isolated "algorithmic" changes to
>> track/invalidate dependencies.
>>
>> Most of the methods that need to know the specific IRUnitT type already
>> take the IRUnit as an argument (e.g. getResult, getCachedResult,
>> invalidate) and so it's actually somewhat natural for the analysis
manager
>> not be templated on IRUnitT (but rather to have just those methods be
>> templated).
>>
>> Also, up until now I hadn't noticed this weird
"registerPass" thing on
>> the analysis manager (AnalysisManagerBase to be specific). Effectively,
it
>> allows analyses to hold state (this is separate from analysis *results*
>> which of course can hold state). But the state is effectively global to
the
>> AnalysisManager (and hence the pass pipeline, and hence (essentially)
the
>> context (since you can currently only run a single pass pipeline
>> concurrently on a single LLVMContext)). The net result is that you can
>> specify a context-global "configuration" for each analysis
type (not to be
>> confused with the analysis *result* type!). Right now, AAManager is the
>> only thing that uses it though (the "configuration" is the AA
pipeline).
>>
>>
>>
>> btw, I've started to keep a log for this like I do for my projects
at
>> home:
>>
https://docs.google.com/document/d/1-nrq2y_hTiZhrsJDmH8TzFjFEeApfccs6wwGyaXjSkg/edit?usp=sharing
>> It's fairly stream-of-consciousness. My log style is mostly
append-only
>> (I rarely edit stuff from a previous day or from too long ago on the
same
>> day; if I do I usually don't overwrite and instead insert something
like
>> "(edit: I was actually totally wrong about this, see
below)"). So if you
>> want to follow this across multiple days just go to the end and scroll
up
>> until you see something that you've already looked at. (right now
there is
>> just one day though so there is not very much).
>>
>> (some interesting things to search for are "interesting",
"okay", "oh",
>> and "?"; I tend to use these a lot)
>>
>> I've gone back to google docs for this log since it is easier to
share on
>> the mailing list. Unfortunately google docs does not have an explicit
>> notion of "cell" like Mathematica does, so I've tried to
just insert lots
>> of line breaks for things where there would have just been a cell
boundary
>> in Mathematica. (I use Mathematica for my logs at home).
>>
>>
>> -- Sean Silva
>>
>> On Fri, Jul 22, 2016 at 1:55 AM, Sean Silva <chisophugis at
gmail.com>
>> wrote:
>>
>>> The more closely I look at this, the more it seems like there may
be a
>>> useful incremental step in the transition to the new PM: use the
new PM
>>> analysis machinery in the old PM. If this is possible, it will
simplify the
>>> old PM and (hopefully) allow an incremental transition to the new
PM
>>> instead of a flag day transition for the switch.
>>>
>>> I.e., AFAICT, the new PM transition is essentially about 2 mostly
>>> orthogonal aspects of running optimization pipelines:
>>> 1. Analysis computation and analysis result lifetime management
>>> (including avoiding analysis recomputation)
>>> 2. Running transformation passes over their respective IRUnit's
in some
>>> order
>>>
>>> These are conflated in the old PM. In reality, the only interaction
>>> between them (with the new PM machinery for 1.) is a small number
of places
>>> within 2. which need to call 'invalidate'.
>>>
>>> I'm pretty sure that 2. is fairly similar in the new PM and old
PM (the
>>> main difference is that the notion of "adapters" is split
out in the new
>>> PM). The analysis handling seems to be what makes the old PM so
difficult
>>> to understand (e.g. it is the cause of the multiple inheritance in
the
>>> implementation). Trying to unify analyses and transformations (and
some
>>> questionable (in hindsight) implementation decisions) seems to be
the main
>>> "problem" with the design of the old PM AFAICT (there are
other issues, but
>>> they are more "nice to have").
>>>
>>> IMO it is an anti-pattern to think of analyses as
"passes". There are
>>> just "analyses" and "transformations" and they
are two separate things. In
>>> fact, the `run` method on analyses should probably be called
>>> `computeResult` or something like that to avoid confusion. And the
`run`
>>> method on transformations could just as easily be called
>>> `performTransformation`.
>>>
>>>
>>> I remember asking and getting and answer from Chandler (
>>>
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150907/299083.html)
>>> about how to coexist the old and new PM compatibly so individual
passes
>>> would be able to work for both and we wouldn't need to have a
flag day. I
>>> wasn't able to find the discussions that Chandler cited, but I
suspect that
>>> the answer is that we didn't have enough analyses ported at
that point to
>>> consider sharing the analysis management between the old and new
PM.
>>>
>>>
>>> If this works out it may be the evolutionary path we have all been
>>> wanting for this pass manager transition. Fingers crossed.
Hopefully I'm
>>> not overlooking some major issue.
>>>
>>> Anyway... back to working on the analysis manager dependency
tracking.
>>>
>>> -- Sean Silva
>>>
>>> On Thu, Jul 21, 2016 at 1:59 AM, Sean Silva <chisophugis at
gmail.com>
>>> wrote:
>>>
>>>> We did some basic sanity checking that memory usage didn't
go out of
>>>> control (it doesn't; at least with with a simple
>>>> preserves-all/preserves-none invalidation scheme and the
current LTO
>>>> pipeline). Also, I did some basic sanity checking for compile
time. The
>>>> simple preserves-all/preserves-none invalidation scheme seems
marginally
>>>> slower, but no real conclusions (besides a simple sanity check)
can be
>>>> drawn without the real analysis preservation semantics in
place.
>>>>
>>>> I'll start working on fixing the analysis managers. There
seem to
>>>> basically be two parts (although they may need to be done
simultaneously to
>>>> make sure all the pieces fit together):
>>>> - unify all the analysis managers into a single analysis
manager for
>>>> all IRUnitT's (requires type-erasing the IRUnit)
>>>> - introduce the dependency tracking machinery
>>>>
>>>> I think I gave a reasonable outline in the two posts above:
>>>> - the one starting with "To clarify, it seems like the
current new PM
>>>> is essentially trying to solve the problem of
maintaining/updating a
>>>> mapping"
>>>> - the one starting with "Yeah, the mechanics of
maintaining this fully
>>>> general mapping are straightforward in the abstract"
>>>>
>>>> I'm happy to do a centralized writeup if anybody wants.
Just let me
>>>> know.
>>>>
>>>> As far as changes to the code, the updates to the new PM passes
should
>>>> hopefully be mechanical (despite there being many of them).
However, from
>>>> what I can tell, fixing this problem will require touching most
lines of
>>>> the analysis manager machinery so the diff will probably be a
bit scary,
>>>> even though I think we can keep the same basic structure (i.e.
a per-IRUnit
>>>> std::list holding one analysis result (at a stable address) per
element,
>>>> combined with a DenseMap from (Analysis, IRUnit) to an element
of the
>>>> per-IRUnit storage list (see AnalysisResultListMapT and
AnalysisResultMapT
>>>> in include/llvm/IR/PassManager.h)).
>>>> The main changes to the analysis manager will be:
>>>> - type-erasing the IRUnit
>>>> - the elements of the AnalysisResultListMapT will need to keep
track
>>>> of any dependents
>>>> - the analysis manager will need to update those dependencies
as it is
>>>> re-entered by analyses getting results of other analyses
>>>> - the analysis manager will need to walk the dependencies to do
>>>> transitive invalidation
>>>>
>>>> I think the most robust approach is for analysis dependencies
to be
>>>> implicitly constructed by the analysis manager via tracking
entry/exit from
>>>> get{,Cached}Result.
>>>> One alternative is for analyses to explicitly pass in their ID
to
>>>> getResult to indicate the dependency, but that seems
error-prone (and also
>>>> verbose). The issue is that we will need a getResult API that
does not
>>>> track dependencies for use by transformation passes (since
there is no
>>>> dependency to track in that case); so an innocuous copy-paste
from a
>>>> transform pass to an analysis can result in a failure to track
dependencies
>>>> and risk of use-after-free (we could fight this with the type
system, but I
>>>> think that would get a bit verbose (I'm willing to try it
though if people
>>>> would prefer))
>>>> One restriction of the implicit tracking approach is that it
requires
>>>> all calls into the analysis manager to occur in the `run`
method of the
>>>> analysis (so that the dependencies are implicitly tracked via
re-entrance
>>>> to get{,Cached}Result); is this a reasonable restriction?
>>>>
>>>>
>>>> One annoying problem is that I think that the dependency links
will
>>>> need to be bidirectional. To use the example analysis cache
from my other
>>>> post:
>>>> (AssumptionAnalysis, function @bar) -> (AssumptionCache for
@bar,
>>>> [(SomeModuleAnalysis, module TheModule)])
>>>> (AssumptionAnalysis, function @baz) -> (AssumptionCache for
@baz,
>>>> [(SomeModuleAnalysis, module TheModule)])
>>>> (SomeModuleAnalysis, module TheModule) ->
(SomeModuleAnalysisResult for
>>>> TheModule, [(SomeFunctionAnalysis, function @baz)])
>>>> (SomeFunctionAnalysis, function @baz) ->
(SomeFunctionAnalysisResult
>>>> for @baz, [])
>>>>
>>>> if we delete function @baz, then the dependent list 
[(SomeFunctionAnalysis,
>>>> function @baz)] for `(SomeModuleAnalysis, module TheModule)`
will now
>>>> have a stale pointer to function @baz. I think that in practice
we can
>>>> check to see if `(SomeFunctionAnalysis, function @baz)` is in
our hash
>>>> table and if it isn't then just ignore the dependency as
"this dependent
>>>> analysis result has already been freed". In the worst case
(memory
>>>> allocator reuses the memory for another function) we may
spuriously free an
>>>> analysis result for a different function. However it is still
unsettling
>>>> (and may actually be UB in C++?).
>>>> Ideally we would track bidirectional links; that way when we
remove an
>>>> analysis result we also have it remove itself from dependence
lists of all
>>>> of its dependencies.
>>>>
>>>> -- Sean Silva
>>>>
>>>> On Fri, Jul 15, 2016 at 8:40 PM, Sean Silva <chisophugis at
gmail.com>
>>>> wrote:
>>>>
>>>>>
>>>>>
>>>>> On Fri, Jul 15, 2016 at 8:39 PM, Sean Silva <chisophugis
at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> It looks like there is really no sane fix within the
current
>>>>>> infrastructure. I've had to essentially trigger
invalidation (except in the
>>>>>> PreservedAnalyses::all() case) in the function pass
manager and function to
>>>>>> loop adapters.
>>>>>>
>>>>>
>>>>> invalidation of *everything* I mean.
>>>>>
>>>>> -- Sean Silva
>>>>>
>>>>>
>>>>>>
>>>>>> So we basically need to get the analysis manager
dependency tracking
>>>>>> fixed.
>>>>>>
>>>>>> Davide and I will get measurements on the resident set
impact of all
>>>>>> this caching (even with the overconservative
invalidation for now) to see
>>>>>> the impact. If there is a big rss impact then we
probably want to consider
>>>>>> that problem at the same time as the rewrite of the
analysis manager.
>>>>>>
>>>>>> -- Sean Silva
>>>>>>
>>>>>> On Thu, Jul 14, 2016 at 12:51 AM, Sean Silva
<chisophugis at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Wed, Jul 13, 2016 at 1:48 AM, Sean Silva
<chisophugis at gmail.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Wed, Jul 13, 2016 at 12:34 AM, Chandler
Carruth <
>>>>>>>> chandlerc at gmail.com> wrote:
>>>>>>>>
>>>>>>>>> On Wed, Jul 13, 2016 at 12:25 AM Sean Silva
<chisophugis at gmail.com>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>> On Tue, Jul 12, 2016 at 11:39 PM,
Chandler Carruth <
>>>>>>>>>> chandlerc at gmail.com> wrote:
>>>>>>>>>>
>>>>>>>>>>> On Tue, Jul 12, 2016 at 11:34 PM
Sean Silva <
>>>>>>>>>>> chisophugis at gmail.com> wrote:
>>>>>>>>>>>
>>>>>>>>>>>> On Tue, Jul 12, 2016 at 11:32
PM, Xinliang David Li <
>>>>>>>>>>>> davidxl at google.com>
wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> On Tue, Jul 12, 2016 at
10:57 PM, Chandler Carruth <
>>>>>>>>>>>>> chandlerc at gmail.com>
wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> Yea, this is a nasty
problem.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> One important thing to
understand is that this is specific to
>>>>>>>>>>>>>> analyses which hold
references to other analyses. While this isn't unheard
>>>>>>>>>>>>>> of, it isn't as
common as it could be. Still, definitely something we need
>>>>>>>>>>>>>> to address.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> We can call this type of
dependencies (holding references)
>>>>>>>>>>>>> hard-dependency. The soft
dependency refers to the case where analysis 'A'
>>>>>>>>>>>>> depends on 'B'
during computation, but does not need 'B' once it is
>>>>>>>>>>>>> computed.
>>>>>>>>>>>>>
>>>>>>>>>>>>> There are actually quite a
few examples of hard-dependency
>>>>>>>>>>>>> case. For instance
LoopAccessInfo, LazyValueInfo etc which hold references
>>>>>>>>>>>>> to other analyses.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Problem involving
hard-dependency is actually easier to
>>>>>>>>>>>>> detect, as it is usually a
compile time problem. Issues involving soft
>>>>>>>>>>>>> dependencies are more
subtle and can lead to wrong code gen.
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Did you mean to say that
soft-dependency problems are easier to
>>>>>>>>>>>> detect? At least my intuition
is that soft-dependency is easier because
>>>>>>>>>>>> there is no risk of dangling
pointers to other analyses.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> The issue is that the fact that
there is *any* dependency isn't
>>>>>>>>>>> clear.
>>>>>>>>>>>
>>>>>>>>>>> However, I think the only real
problem here are these "hard
>>>>>>>>>>> dependencies" (I don't
really like that term though). For others, only an
>>>>>>>>>>> analysis that is *explicitly*
preserved survives. So I'm not worried about
>>>>>>>>>>> the fact that people have to
remember this.
>>>>>>>>>>>
>>>>>>>>>>> The question is how often there are
cross-data-structure
>>>>>>>>>>> references. David mentions a few
examples, and I'm sure there are more, but
>>>>>>>>>>> it isn't clear to me yet
whether this is pervasive or occasional.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> I just did a quick run-through of
PassRegistry.def and this is
>>>>>>>>>> what I found:
>>>>>>>>>>
>>>>>>>>>> Module analyses: 0/5 hold pointers to
other analyses
>>>>>>>>>> CallGraph: No pointers to other
analyses.
>>>>>>>>>> LazyCallGraph: No pointers to other
analyses.
>>>>>>>>>> ProfileSummaryAnalysis: No pointers to
other analyses.
>>>>>>>>>> TargetLibraryAnalysis: No pointers to
other analyses.
>>>>>>>>>> VerifierAnalysis: No pointers to other
analyses.
>>>>>>>>>>
>>>>>>>>>> Module alias analyses: 1/1 keeps
pointer to other analysis.
>>>>>>>>>> GlobalsAA: Result keeps pointer to TLI
(this is a function
>>>>>>>>>> analysis).
>>>>>>>>>>
>>>>>>>>>> Function analyses: 9/17 keep pointers
to other analysis
>>>>>>>>>> AAManager: Its Result holds TLI pointer
and pointers to
>>>>>>>>>> individual AA result objects.
>>>>>>>>>> AssumptionAnalysis: No pointers to
other analyses.
>>>>>>>>>> BlockFrequencyAnalysis: Its Result
holds pointers to LoopInfo and
>>>>>>>>>> BPI.
>>>>>>>>>> BranchProbabilityAnalysis: Stores no
pointers to other analyses.
>>>>>>>>>> (uses LoopInfo to
"recalculate" though)
>>>>>>>>>> DominatorTreeAnalysis: Stores no
pointers to other analyses.
>>>>>>>>>> PostDominatorTreeAnalysis: Stores no
pointers to other analyses.
>>>>>>>>>> DemandedBitsAnalysis: Stores pointers
to AssumptionCache
>>>>>>>>>> and DominatorTree
>>>>>>>>>> DominanceFrontierAnalysis: Stores no
pointers to other analyses.
>>>>>>>>>> (uses DominatorTreeAnalysis for
"recalculate" though).
>>>>>>>>>> LoopInfo: Uses DominatorTreeAnalysis
for "recalculate" but stores
>>>>>>>>>> no pointers.
>>>>>>>>>> LazyValueAnalysis: Stores pointers to
AssumptionCache,
>>>>>>>>>> TargetLibraryInfo, DominatorTree.
>>>>>>>>>> DependenceAnalysis: Stores pointers to
AliasAnalysis,
>>>>>>>>>> ScalarEvolution, LoopInfo
>>>>>>>>>> MemoryDependenceAnalysis: Stores
pointers to AliasAnalysis,
>>>>>>>>>> AssumptionCache, TargetLibraryInfo,
DominatorTree
>>>>>>>>>> MemorySSAAnalysis: Stores pointers to
AliasAnalysis, DominatorTree
>>>>>>>>>> RegionInfoAnalysis: Stores pointers to
DomTree, PostDomTree,
>>>>>>>>>> DomFrontier
>>>>>>>>>> ScalarEvolutionAnalysis: Stores
pointers to TargetLibraryInfo,
>>>>>>>>>> AssumptionCache, DominatorTree,
LoopInfo
>>>>>>>>>> TargetLibraryAnalysis: Has no
dependencies
>>>>>>>>>> TargetIRAnalysis: Has no dependencies.
>>>>>>>>>>
>>>>>>>>>> Function alias analyses: 3/5 keep
pointers to other analyses
>>>>>>>>>> BasicAA: Keeps pointers to
TargetLibraryInfo, AssumptionCache,
>>>>>>>>>> DominatorTree, LoopInfo
>>>>>>>>>> CFLAA: Keeps pointer to
TargetLibraryInfo
>>>>>>>>>> SCEVAA: Keeps pointer to
ScalarEvolution
>>>>>>>>>> ScopedNoAliasAA: No dependencies
>>>>>>>>>> TypeBasedAA: No dependencies
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Total: 13/28 analyses (~50%) hold
pointers to other analyses.
>>>>>>>>>> Of the 15/28 analyses that don't
hold pointers, 12/15 simply have
>>>>>>>>>> no dependencies. Only 3/15 (BPI,
LoopInfo, DominanceFrontier) have
>>>>>>>>>> dependencies that are used just for a
"recalculate" step that retains no
>>>>>>>>>> pointers.
>>>>>>>>>> So I think it is fair to say that
analyses which hold pointers to
>>>>>>>>>> other analyses is not an exceptional
case. In fact, analyses that use other
>>>>>>>>>> analyses just for a
"recalculate" step seems to be the exceptional case
>>>>>>>>>> (only 3/28 or about 10%)
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Interesting!
>>>>>>>>>
>>>>>>>>> Most of these look like they hold a pointer
to the root analysis
>>>>>>>>> as opposed to detailed objects *inside* the
analysis?
>>>>>>>>>
>>>>>>>>> It might make sense to try to handle this
very specific pattern in
>>>>>>>>> a special way of overriding the invalidate
routines is too error prone.... We
>>>>>>>>> could try to make this work
"automatically" but I'm worried this would be
>>>>>>>>> challenging to get right. Open to
suggestions of course.
>>>>>>>>>
>>>>>>>>> Any other ideas about what would make sense
to handle this?
>>>>>>>>>
>>>>>>>>> Does it make sense to override the
invalidate routines now and
>>>>>>>>> iterate from there? I feel like you've
done a lot of the research necessary
>>>>>>>>> for this already...
>>>>>>>>>
>>>>>>>>
>>>>>>>> I'll keep pushing forward tomorrow with
building test-suite
>>>>>>>> successfully using the new PM for the LTO
pipeline (I was doing some
>>>>>>>> unrelated LLD stuff for most of today). It will
be interesting to see how
>>>>>>>> many `invalidate` overrides will be needed to
avoid these issues for just
>>>>>>>> the LTO pipeline on test-suite.
>>>>>>>>
>>>>>>>
>>>>>>> I spent the better part of today working on this
and will continue
>>>>>>> tomorrow; this problem seems nastier than I
thought. For some reason the
>>>>>>> LTO pipeline (or something about LTO) seems to hit
on these issues much
>>>>>>> more (I'm talking like 40k lines of ASan error
reports from building
>>>>>>> test-suite with the LTO pipeline in the new PM;
per-TU steps still using
>>>>>>> the old PM). Some notes:
>>>>>>>
>>>>>>> - BasicAA's dependence on domtree and loopinfo
in the new PM seems
>>>>>>> to account for quite a few of the problems.
>>>>>>> - BasicAA and other stuff are marked (by overriding
`invalidate` to
>>>>>>> return false) to never be invalidated because they
are "stateless". However
>>>>>>> they still hold pointers and so they do need to be
invalidated.
>>>>>>> - CallGraph uses AssertingVH (PR28400) and so I
needed a workaround
>>>>>>> similar to r274656 in various passes.
>>>>>>> - D21921 is holding up -- I haven't hit any
issues with the core
>>>>>>> logic of that patch.
>>>>>>> - AAResults holds handles to various AA result
objects. This means
>>>>>>> it pretty much always needs to be invalidated
unless you are sure that none
>>>>>>> of the AA's will get invalidated.
>>>>>>>
>>>>>>>
>>>>>>> The existing `invalidate` method doesn't have
the right semantics
>>>>>>> for even an error-prone solution :( We are going to
need to make some
>>>>>>> significant changes to even get basic sanity I
think. Perhaps each analysis
>>>>>>> can expose a "preserve" static function.
E.g. instead of
>>>>>>> `PA.preserve<Foo>();` you have to do
`Foo::setPreserved(PA);`.
>>>>>>> I'm actually not quite sure that that will even
work. Once I have
>>>>>>> test-suite fully building successfully with the LTO
pipeline in the new PM
>>>>>>> I'll be able to give a more confident answer
(esp. w.r.t. the manager for
>>>>>>> different IRUnitT's).
>>>>>>> But at this point I'm not confident running
*any* pass pipeline in
>>>>>>> the new PM without at least assertions+ASan.
>>>>>>>
>>>>>>> We may want to have a proper design discussion
around this problem
>>>>>>> though.
>>>>>>>
>>>>>>> Also I'd like to have test-suite working (by
hook or by crook) with
>>>>>>> LTO in the new PM so we can get some numbers on the
resident set impact of
>>>>>>> all this caching; if it is really problematic then
we may need to start
>>>>>>> talking front-and-center about different
invalidation policies for keeping
>>>>>>> this in check instead of leaving it as something
that we will be able to
>>>>>>> patch later.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> The more I think about it, the more I'm
convinced that the real
>>>>>>> "hard" problem that the new PM is
exposing us to is having the ability for
>>>>>>> any pass to ask for any analysis on any IRUnitT
(and any specific IRUnit of
>>>>>>> that IRUnitT) and have the result stored somewhere
and then invalidated.
>>>>>>> This means that "getAnalysisUsage" is not
just a list of passes, but much
>>>>>>> more complicated and is essentially a set of
arbitrary pairs "(analysis,
>>>>>>> IRUnit)" (and the associated potential tangle
of dependencies between the
>>>>>>> state cached on these tuples). With the old PM, you
essentially are looking
>>>>>>> at a problem of scheduling the lifetime of analyses
of the same IRUnit
>>>>>>> intermingled with transformation passes on that
same IRUnit, so you only
>>>>>>> have the "analysis" part of the tuple
above, making things much simpler
>>>>>>> (and handling dependencies is much simpler too).
We've obviously outgrown
>>>>>>> this model with examples like LAA,
AssumptionCacheTracker, etc. that hack
>>>>>>> around this in the old PM. We may want to have a
fresh re-examination of
>>>>>>> what problems we are exactly trying to solve.
>>>>>>>
>>>>>>> For me, my main concern now is what changes need to
be made in order
>>>>>>> to feel confident running a pipeline in the new PM
without assertions+ASan.
>>>>>>>
>>>>>>>
>>>>>>> Sorry for the long post, just brain-dumping before
heading home.
>>>>>>>
>>>>>>> -- Sean Silva
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> -- Sean Silva
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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I have the unified analysis manager implemented now and passing all tests
at: https://github.com/chisophugis/llvm/commits/analysis-manager

One caveat:
It would be a layering violation for the analysis manager to know about
Loop and SCC since those live in libAnalysis. So currently I have some
stuff commented out for imitating the proxy downward invalidation for them.
(and generally the proxy downward invalidation imitation stuff is somewhat
ugly; it is in invalidatImpl:
https://github.com/chisophugis/llvm/commit/23a65b884d3f15a928ceaef63a8293ab2eda2348#diff-c94a14214ca2d8403849baf9bb4d90e1R559
).
It more or less imitates what the proxy was doing, albeit that it does it
by iterating the map and invalidating any lower IRUnit's instead of just
clearing the other analysis manager.
I'm happy to add an indirect interface to make it work with all the
IRUnit's.
Down the road, if we have pass manager cooperation (such as the push/pop
thing I mentioned for tracking outer/inner IRUnit relationships) we can
make this much smarter and support IRUnit's more generically without a
hardcoded list.

I'll start working on the inter-analysis dependency stuff tomorrow.

-- Sean Silva

On Wed, Jul 27, 2016 at 3:58 AM, Sean Silva <chisophugis at gmail.com>
wrote:
> Okay, did the big renaming and de-templating today (and removing the
> proxies).
>
> check-llvm worked fine, except that this test caught a bug (yay!):
> unittests/IR/PassManagerTest.cpp:326
>
> After removing the proxies, the module pass is not invalidating the
> function passes (since the invalidation is on the module IRUnit). we now
> don't have a method for delegating the invalidation from outer
IRUnit's to
> inner IRUnit's.
> I'll try to add a mechanism for the analysis manager to do this itself
if
> there's a straightforward way to do so. Not sure the best way. We
can't
> just iterate over all the functions calling `invalidate`; a module pass may
> have e.g. deleted a function, so we would never call `invalidate` on the
> deleted function. A simple solution is to add another member to the
> analysis manager which tracks, for each IRUnitT, all IRUnit's that we
have
> state cached for (so we can easily walk them).
>
> Interestingly, it seems like the current state in trunk is that a module
> analysis can't preserve a specific function analysis. The only way that
> analysis invalidation can propagate downwards is by clearing the entire
> inner analysis manager.
> So I think this means that the current state in trunk is that if a
> function transformation returns PreservedAnalyses::none(), it will
> invalidate all loop analyses for all functions.
> (I think Chandler mentioned this issue in passing here:
> https://reviews.llvm.org/D21921#inline-187608)
>
> (slightly more info in my log)
>
> Anyway, for now, I'll try to do something that is as NFC as possible.
> Hopefully just tracking all the IRUnit's per IRUnitT will allow us to
> emulate the proxy-based invalidation fairly well.
>
>
> It is probably relevant to discuss how we want to "properly" deal
with
> this with the unified analysis manager for all IRUnitT's.
>
> One solution is to have the pass managers (actually, the adaptors),
> push/pop a stack of the current IRUnit nest (e.g. the stack might contain
> at a given point in time something like `[Module foo, Function @bar]` or
> `[Module foo, SCC bar, Function @baz, Loop %qux]`).
> The key information that is needed is for the analysis manager to be able
> to reconstruct a parent-child relationship of IRUnit's (which might
contain
> parent-child links that are no longer obtainable from the parent IRUnit;
> e.g. when you go to invalidate analyses after running a module pass that
> deleted a function; the analysis manager must still track that relationship
> even if the module has already forgotten about the function).
>
> I don't see any reasonable way to do this without cooperation from the
> adaptors, because (like the example "IRUnit stacks" I showed
above) the
> analysis manager would need a way to know whether to map back to a
> particular SCC or to a particular Module (or potentially both).
>
> -- Sean Silva
>
>
>
> On Tue, Jul 26, 2016 at 1:32 AM, Sean Silva <chisophugis at
gmail.com> wrote:
>
>> I'm not quite sure what post to respond to for a status update, but
I
>> guess this one will do (and you can check my log for more info of
course).
>>
>> My current working branch for the analysis manager stuff is at
>> https://github.com/chisophugis/llvm/commits/analysis-manager
>> (4ecf6115890bd01caa52c0b99424974e3469291e)
>>
>> I described in my log a bit more my thought process and how to do this
>> without having to churn every single new PM pass (search for
"Okay, so now
>> that I think about it, adding the dependency management is a second
step
>> after making the analysis manager work for all IRUnit’s.").
Essentially,
>> the first step is to still have AnalysisManager<Function>,
>> AnalysisManager<Module>, etc. but the template argument is just a
dummy.
>> The real templating is on the methods, which can accept any IRUnit. The
>> advantage of this is that it is source compatible with all the existing
>> code using multiple manager, proxies, etc.
>>
>> The code on my branch at least passes check-llvm. But of course the
>> existing code doesn't test any of the situations where being able
to handle
>> multiple IRUnitT's in a single analysis manager would matter.
>> Tomorrow, I get to churn all the passes that have been ported so far,
>> removing the proxies etc.
>>
>>
>>
>>
>>
>> Another thing that just came to me: the current way things work with
>> adaptors means that if a function transformation invalidates a module
>> analysis, the module analysis won't be invalidated until after the
>> ModuleToFunctionPassAdapter returns.
>>
>> So we can end up running an arbitrary number of function
transformations
>> that query that stale module analysis. This is (I think?) mostly benign
>> given our current set of module analyses (more info in the log; search
for
>> "We may be getting by simply because we seem to not have many
module
>> analyses"). But if we invalidate more "correctly" (after
every
>> transformation on any contained IRUnit), there's a potential
quadratic
>> compile time issue lurking here once have the unified analysis manager:
we
>> could easily end up invalidating/recomputing a module analysis once per
>> function visitation.
>>
>> For a pipeline that just does a lot of churn (e.g. function
>> simplification passes we run as part of the regular O3 pipeline; that
can
>> delete most code in a function, change the cfg, etc.) it's not
clear what
>> useful properties we can guarantee will be preserved which would
prevent us
>> from invalidating pretty much all non-immutable outer analyses (module
or
>> (theoretically)CGSCC). So for any really nontrivial module analysis, we
may
>> end up having to change the interface of module passes to have some way
to
>> incrementally recompute themselves as function passes mutate the IR?
>>
>> In some sense, Chandler's CGSCC patch
https://reviews.llvm.org/D21464 is
>> trying to do this for a specific module analysis (lazy call graph) and
even
>> for just that one (and lots of special handling in the adaptors etc.)
it
>> still only gets incrementally updated it after a potentially large set
of
>> function passes have run. And it's already quite tricky.
>>
>> Broadly speaking, I can think of two major kinds of
"solutions" to this
>> problem:
>> - severely restrict the interaction of function transformations and
>> module analyses in some way. Essentially, we could define a restricted
>> class of module analyses that "are conservatively correct in the
face of
>> any transformation that can be made by a function transformation"
(this
>> includes immutable module analyses and IIRC things like GlobalsModRef).
But
>> the issue is a bit deeper, because it is really about transformations
at
>> any smaller IRUnitT. This includes CGSCC. And CGSCC can do substantial
>> interprocedural transformation (e.g. delete a wholefunction IIRC?); we
>> would then need to have another class of module analyses that are
>> "conservatively correct in the face of CGSCC transformations"
which seems
>> quite restrictive.
>> -- alternatively or in addition, this can involve restricting what kind
>> of information a function pass can get out of a module analysis
>> - provide some sort of update callback for the outer IRUnit analysis to
>> update itself if the inner one is modified (not sure how practical this
>> will be). E.g. `AM.invalidate<FooModuleAnalysis>(F);` doesn't
actually
>> invalidate FooModuleAnalysis, but rather invokes a callback for it to
>> update itself (possibly as an opt-in for the module analysis).
>> -- The current analysis manager does have a `invalidate` hook that it
>> will call, but the argument is always inherently the IRUnit that the
>> analysis itself is cached on so it isn't useful for partial
recomputation.
>> We would need to extend that, which requires making the
>> AnalysisResultConcept more complicated (and potentially having to have
>> centralized awareness of all the different IRUnitT's, which until
now are
>> fairly decoupled in the new PM).
>>
>>
>> I'm curious what others' thoughts are on this issue.
>>
>> -- Sean Silva
>>
>> On Sun, Jul 24, 2016 at 2:58 AM, Sean Silva <chisophugis at
gmail.com>
>> wrote:
>>
>>> I've started looking specifically at the existing code and how
it needs
>>> to be changed. It seems like the concept-based polymorphism stuff
in
>>> PassManagerInternal.h actually don't need to be changed that
much.
>>> AnalysisPassConcept and AnalysisResultConcept need to be changed to
take a
>>> type-erased IRUnit (e.g. a void* or something). AnalysisPassModel
and
>>> AnalysisResultModel (which inherit from their respective abstract
base
>>> classes I mentioned above) are already templated on the IRUnitT and
so they
>>> can just cast the void* back to the right type.
>>>
>>> Adding the dependency tracking seems like it will be mostly a data
>>> structure change with some isolated "algorithmic" changes
to
>>> track/invalidate dependencies.
>>>
>>> Most of the methods that need to know the specific IRUnitT type
already
>>> take the IRUnit as an argument (e.g. getResult, getCachedResult,
>>> invalidate) and so it's actually somewhat natural for the
analysis manager
>>> not be templated on IRUnitT (but rather to have just those methods
be
>>> templated).
>>>
>>> Also, up until now I hadn't noticed this weird
"registerPass" thing on
>>> the analysis manager (AnalysisManagerBase to be specific).
Effectively, it
>>> allows analyses to hold state (this is separate from analysis
*results*
>>> which of course can hold state). But the state is effectively
global to the
>>> AnalysisManager (and hence the pass pipeline, and hence
(essentially) the
>>> context (since you can currently only run a single pass pipeline
>>> concurrently on a single LLVMContext)). The net result is that you
can
>>> specify a context-global "configuration" for each
analysis type (not to be
>>> confused with the analysis *result* type!). Right now, AAManager is
the
>>> only thing that uses it though (the "configuration" is
the AA pipeline).
>>>
>>>
>>>
>>> btw, I've started to keep a log for this like I do for my
projects at
>>> home:
>>>
https://docs.google.com/document/d/1-nrq2y_hTiZhrsJDmH8TzFjFEeApfccs6wwGyaXjSkg/edit?usp=sharing
>>> It's fairly stream-of-consciousness. My log style is mostly
append-only
>>> (I rarely edit stuff from a previous day or from too long ago on
the same
>>> day; if I do I usually don't overwrite and instead insert
something like
>>> "(edit: I was actually totally wrong about this, see
below)"). So if you
>>> want to follow this across multiple days just go to the end and
scroll up
>>> until you see something that you've already looked at. (right
now there is
>>> just one day though so there is not very much).
>>>
>>> (some interesting things to search for are "interesting",
"okay", "oh",
>>> and "?"; I tend to use these a lot)
>>>
>>> I've gone back to google docs for this log since it is easier
to share
>>> on the mailing list. Unfortunately google docs does not have an
explicit
>>> notion of "cell" like Mathematica does, so I've tried
to just insert lots
>>> of line breaks for things where there would have just been a cell
boundary
>>> in Mathematica. (I use Mathematica for my logs at home).
>>>
>>>
>>> -- Sean Silva
>>>
>>> On Fri, Jul 22, 2016 at 1:55 AM, Sean Silva <chisophugis at
gmail.com>
>>> wrote:
>>>
>>>> The more closely I look at this, the more it seems like there
may be a
>>>> useful incremental step in the transition to the new PM: use
the new PM
>>>> analysis machinery in the old PM. If this is possible, it will
simplify the
>>>> old PM and (hopefully) allow an incremental transition to the
new PM
>>>> instead of a flag day transition for the switch.
>>>>
>>>> I.e., AFAICT, the new PM transition is essentially about 2
mostly
>>>> orthogonal aspects of running optimization pipelines:
>>>> 1. Analysis computation and analysis result lifetime management
>>>> (including avoiding analysis recomputation)
>>>> 2. Running transformation passes over their respective
IRUnit's in some
>>>> order
>>>>
>>>> These are conflated in the old PM. In reality, the only
interaction
>>>> between them (with the new PM machinery for 1.) is a small
number of places
>>>> within 2. which need to call 'invalidate'.
>>>>
>>>> I'm pretty sure that 2. is fairly similar in the new PM and
old PM (the
>>>> main difference is that the notion of "adapters" is
split out in the new
>>>> PM). The analysis handling seems to be what makes the old PM so
difficult
>>>> to understand (e.g. it is the cause of the multiple inheritance
in the
>>>> implementation). Trying to unify analyses and transformations
(and some
>>>> questionable (in hindsight) implementation decisions) seems to
be the main
>>>> "problem" with the design of the old PM AFAICT (there
are other issues, but
>>>> they are more "nice to have").
>>>>
>>>> IMO it is an anti-pattern to think of analyses as
"passes". There are
>>>> just "analyses" and "transformations" and
they are two separate things. In
>>>> fact, the `run` method on analyses should probably be called
>>>> `computeResult` or something like that to avoid confusion. And
the `run`
>>>> method on transformations could just as easily be called
>>>> `performTransformation`.
>>>>
>>>>
>>>> I remember asking and getting and answer from Chandler (
>>>>
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150907/299083.html)
>>>> about how to coexist the old and new PM compatibly so
individual passes
>>>> would be able to work for both and we wouldn't need to have
a flag day. I
>>>> wasn't able to find the discussions that Chandler cited,
but I suspect that
>>>> the answer is that we didn't have enough analyses ported at
that point to
>>>> consider sharing the analysis management between the old and
new PM.
>>>>
>>>>
>>>> If this works out it may be the evolutionary path we have all
been
>>>> wanting for this pass manager transition. Fingers crossed.
Hopefully I'm
>>>> not overlooking some major issue.
>>>>
>>>> Anyway... back to working on the analysis manager dependency
tracking.
>>>>
>>>> -- Sean Silva
>>>>
>>>> On Thu, Jul 21, 2016 at 1:59 AM, Sean Silva <chisophugis at
gmail.com>
>>>> wrote:
>>>>
>>>>> We did some basic sanity checking that memory usage
didn't go out of
>>>>> control (it doesn't; at least with with a simple
>>>>> preserves-all/preserves-none invalidation scheme and the
current LTO
>>>>> pipeline). Also, I did some basic sanity checking for
compile time. The
>>>>> simple preserves-all/preserves-none invalidation scheme
seems marginally
>>>>> slower, but no real conclusions (besides a simple sanity
check) can be
>>>>> drawn without the real analysis preservation semantics in
place.
>>>>>
>>>>> I'll start working on fixing the analysis managers.
There seem to
>>>>> basically be two parts (although they may need to be done
simultaneously to
>>>>> make sure all the pieces fit together):
>>>>> - unify all the analysis managers into a single analysis
manager for
>>>>> all IRUnitT's (requires type-erasing the IRUnit)
>>>>> - introduce the dependency tracking machinery
>>>>>
>>>>> I think I gave a reasonable outline in the two posts above:
>>>>> - the one starting with "To clarify, it seems like the
current new PM
>>>>> is essentially trying to solve the problem of
maintaining/updating a
>>>>> mapping"
>>>>> - the one starting with "Yeah, the mechanics of
maintaining this
>>>>> fully general mapping are straightforward in the
abstract"
>>>>>
>>>>> I'm happy to do a centralized writeup if anybody wants.
Just let me
>>>>> know.
>>>>>
>>>>> As far as changes to the code, the updates to the new PM
passes
>>>>> should hopefully be mechanical (despite there being many of
them). However,
>>>>> from what I can tell, fixing this problem will require
touching most lines
>>>>> of the analysis manager machinery so the diff will probably
be a bit scary,
>>>>> even though I think we can keep the same basic structure
(i.e. a per-IRUnit
>>>>> std::list holding one analysis result (at a stable address)
per element,
>>>>> combined with a DenseMap from (Analysis, IRUnit) to an
element of the
>>>>> per-IRUnit storage list (see AnalysisResultListMapT and
AnalysisResultMapT
>>>>> in include/llvm/IR/PassManager.h)).
>>>>> The main changes to the analysis manager will be:
>>>>> - type-erasing the IRUnit
>>>>> - the elements of the AnalysisResultListMapT will need to
keep track
>>>>> of any dependents
>>>>> - the analysis manager will need to update those
dependencies as it is
>>>>> re-entered by analyses getting results of other analyses
>>>>> - the analysis manager will need to walk the dependencies
to do
>>>>> transitive invalidation
>>>>>
>>>>> I think the most robust approach is for analysis
dependencies to be
>>>>> implicitly constructed by the analysis manager via tracking
entry/exit from
>>>>> get{,Cached}Result.
>>>>> One alternative is for analyses to explicitly pass in their
ID to
>>>>> getResult to indicate the dependency, but that seems
error-prone (and also
>>>>> verbose). The issue is that we will need a getResult API
that does not
>>>>> track dependencies for use by transformation passes (since
there is no
>>>>> dependency to track in that case); so an innocuous
copy-paste from a
>>>>> transform pass to an analysis can result in a failure to
track dependencies
>>>>> and risk of use-after-free (we could fight this with the
type system, but I
>>>>> think that would get a bit verbose (I'm willing to try
it though if people
>>>>> would prefer))
>>>>> One restriction of the implicit tracking approach is that
it requires
>>>>> all calls into the analysis manager to occur in the `run`
method of the
>>>>> analysis (so that the dependencies are implicitly tracked
via re-entrance
>>>>> to get{,Cached}Result); is this a reasonable restriction?
>>>>>
>>>>>
>>>>> One annoying problem is that I think that the dependency
links will
>>>>> need to be bidirectional. To use the example analysis cache
from my other
>>>>> post:
>>>>> (AssumptionAnalysis, function @bar) -> (AssumptionCache
for @bar,
>>>>> [(SomeModuleAnalysis, module TheModule)])
>>>>> (AssumptionAnalysis, function @baz) -> (AssumptionCache
for @baz,
>>>>> [(SomeModuleAnalysis, module TheModule)])
>>>>> (SomeModuleAnalysis, module TheModule) ->
(SomeModuleAnalysisResult
>>>>> for TheModule, [(SomeFunctionAnalysis, function @baz)])
>>>>> (SomeFunctionAnalysis, function @baz) ->
(SomeFunctionAnalysisResult
>>>>> for @baz, [])
>>>>>
>>>>> if we delete function @baz, then the dependent list 
[(SomeFunctionAnalysis,
>>>>> function @baz)] for `(SomeModuleAnalysis, module
TheModule)` will now
>>>>> have a stale pointer to function @baz. I think that in
practice we can
>>>>> check to see if `(SomeFunctionAnalysis, function @baz)` is
in our
>>>>> hash table and if it isn't then just ignore the
dependency as "this
>>>>> dependent analysis result has already been freed". In
the worst case
>>>>> (memory allocator reuses the memory for another function)
we may spuriously
>>>>> free an analysis result for a different function. However
it is still
>>>>> unsettling (and may actually be UB in C++?).
>>>>> Ideally we would track bidirectional links; that way when
we remove an
>>>>> analysis result we also have it remove itself from
dependence lists of all
>>>>> of its dependencies.
>>>>>
>>>>> -- Sean Silva
>>>>>
>>>>> On Fri, Jul 15, 2016 at 8:40 PM, Sean Silva <chisophugis
at gmail.com>
>>>>> wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On Fri, Jul 15, 2016 at 8:39 PM, Sean Silva
<chisophugis at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> It looks like there is really no sane fix within
the current
>>>>>>> infrastructure. I've had to essentially trigger
invalidation (except in the
>>>>>>> PreservedAnalyses::all() case) in the function pass
manager and function to
>>>>>>> loop adapters.
>>>>>>>
>>>>>>
>>>>>> invalidation of *everything* I mean.
>>>>>>
>>>>>> -- Sean Silva
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> So we basically need to get the analysis manager
dependency tracking
>>>>>>> fixed.
>>>>>>>
>>>>>>> Davide and I will get measurements on the resident
set impact of all
>>>>>>> this caching (even with the overconservative
invalidation for now) to see
>>>>>>> the impact. If there is a big rss impact then we
probably want to consider
>>>>>>> that problem at the same time as the rewrite of the
analysis manager.
>>>>>>>
>>>>>>> -- Sean Silva
>>>>>>>
>>>>>>> On Thu, Jul 14, 2016 at 12:51 AM, Sean Silva
<chisophugis at gmail.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Wed, Jul 13, 2016 at 1:48 AM, Sean Silva
<chisophugis at gmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Wed, Jul 13, 2016 at 12:34 AM, Chandler
Carruth <
>>>>>>>>> chandlerc at gmail.com> wrote:
>>>>>>>>>
>>>>>>>>>> On Wed, Jul 13, 2016 at 12:25 AM Sean
Silva <
>>>>>>>>>> chisophugis at gmail.com> wrote:
>>>>>>>>>>
>>>>>>>>>>> On Tue, Jul 12, 2016 at 11:39 PM,
Chandler Carruth <
>>>>>>>>>>> chandlerc at gmail.com> wrote:
>>>>>>>>>>>
>>>>>>>>>>>> On Tue, Jul 12, 2016 at 11:34
PM Sean Silva <
>>>>>>>>>>>> chisophugis at gmail.com>
wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> On Tue, Jul 12, 2016 at
11:32 PM, Xinliang David Li <
>>>>>>>>>>>>> davidxl at google.com>
wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> On Tue, Jul 12, 2016 at
10:57 PM, Chandler Carruth <
>>>>>>>>>>>>>> chandlerc at
gmail.com> wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Yea, this is a
nasty problem.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> One important thing
to understand is that this is specific
>>>>>>>>>>>>>>> to analyses which
hold references to other analyses. While this isn't
>>>>>>>>>>>>>>> unheard of, it
isn't as common as it could be. Still, definitely something
>>>>>>>>>>>>>>> we need to address.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> We can call this type
of dependencies (holding references)
>>>>>>>>>>>>>> hard-dependency. The
soft dependency refers to the case where analysis 'A'
>>>>>>>>>>>>>> depends on 'B'
during computation, but does not need 'B' once it is
>>>>>>>>>>>>>> computed.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> There are actually
quite a few examples of hard-dependency
>>>>>>>>>>>>>> case. For instance
LoopAccessInfo, LazyValueInfo etc which hold references
>>>>>>>>>>>>>> to other analyses.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Problem involving
hard-dependency is actually easier to
>>>>>>>>>>>>>> detect, as it is
usually a compile time problem. Issues involving soft
>>>>>>>>>>>>>> dependencies are more
subtle and can lead to wrong code gen.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Did you mean to say that
soft-dependency problems are easier
>>>>>>>>>>>>> to detect? At least my
intuition is that soft-dependency is easier because
>>>>>>>>>>>>> there is no risk of
dangling pointers to other analyses.
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> The issue is that the fact that
there is *any* dependency isn't
>>>>>>>>>>>> clear.
>>>>>>>>>>>>
>>>>>>>>>>>> However, I think the only real
problem here are these "hard
>>>>>>>>>>>> dependencies" (I don't
really like that term though). For others, only an
>>>>>>>>>>>> analysis that is *explicitly*
preserved survives. So I'm not worried about
>>>>>>>>>>>> the fact that people have to
remember this.
>>>>>>>>>>>>
>>>>>>>>>>>> The question is how often there
are cross-data-structure
>>>>>>>>>>>> references. David mentions a
few examples, and I'm sure there are more, but
>>>>>>>>>>>> it isn't clear to me yet
whether this is pervasive or occasional.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> I just did a quick run-through of
PassRegistry.def and this is
>>>>>>>>>>> what I found:
>>>>>>>>>>>
>>>>>>>>>>> Module analyses: 0/5 hold pointers
to other analyses
>>>>>>>>>>> CallGraph: No pointers to other
analyses.
>>>>>>>>>>> LazyCallGraph: No pointers to other
analyses.
>>>>>>>>>>> ProfileSummaryAnalysis: No pointers
to other analyses.
>>>>>>>>>>> TargetLibraryAnalysis: No pointers
to other analyses.
>>>>>>>>>>> VerifierAnalysis: No pointers to
other analyses.
>>>>>>>>>>>
>>>>>>>>>>> Module alias analyses: 1/1 keeps
pointer to other analysis.
>>>>>>>>>>> GlobalsAA: Result keeps pointer to
TLI (this is a function
>>>>>>>>>>> analysis).
>>>>>>>>>>>
>>>>>>>>>>> Function analyses: 9/17 keep
pointers to other analysis
>>>>>>>>>>> AAManager: Its Result holds TLI
pointer and pointers to
>>>>>>>>>>> individual AA result objects.
>>>>>>>>>>> AssumptionAnalysis: No pointers to
other analyses.
>>>>>>>>>>> BlockFrequencyAnalysis: Its Result
holds pointers to LoopInfo
>>>>>>>>>>> and BPI.
>>>>>>>>>>> BranchProbabilityAnalysis: Stores
no pointers to other analyses.
>>>>>>>>>>> (uses LoopInfo to
"recalculate" though)
>>>>>>>>>>> DominatorTreeAnalysis: Stores no
pointers to other analyses.
>>>>>>>>>>> PostDominatorTreeAnalysis: Stores
no pointers to other analyses.
>>>>>>>>>>> DemandedBitsAnalysis: Stores
pointers to AssumptionCache
>>>>>>>>>>> and DominatorTree
>>>>>>>>>>> DominanceFrontierAnalysis: Stores
no pointers to other analyses.
>>>>>>>>>>> (uses DominatorTreeAnalysis for
"recalculate" though).
>>>>>>>>>>> LoopInfo: Uses
DominatorTreeAnalysis for "recalculate" but
>>>>>>>>>>> stores no pointers.
>>>>>>>>>>> LazyValueAnalysis: Stores pointers
to AssumptionCache,
>>>>>>>>>>> TargetLibraryInfo, DominatorTree.
>>>>>>>>>>> DependenceAnalysis: Stores pointers
to AliasAnalysis,
>>>>>>>>>>> ScalarEvolution, LoopInfo
>>>>>>>>>>> MemoryDependenceAnalysis: Stores
pointers to AliasAnalysis,
>>>>>>>>>>> AssumptionCache, TargetLibraryInfo,
DominatorTree
>>>>>>>>>>> MemorySSAAnalysis: Stores pointers
to AliasAnalysis,
>>>>>>>>>>> DominatorTree
>>>>>>>>>>> RegionInfoAnalysis: Stores pointers
to DomTree, PostDomTree,
>>>>>>>>>>> DomFrontier
>>>>>>>>>>> ScalarEvolutionAnalysis: Stores
pointers to TargetLibraryInfo,
>>>>>>>>>>> AssumptionCache, DominatorTree,
LoopInfo
>>>>>>>>>>> TargetLibraryAnalysis: Has no
dependencies
>>>>>>>>>>> TargetIRAnalysis: Has no
dependencies.
>>>>>>>>>>>
>>>>>>>>>>> Function alias analyses: 3/5 keep
pointers to other analyses
>>>>>>>>>>> BasicAA: Keeps pointers to
TargetLibraryInfo, AssumptionCache,
>>>>>>>>>>> DominatorTree, LoopInfo
>>>>>>>>>>> CFLAA: Keeps pointer to
TargetLibraryInfo
>>>>>>>>>>> SCEVAA: Keeps pointer to
ScalarEvolution
>>>>>>>>>>> ScopedNoAliasAA: No dependencies
>>>>>>>>>>> TypeBasedAA: No dependencies
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Total: 13/28 analyses (~50%) hold
pointers to other analyses.
>>>>>>>>>>> Of the 15/28 analyses that
don't hold pointers, 12/15 simply
>>>>>>>>>>> have no dependencies. Only 3/15
(BPI, LoopInfo, DominanceFrontier) have
>>>>>>>>>>> dependencies that are used just for
a "recalculate" step that retains no
>>>>>>>>>>> pointers.
>>>>>>>>>>> So I think it is fair to say that
analyses which hold pointers
>>>>>>>>>>> to other analyses is not an
exceptional case. In fact, analyses that use
>>>>>>>>>>> other analyses just for a
"recalculate" step seems to be the exceptional
>>>>>>>>>>> case (only 3/28 or about 10%)
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Interesting!
>>>>>>>>>>
>>>>>>>>>> Most of these look like they hold a
pointer to the root analysis
>>>>>>>>>> as opposed to detailed objects *inside*
the analysis?
>>>>>>>>>>
>>>>>>>>>> It might make sense to try to handle
this very specific pattern
>>>>>>>>>> in a special way of overriding the
invalidate routines is too error
>>>>>>>>>> prone.... We could try to make this
work "automatically" but I'm
>>>>>>>>>> worried this would be challenging to
get right. Open to suggestions of
>>>>>>>>>> course.
>>>>>>>>>>
>>>>>>>>>> Any other ideas about what would make
sense to handle this?
>>>>>>>>>>
>>>>>>>>>> Does it make sense to override the
invalidate routines now and
>>>>>>>>>> iterate from there? I feel like
you've done a lot of the research necessary
>>>>>>>>>> for this already...
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I'll keep pushing forward tomorrow with
building test-suite
>>>>>>>>> successfully using the new PM for the LTO
pipeline (I was doing some
>>>>>>>>> unrelated LLD stuff for most of today). It
will be interesting to see how
>>>>>>>>> many `invalidate` overrides will be needed
to avoid these issues for just
>>>>>>>>> the LTO pipeline on test-suite.
>>>>>>>>>
>>>>>>>>
>>>>>>>> I spent the better part of today working on
this and will continue
>>>>>>>> tomorrow; this problem seems nastier than I
thought. For some reason the
>>>>>>>> LTO pipeline (or something about LTO) seems to
hit on these issues much
>>>>>>>> more (I'm talking like 40k lines of ASan
error reports from building
>>>>>>>> test-suite with the LTO pipeline in the new PM;
per-TU steps still using
>>>>>>>> the old PM). Some notes:
>>>>>>>>
>>>>>>>> - BasicAA's dependence on domtree and
loopinfo in the new PM seems
>>>>>>>> to account for quite a few of the problems.
>>>>>>>> - BasicAA and other stuff are marked (by
overriding `invalidate` to
>>>>>>>> return false) to never be invalidated because
they are "stateless". However
>>>>>>>> they still hold pointers and so they do need to
be invalidated.
>>>>>>>> - CallGraph uses AssertingVH (PR28400) and so I
needed a workaround
>>>>>>>> similar to r274656 in various passes.
>>>>>>>> - D21921 is holding up -- I haven't hit any
issues with the core
>>>>>>>> logic of that patch.
>>>>>>>> - AAResults holds handles to various AA result
objects. This means
>>>>>>>> it pretty much always needs to be invalidated
unless you are sure that none
>>>>>>>> of the AA's will get invalidated.
>>>>>>>>
>>>>>>>>
>>>>>>>> The existing `invalidate` method doesn't
have the right semantics
>>>>>>>> for even an error-prone solution :( We are
going to need to make some
>>>>>>>> significant changes to even get basic sanity I
think. Perhaps each analysis
>>>>>>>> can expose a "preserve" static
function. E.g. instead of
>>>>>>>> `PA.preserve<Foo>();` you have to do
`Foo::setPreserved(PA);`.
>>>>>>>> I'm actually not quite sure that that will
even work. Once I have
>>>>>>>> test-suite fully building successfully with the
LTO pipeline in the new PM
>>>>>>>> I'll be able to give a more confident
answer (esp. w.r.t. the manager for
>>>>>>>> different IRUnitT's).
>>>>>>>> But at this point I'm not confident running
*any* pass pipeline in
>>>>>>>> the new PM without at least assertions+ASan.
>>>>>>>>
>>>>>>>> We may want to have a proper design discussion
around this problem
>>>>>>>> though.
>>>>>>>>
>>>>>>>> Also I'd like to have test-suite working
(by hook or by crook) with
>>>>>>>> LTO in the new PM so we can get some numbers on
the resident set impact of
>>>>>>>> all this caching; if it is really problematic
then we may need to start
>>>>>>>> talking front-and-center about different
invalidation policies for keeping
>>>>>>>> this in check instead of leaving it as
something that we will be able to
>>>>>>>> patch later.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> The more I think about it, the more I'm
convinced that the real
>>>>>>>> "hard" problem that the new PM is
exposing us to is having the ability for
>>>>>>>> any pass to ask for any analysis on any IRUnitT
(and any specific IRUnit of
>>>>>>>> that IRUnitT) and have the result stored
somewhere and then invalidated.
>>>>>>>> This means that "getAnalysisUsage" is
not just a list of passes, but much
>>>>>>>> more complicated and is essentially a set of
arbitrary pairs "(analysis,
>>>>>>>> IRUnit)" (and the associated potential
tangle of dependencies between the
>>>>>>>> state cached on these tuples). With the old PM,
you essentially are looking
>>>>>>>> at a problem of scheduling the lifetime of
analyses of the same IRUnit
>>>>>>>> intermingled with transformation passes on that
same IRUnit, so you only
>>>>>>>> have the "analysis" part of the tuple
above, making things much simpler
>>>>>>>> (and handling dependencies is much simpler
too). We've obviously outgrown
>>>>>>>> this model with examples like LAA,
AssumptionCacheTracker, etc. that hack
>>>>>>>> around this in the old PM. We may want to have
a fresh re-examination of
>>>>>>>> what problems we are exactly trying to solve.
>>>>>>>>
>>>>>>>> For me, my main concern now is what changes
need to be made in
>>>>>>>> order to feel confident running a pipeline in
the new PM
>>>>>>>> without assertions+ASan.
>>>>>>>>
>>>>>>>>
>>>>>>>> Sorry for the long post, just brain-dumping
before heading home.
>>>>>>>>
>>>>>>>> -- Sean Silva
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>> -- Sean Silva
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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