Sean Silva via llvm-dev
2016-Aug-08 09:51 UTC
[llvm-dev] [PM] I think that the new PM needs to learn about inter-analysis dependencies...
On Sun, Aug 7, 2016 at 4:55 PM, Philip Reames <listmail at philipreames.com> wrote:> Skimming the thread, this post is the clearest path forward I've seen. > Minor comments inline, but I generally like this framing. > > On 07/14/2016 08:04 PM, Chandler Carruth via llvm-dev wrote: > > We need better terminology to talk about this. I propose: > > analysis-dependencies: analysis A uses result of analysis B when *running* > an analysis and not used by the result > query-dependencies: result of analysis A uses result of analysis B when > evaluating a query > data-structure-depnedencies: result of analysis A uses data structures > from the result of analysis B inside its own data structures > > I think these are much more precise than "hard" or "soft" and expose more > facets. > > For analysis-depnedencies, I continue to think they work correctly. If a > transformation claims it preserves an analysis, it must actually know this > to be true. I don't see any actual problems here in practice today, and > this isn't actually something changed in the new PM. > > For data-structure-dependencies, the investigation done by Sean seems to > clearly show these to be rare, and I think having their invalidate methods > be overridden to test that *all* of the data structures they depend on are > preserved is the correct approach. > > This may end up being too error prone. That seems to be Sean's concern > down thread. I am also worried about that, but assuming the number of such > occurrences are low, it seems reasonable to start with this approach and > revisit if needed. >Chandler's suggestion here doesn't actually avoid the issue. A simple proof that this approach cannot work is that the issue at hand is one of insufficient invalidation. The `invalidate` callback can only prevent invalidation, so Chandler's suggestion of overriding it can only prevent invalidation -- it cannot trigger additional invalidation and therefore cannot solve a problem of insufficient invalidation. -- Sean Silva> > The primary issue seems to be with query-dependencies. These in turn break > down into two categories: > > 1) query-dependencies on "immutable" analysis results. These are results > that we *never* expect to be invalidated and we just want easy access to. > TargetLibraryInfo is the classic example here. > > 2) query-dependencies on normal analysis results. DominatorTree and and > AAResults are the primary examples here. > > I would like to have a simple way to handle #1 by ensuring invalidation > doesn't occur. I think this already works, but I'm interested if there is > actually an issue here. > > We *could* handle #2 much like data-structure-dependencies, but I hear Hal > and others that this would become burdensome. However, my preference would > be to instead handle #2 by making result APIs accept direct handles to the > analysis results they rely on. > > The reason for preferring this approach is because I think it makes the > relationship between these things much more clear to users of the analyses. > > +1 to this. Having the query dependencies explicit at the call site would > generally make the code much easier to understand and thus much more likely > to be correct. I recently ran across an issue in LVI under the old pass > manager that looks highly suspicious, but because the code is quite complex > (putting it mildly), I wasn't sure whether it was correct or not. Having > the additional analyzes passed in explicitly through the query would make > many of these cases substantially easier to audit. > > > I think the most annoying of these to handle are aggregation-style > analyses results like AAResults. There, I think it might make more sense to > handle them along the lines of data-structure-dependencies. I don't think > we have so many of those that this would be a significant burden IMO. > > On Thu, Jul 14, 2016 at 7:48 PM Sean Silva <chisophugis at gmail.com> wrote: > >> On Thu, Jul 14, 2016 at 7:21 PM, Hal Finkel <hfinkel at anl.gov> wrote: >> >>> Hi Sean, >>> >>> Thanks for writing all of this up. I'll go back to my previous position: >>> we need a general dependency graph built as the analysis cache is used. It >>> should have the following properties: >>> >>> 1. When we call getResult or getCachedResult on an analysis manager, we >>> record a dependency of the current pass on the returned result. >>> 2. This dependency needs to be stored such that it can be used to >>> invalidate the current result when the returned result is invalidates and >>> so that the dependency can be deleted when the current result is >>> invalidated. >>> >>> As I understand the problem, this is a fully-general solution. I see no >>> reason not to have a fully-general solution. >>> >> >> Yeah, the mechanics of maintaining this fully general mapping are >> straightforward in the abstract (once we have a single analysis manager for >> all IRUnitT's). Essentially, the analysis manager maintains a stack of (Analysis, >> IRUnit) that it is currently computing and pushes/pops the stack as it >> (re-)enters/exits get{,Cached}Result. If the stack is not empty (suppose >> top of stack is `(AnalysisFoo, IRUnitBar)`), then when we go to push >> `(AnalysisBaz, IRUnitQux)` we record a dependency that the result cached on >> key `(AnalysisBaz, IRUnitQux)` must be invalidated if the result cached on >> key `(AnalysisFoo, IRUnitBar)` is invalidated. >> >> The difficult part is what guarantees we provide about things being stale >> or not and how we invalidate when IRUnit's are deleted or created. >> For example, suppose a function pass DCE's a call which causes an SCC Foo >> of 3 functions to no longer be an SCC. When/how do analyses cached on Foo >> get invalidated? And is it valid to query them? One of the expected use >> cases (I'm told) for CGSCC passes is to propagate function-attribute like >> things, so these are being potentially queried by that same function pass. >> >> -- Sean Silva >> >> >>> >>> Thanks again, >>> Hal >>> >>> ------------------------------ >>> >>> *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: *Thursday, July 14, 2016 4:11:58 AM >>> *Subject: *Re: [PM] I think that the new PM needs to learn about >>> inter-analysis dependencies... >>> >>> >>> >>> 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). >>>> >>> >>> To clarify, it seems like the current new PM is essentially trying to >>> solve the problem of maintaining/updating a mapping: >>> (Analysis, IRUnit) -> AnalysisResult >>> where the AnalysisResult's can have an arbitrary dependency on an >>> arbitrary set of other AnalysisResult's currently maintained in this >>> mapping. In order to invalidate any AnalysisResult you need to invalidate >>> all AnalysisResult's that transitively depend on it. Therefore the >>> right-hand side of this mapping needs to be something like >>> `(AnalysisResult, SetOfDependents)`. >>> So the mapping is really `(Analysis, IRUnit) -> (AnalysisResult, >>> SetOfDependents)` >>> Also, this mapping can be updated at any point during the execution of a >>> transformation pass (and various other places) and must stay correct as the >>> IR is changed (more on this below). >>> For example, you might have something like: >>> (DominatorTreeAnalysis, function @foo) -> (DominatorTree for @foo, >>> [(DemandedBitsAnalysis, function @foo)]) >>> (AssumptionAnalysis, function @foo) -> (AssumptionCache for @foo, >>> [(DemandedBitsAnalysis, function @foo)]) >>> (DemandedBitsAnalysis, function @foo) -> (DemandedBits for @foo, []) >>> (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, []) >>> >>> So for example, when a transformation pass invalidates >>> `(AssumptionAnalysis, function @bar)`, we need to walk >>> `(SomeModuleAnalysis, module TheModule)` and `(SomeFunctionAnalysis, >>> function @baz)` to invalidate them. >>> >>> >>> Compare this with the old PM (although like I said we have outgrown this >>> model). Essentially you take the previous mapping, and require IRUnit to be >>> a constant at any given point in time. Hence the mapping is essentially >>> Analysis -> AnalysisResult >>> Since this is 1:1 there is no real distinction between the Analysis and >>> the AnalysisResult (and as part of transitioning to the new PM this has had >>> to be untangled). >>> This also makes the dependencies simpler since you just have a set of >>> "what analyses have been run at this point". You just need to run the >>> analyses individually and make sure they are in the right order. Also, >>> getAnalysis just takes the Analysis to get the AnalysisResult which makes >>> it simpler -- you just query which analyses are live. >>> >>> >>> Also, the mapping `(Analysis, IRUnit) -> (AnalysisResult, >>> SetOfDependents)` that the new PM is essentially trying to keep is even >>> more complicated because for e.g. Loop and CGSCC passes the IRUnit itself >>> is an object created by an analysis and subject to invalidation of that >>> analysis as the IR changes underneath it. >>> >>> And then there is the question of at what points must this mapping be >>> valid (i.e. no stale analysis results, no dangling pointers, etc.) and when >>> the transitive invalidation walking happens. Evidently while a >>> transformation pass is running, things might temporarily be stale; what are >>> the "checkpoints" where the mapping is guaranteed to be valid? At the start >>> of each transformation pass? At least Chandler's D21464 does not stick to >>> this because the IRUnit's (SCC's) are only updated at the end of running >>> potentially many function transformation passes. I.e. all but the first >>> function transformation pass might observe stale IRUnit's (SCC's). >>> >>> One other thing to note is that soft-dependencies (using David's >>> terminology) don't require this kind of dependency tracking. An analysis >>> result can be cached even though its soft-dependencies are not cached. And >>> invalidation of soft-dependencies does not require invalidating the >>> soft-dependents. Actually, this makes it the terminology "soft" and "hard' >>> quite natural; "hard" requires an edge to track the dependency for >>> invalidation purposes, "soft" does not. >>> >>> This is all quite general. Perhaps too much. We clearly need to go >>> beyond the old PM's model, but we may not need to go to the fully general >>> case. Is there a good middle-ground that meets our needs? What restrictions >>> would we be willing to live with in order to make it easier? The first one >>> on my list is to not have the IRUnit's themselves depend on analyses. Like >>> Chandler mentioned on D21921 this has the effect of e.g. preventing caching >>> across the intervening module pass in a case like >>> `module(cgscc(require<foo-cgscc-analysis>),some-module-pass- >>> that-makes-no-changes,cgscc(some-cgscc-pass-that-uses-foo-cgscc-analysis))` >>> but that seems like a restriction we can live with. >>> >>> >>> Again, sorry for the braindump. >>> >>> >>> -- Sean Silva >>> >>> >>>> 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 >>> >> > > _______________________________________________ > LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160808/77752a68/attachment-0001.html>
Jan Sjodin via llvm-dev
2016-Aug-09 15:29 UTC
[llvm-dev] [PM] I think that the new PM needs to learn about inter-analysis dependencies...
When we talk about stale information, what does it mean exactly? This goes back
to the point about how what guarantees are made about the analysis information,
and when it is invalidated. If some information is wrong for the current IR
(during a transform), but still fully correct with regards to the IR that was
given to the analysis before the transform, this is information that some
transforms actually need. The other case is if the analysis info directly points
to the IR that is being modified, and some information is wrong for both the old
and current IR, which could cause the transform to fail. What should the policy
be for analyses, and how would someone writing a transform know when information
becomes stale and how?
- Jan
On Monday, August 8, 2016 1:42 PM, Sean Silva via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
On Sun, Aug 7, 2016 at 4:55 PM, Philip Reames <listmail at
philipreames.com> wrote:
Skimming the thread, this post is the clearest path forward I've seen.
Minor comments inline, but I generally like this framing.
On 07/14/2016 08:04 PM, Chandler Carruth via llvm-dev wrote:
We need better terminology to talk about this. I propose:
analysis-dependencies: analysis A uses result of analysis B when *running* an
analysis and not used by the result query-dependencies: result of analysis A
uses result of analysis B when evaluating a query data-structure-depnedencies:
result of analysis A uses data structures from the result of analysis B inside
its own data structures
I think these are much more precise than "hard" or "soft"
and expose more facets.
For analysis-depnedencies, I continue to think they work correctly. If a
transformation claims it preserves an analysis, it must actually know this to be
true. I don't see any actual problems here in practice today, and this
isn't actually something changed in the new PM.
For data-structure-dependencies, the investigation done by Sean seems to
clearly show these to be rare, and I think having their invalidate methods be
overridden to test that *all* of the data structures they depend on are
preserved is the correct approach.
This may end up being too error prone. That seems to be Sean's concern
down thread. I am also worried about that, but assuming the number of such
occurrences are low, it seems reasonable to start with this approach and revisit
if needed.
Chandler's suggestion here doesn't actually avoid the issue.
A simple proof that this approach cannot work is that the issue at hand is one
of insufficient invalidation. The `invalidate` callback can only prevent
invalidation, so Chandler's suggestion of overriding it can only prevent
invalidation -- it cannot trigger additional invalidation and therefore cannot
solve a problem of insufficient invalidation.
-- Sean Silva
The primary issue seems to be with query-dependencies. These in turn break
down into two categories:
1) query-dependencies on "immutable" analysis results. These are
results that we *never* expect to be invalidated and we just want easy access
to. TargetLibraryInfo is the classic example here.
2) query-dependencies on normal analysis results. DominatorTree and and
AAResults are the primary examples here.
I would like to have a simple way to handle #1 by ensuring invalidation
doesn't occur. I think this already works, but I'm interested if there
is actually an issue here.
We *could* handle #2 much like data-structure-dependencies, but I hear Hal and
others that this would become burdensome. However, my preference would be to
instead handle #2 by making result APIs accept direct handles to the analysis
results they rely on.
The reason for preferring this approach is because I think it makes the
relationship between these things much more clear to users of the analyses.
+1 to this. Having the query dependencies explicit at the call site would
generally make the code much easier to understand and thus much more likely to
be correct. I recently ran across an issue in LVI under the old pass manager
that looks highly suspicious, but because the code is quite complex (putting it
mildly), I wasn't sure whether it was correct or not. Having the
additional analyzes passed in explicitly through the query would make many of
these cases substantially easier to audit.
I think the most annoying of these to handle are aggregation-style analyses
results like AAResults. There, I think it might make more sense to handle them
along the lines of data-structure-dependencies. I don't think we have so
many of those that this would be a significant burden IMO.
On Thu, Jul 14, 2016 at 7:48 PM Sean Silva <chisophugis at gmail.com>
wrote:
On Thu, Jul 14, 2016 at 7:21 PM, Hal Finkel <hfinkel at anl.gov> wrote:
Hi Sean,
Thanks for writing all of this up. I'll go back to my previous position: we
need a general dependency graph built as the analysis cache is used. It should
have the following properties:
1. When we call getResult or getCachedResult on an analysis manager, we record
a dependency of the current pass on the returned result.
2. This dependency needs to be stored such that it can be used to invalidate
the current result when the returned result is invalidates and so that the
dependency can be deleted when the current result is invalidated.
As I understand the problem, this is a fully-general solution. I see no reason
not to have a fully-general solution.
Yeah, the mechanics of maintaining this fully general mapping are
straightforward in the abstract (once we have a single analysis manager for all
IRUnitT's). Essentially, the analysis manager maintains a stack of
(Analysis, IRUnit) that it is currently computing and pushes/pops the stack as
it (re-)enters/exits get{,Cached}Result. If the stack is not empty (suppose top
of stack is `(AnalysisFoo, IRUnitBar)`), then when we go to push `(AnalysisBaz,
IRUnitQux)` we record a dependency that the result cached on key `(AnalysisBaz,
IRUnitQux)` must be invalidated if the result cached on key `(AnalysisFoo,
IRUnitBar)` is invalidated.
The difficult part is what guarantees we provide about things being stale or
not and how we invalidate when IRUnit's are deleted or created. For
example, suppose a function pass DCE's a call which causes an SCC Foo of 3
functions to no longer be an SCC. When/how do analyses cached on Foo get
invalidated? And is it valid to query them? One of the expected use cases
(I'm told) for CGSCC passes is to propagate function-attribute like things,
so these are being potentially queried by that same function pass.
-- Sean Silva
Thanks again,
Hal
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.co m>,
"Pete Cooper" <peter_cooper at apple.com>
Sent: Thursday, July 14, 2016 4:11:58 AM
Subject: Re: [PM] I think that the new PM needs to learn about inter-analysis
dependencies...
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 ofPassRegistry.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 AssumptionCacheand 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).
To clarify, it seems like the current new PM is essentially trying to solve
the problem of maintaining/updating a mapping: (Analysis, IRUnit) ->
AnalysisResult where the AnalysisResult's can have an arbitrary dependency
on an arbitrary set of other AnalysisResult's currently maintained in this
mapping. In order to invalidate any AnalysisResult you need to invalidate all
AnalysisResult's that transitively depend on it. Therefore the right-hand
side of this mapping needs to be something like `(AnalysisResult,
SetOfDependents)`. So the mapping is really `(Analysis, IRUnit)
-> (AnalysisResult, SetOfDependents)` Also, this mapping can be updated at
any point during the execution of a transformation pass (and various other
places) and must stay correct as the IR is changed (more on this below). For
example, you might have something like: (DominatorTreeAnalysis, function @foo)
-> (DominatorTree for @foo, [(DemandedBitsAnalysis, function @foo)])
(AssumptionAnalysis, function @foo) -> (AssumptionCache for @foo,
[(DemandedBitsAnalysis, function @foo)]) (DemandedBitsAnalysis, function @foo)
-> (DemandedBits for @foo, []) (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, [])
So for example, when a transformation pass invalidates `(AssumptionAnalysis,
function @bar)`, we need to walk `(SomeModuleAnalysis, module TheModule)` and
`(SomeFunctionAnalysis, function @baz)` to invalidate them.
Compare this with the old PM (although like I said we have outgrown this
model). Essentially you take the previous mapping, and require IRUnit to be a
constant at any given point in time. Hence the mapping is essentially
Analysis -> AnalysisResult Since this is 1:1 there is no real distinction
between the Analysis and the AnalysisResult (and as part of transitioning to the
new PM this has had to be untangled). This also makes the dependencies simpler
since you just have a set of "what analyses have been run at this
point". You just need to run the analyses individually and make sure they
are in the right order. Also, getAnalysis just takes the Analysis to get the
AnalysisResult which makes it simpler -- you just query which analyses are live.
Also, the mapping `(Analysis, IRUnit) -> (AnalysisResult, SetOfDependents)`
that the new PM is essentially trying to keep is even more complicated because
for e.g. Loop and CGSCC passes the IRUnit itself is an object created by an
analysis and subject to invalidation of that analysis as the IR changes
underneath it.
And then there is the question of at what points must this mapping be valid
(i.e. no stale analysis results, no dangling pointers, etc.) and when the
transitive invalidation walking happens. Evidently while a transformation pass
is running, things might temporarily be stale; what are the
"checkpoints" where the mapping is guaranteed to be valid? At the
start of each transformation pass? At least Chandler's D21464 does not
stick to this because the IRUnit's (SCC's) are only updated at the end
of running potentially many function transformation passes. I.e. all but the
first function transformation pass might observe stale IRUnit's (SCC's).
One other thing to note is that soft-dependencies (using David's
terminology) don't require this kind of dependency tracking. An analysis
result can be cached even though its soft-dependencies are not cached. And
invalidation of soft-dependencies does not require invalidating the
soft-dependents. Actually, this makes it the terminology "soft" and
"hard' quite natural; "hard" requires an edge to track the
dependency for invalidation purposes, "soft" does not.
This is all quite general. Perhaps too much. We clearly need to go beyond the
old PM's model, but we may not need to go to the fully general case. Is
there a good middle-ground that meets our needs? What restrictions would we be
willing to live with in order to make it easier? The first one on my list is to
not have the IRUnit's themselves depend on analyses. Like Chandler
mentioned on D21921 this has the effect of e.g. preventing caching across the
intervening module pass in a case like`module(cgscc(require<foo-cgsc
c-analysis>),some-module-pass- that-makes-no-changes,cgscc(so
me-cgscc-pass-that-uses-foo-cg scc-analysis))` but that seems like a restriction
we can live with.
Again, sorry for the braindump.
-- Sean Silva
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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Sean Silva via llvm-dev
2016-Aug-10 00:28 UTC
[llvm-dev] [PM] I think that the new PM needs to learn about inter-analysis dependencies...
On Tue, Aug 9, 2016 at 8:29 AM, Jan Sjodin <jan_sjodin at yahoo.com> wrote:> When we talk about stale information, what does it mean exactly? This goes > back to the point about how what guarantees are made about the analysis > information, and when it is invalidated. If some information is wrong for > the current IR (during a transform), but still fully correct with regards > to the IR that was given to the analysis before the transform, this is > information that some transforms actually need. The other case is if the > analysis info directly points to the IR that is being modified, and some > information is wrong for both the old and current IR, which could cause the > transform to fail. What should the policy be for analyses, and how would > someone writing a transform know when information becomes stale and how? >Those are more subtle issues. The notion of "stale" we are talking about right now is literally just use-after-free. I.e. an analysis result A holds a pointer to an object that is freed when analysis result B is invalidated. When B is invalidated, A must be invalidated too. -- Sean Silva> > - Jan > > > On Monday, August 8, 2016 1:42 PM, Sean Silva via llvm-dev < > llvm-dev at lists.llvm.org> wrote: > > > > > On Sun, Aug 7, 2016 at 4:55 PM, Philip Reames <listmail at philipreames.com> > wrote: > > Skimming the thread, this post is the clearest path forward I've seen. > Minor comments inline, but I generally like this framing. > > On 07/14/2016 08:04 PM, Chandler Carruth via llvm-dev wrote: > > We need better terminology to talk about this. I propose: > > analysis-dependencies: analysis A uses result of analysis B when *running* > an analysis and not used by the result > query-dependencies: result of analysis A uses result of analysis B when > evaluating a query > data-structure-depnedencies: result of analysis A uses data structures > from the result of analysis B inside its own data structures > > I think these are much more precise than "hard" or "soft" and expose more > facets. > > For analysis-depnedencies, I continue to think they work correctly. If a > transformation claims it preserves an analysis, it must actually know this > to be true. I don't see any actual problems here in practice today, and > this isn't actually something changed in the new PM. > > For data-structure-dependencies, the investigation done by Sean seems to > clearly show these to be rare, and I think having their invalidate methods > be overridden to test that *all* of the data structures they depend on are > preserved is the correct approach. > > This may end up being too error prone. That seems to be Sean's concern > down thread. I am also worried about that, but assuming the number of such > occurrences are low, it seems reasonable to start with this approach and > revisit if needed. > > > Chandler's suggestion here doesn't actually avoid the issue. > > A simple proof that this approach cannot work is that the issue at hand is > one of insufficient invalidation. The `invalidate` callback can only > prevent invalidation, so Chandler's suggestion of overriding it can only > prevent invalidation -- it cannot trigger additional invalidation and > therefore cannot solve a problem of insufficient invalidation. > > -- Sean Silva > > > > The primary issue seems to be with query-dependencies. These in turn break > down into two categories: > > 1) query-dependencies on "immutable" analysis results. These are results > that we *never* expect to be invalidated and we just want easy access to. > TargetLibraryInfo is the classic example here. > > 2) query-dependencies on normal analysis results. DominatorTree and and > AAResults are the primary examples here. > > I would like to have a simple way to handle #1 by ensuring invalidation > doesn't occur. I think this already works, but I'm interested if there is > actually an issue here. > > We *could* handle #2 much like data-structure-dependencies, but I hear Hal > and others that this would become burdensome. However, my preference would > be to instead handle #2 by making result APIs accept direct handles to the > analysis results they rely on. > > The reason for preferring this approach is because I think it makes the > relationship between these things much more clear to users of the analyses. > > +1 to this. Having the query dependencies explicit at the call site would > generally make the code much easier to understand and thus much more likely > to be correct. I recently ran across an issue in LVI under the old pass > manager that looks highly suspicious, but because the code is quite complex > (putting it mildly), I wasn't sure whether it was correct or not. Having > the additional analyzes passed in explicitly through the query would make > many of these cases substantially easier to audit. > > > I think the most annoying of these to handle are aggregation-style > analyses results like AAResults. There, I think it might make more sense to > handle them along the lines of data-structure-dependencies. I don't think > we have so many of those that this would be a significant burden IMO. > > On Thu, Jul 14, 2016 at 7:48 PM Sean Silva <chisophugis at gmail.com> wrote: > > On Thu, Jul 14, 2016 at 7:21 PM, Hal Finkel <hfinkel at anl.gov> wrote: > > Hi Sean, > > Thanks for writing all of this up. I'll go back to my previous position: > we need a general dependency graph built as the analysis cache is used. It > should have the following properties: > > 1. When we call getResult or getCachedResult on an analysis manager, we > record a dependency of the current pass on the returned result. > 2. This dependency needs to be stored such that it can be used to > invalidate the current result when the returned result is invalidates and > so that the dependency can be deleted when the current result is > invalidated. > > As I understand the problem, this is a fully-general solution. I see no > reason not to have a fully-general solution. > > > Yeah, the mechanics of maintaining this fully general mapping are > straightforward in the abstract (once we have a single analysis manager for > all IRUnitT's). Essentially, the analysis manager maintains a stack of (Analysis, > IRUnit) that it is currently computing and pushes/pops the stack as it > (re-)enters/exits get{,Cached}Result. If the stack is not empty (suppose > top of stack is `(AnalysisFoo, IRUnitBar)`), then when we go to push > `(AnalysisBaz, IRUnitQux)` we record a dependency that the result cached on > key `(AnalysisBaz, IRUnitQux)` must be invalidated if the result cached on > key `(AnalysisFoo, IRUnitBar)` is invalidated. > > The difficult part is what guarantees we provide about things being stale > or not and how we invalidate when IRUnit's are deleted or created. > For example, suppose a function pass DCE's a call which causes an SCC Foo > of 3 functions to no longer be an SCC. When/how do analyses cached on Foo > get invalidated? And is it valid to query them? One of the expected use > cases (I'm told) for CGSCC passes is to propagate function-attribute like > things, so these are being potentially queried by that same function pass. > > -- Sean Silva > > > > Thanks again, > Hal > > ------------------------------ > > *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.co m > <sanjoy at playingwithpointers.com>>, "Pete Cooper" <peter_cooper at apple.com> > *Sent: *Thursday, July 14, 2016 4:11:58 AM > *Subject: *Re: [PM] I think that the new PM needs to learn about > inter-analysis dependencies... > > > > 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). > > > To clarify, it seems like the current new PM is essentially trying to > solve the problem of maintaining/updating a mapping: > (Analysis, IRUnit) -> AnalysisResult > where the AnalysisResult's can have an arbitrary dependency on an > arbitrary set of other AnalysisResult's currently maintained in this > mapping. In order to invalidate any AnalysisResult you need to invalidate > all AnalysisResult's that transitively depend on it. Therefore the > right-hand side of this mapping needs to be something like > `(AnalysisResult, SetOfDependents)`. > So the mapping is really `(Analysis, IRUnit) -> (AnalysisResult, > SetOfDependents)` > Also, this mapping can be updated at any point during the execution of a > transformation pass (and various other places) and must stay correct as the > IR is changed (more on this below). > For example, you might have something like: > (DominatorTreeAnalysis, function @foo) -> (DominatorTree for @foo, > [(DemandedBitsAnalysis, function @foo)]) > (AssumptionAnalysis, function @foo) -> (AssumptionCache for @foo, > [(DemandedBitsAnalysis, function @foo)]) > (DemandedBitsAnalysis, function @foo) -> (DemandedBits for @foo, []) > (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, []) > > So for example, when a transformation pass invalidates > `(AssumptionAnalysis, function @bar)`, we need to walk > `(SomeModuleAnalysis, module TheModule)` and `(SomeFunctionAnalysis, > function @baz)` to invalidate them. > > > Compare this with the old PM (although like I said we have outgrown this > model). Essentially you take the previous mapping, and require IRUnit to be > a constant at any given point in time. Hence the mapping is essentially > Analysis -> AnalysisResult > Since this is 1:1 there is no real distinction between the Analysis and > the AnalysisResult (and as part of transitioning to the new PM this has had > to be untangled). > This also makes the dependencies simpler since you just have a set of > "what analyses have been run at this point". You just need to run the > analyses individually and make sure they are in the right order. Also, > getAnalysis just takes the Analysis to get the AnalysisResult which makes > it simpler -- you just query which analyses are live. > > > Also, the mapping `(Analysis, IRUnit) -> (AnalysisResult, > SetOfDependents)` that the new PM is essentially trying to keep is even > more complicated because for e.g. Loop and CGSCC passes the IRUnit itself > is an object created by an analysis and subject to invalidation of that > analysis as the IR changes underneath it. > > And then there is the question of at what points must this mapping be > valid (i.e. no stale analysis results, no dangling pointers, etc.) and when > the transitive invalidation walking happens. Evidently while a > transformation pass is running, things might temporarily be stale; what are > the "checkpoints" where the mapping is guaranteed to be valid? At the start > of each transformation pass? At least Chandler's D21464 does not stick to > this because the IRUnit's (SCC's) are only updated at the end of running > potentially many function transformation passes. I.e. all but the first > function transformation pass might observe stale IRUnit's (SCC's). > > One other thing to note is that soft-dependencies (using David's > terminology) don't require this kind of dependency tracking. An analysis > result can be cached even though its soft-dependencies are not cached. And > invalidation of soft-dependencies does not require invalidating the > soft-dependents. Actually, this makes it the terminology "soft" and "hard' > quite natural; "hard" requires an edge to track the dependency for > invalidation purposes, "soft" does not. > > This is all quite general. Perhaps too much. We clearly need to go beyond > the old PM's model, but we may not need to go to the fully general case. Is > there a good middle-ground that meets our needs? What restrictions would we > be willing to live with in order to make it easier? The first one on my > list is to not have the IRUnit's themselves depend on analyses. Like > Chandler mentioned on D21921 this has the effect of e.g. preventing caching > across the intervening module pass in a case like > `module(cgscc(require<foo-cgsc c-analysis>),some-module-pass- > that-makes-no-changes,cgscc(so me-cgscc-pass-that-uses-foo-cg > scc-analysis))` but that seems like a restriction we can live with. > > > Again, sorry for the braindump. > > > -- Sean Silva > > > 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 > > > > ______________________________ _________________ > LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/ mailman/listinfo/llvm-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev> > > > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > > >-------------- next part -------------- An HTML attachment was scrubbed... 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