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

----- Original Message -----
> From: "Mehdi Amini" <mehdi.amini at apple.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Sean Silva" <chisophugis at gmail.com>, "Xinliang
David Li"
> <davidxl at google.com>, "llvm-dev" <llvm-dev at
lists.llvm.org>, "Davide
> Italiano" <dccitaliano at gmail.com>, "Sanjoy Das"
> <sanjoy at playingwithpointers.com>, "Pete Cooper"
> <peter_cooper at apple.com>, "Chandler Carruth"
<chandlerc at gmail.com>
> Sent: Thursday, July 14, 2016 9:37:38 PM
> Subject: Re: [PM] I think that the new PM needs to learn about
> inter-analysis dependencies...
> Sent from my iPhone
> On 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.
> 
> I share the same feeling.
> > 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.
> 
> Because of the hard vs soft dependency, I'm not sure if this has to
> be implicit or if it'd be better made explicit though, i.e. with a
> separate API call.
Can you explain? You still need to invalidate for soft dependencies. 

-Hal 
> --
> Mehdi
> > 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.
> 
> > Thanks again,
> 
> > Hal
> 
> > ----- 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: 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
> 
-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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On Thu, Jul 14, 2016 at 7:43 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>
> ------------------------------
>
> *From: *"Mehdi Amini" <mehdi.amini at apple.com>
> *To: *"Hal Finkel" <hfinkel at anl.gov>
> *Cc: *"Sean Silva" <chisophugis at gmail.com>,
"Xinliang David Li" <
> davidxl at google.com>, "llvm-dev" <llvm-dev at
lists.llvm.org>, "Davide
> Italiano" <dccitaliano at gmail.com>, "Sanjoy Das"
<
> sanjoy at playingwithpointers.com>, "Pete Cooper"
<peter_cooper at apple.com>,
> "Chandler Carruth" <chandlerc at gmail.com>
> *Sent: *Thursday, July 14, 2016 9:37:38 PM
> *Subject: *Re: [PM] I think that the new PM needs to learn about
> inter-analysis dependencies...
>
>
>
> Sent from my iPhone
>
> On 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.
>
>
> I share the same feeling.
>
> 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.
>
>
> Because of the hard vs soft dependency, I'm not sure if this has to be
> implicit or if it'd be better made explicit though, i.e. with a
separate
> API call.
>
> Can you explain? You still need to invalidate for soft dependencies.
>
You don't necessarily need too. For example, a pass may manually modify
LoopInfo to keep it up to date but not update DominatorTree.
On the other hand, invalidation in the case of soft dependencies is still
correct, so we may decide to not worry about the hard/soft distinction
unless there are real compile time savings to be gotten by adding special
handling for soft dependencies.
I.e. the PreservedAnalyses set is about what is preserved, not about what
is invalidated. For a PreservedAnalyses set PA, the set that needs to be
invalidated is something like:
union(transitiveHardDeps(x) for x in ~PA) & ~PA

I.e. in response to Mehdi's question, the soft case can be phrased as an
explicit opt-out of dependency tracking.

-- Sean Silva

>
>
>  -Hal
>
>
> --
> Mehdi
>
>
>  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.
>
> 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
>
>
>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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On Thu, Jul 14, 2016 at 8:41 PM, Sean Silva <chisophugis at gmail.com>
wrote:
>
>
> On Thu, Jul 14, 2016 at 7:43 PM, Hal Finkel <hfinkel at anl.gov>
wrote:
>
>>
>> ------------------------------
>>
>> *From: *"Mehdi Amini" <mehdi.amini at apple.com>
>> *To: *"Hal Finkel" <hfinkel at anl.gov>
>> *Cc: *"Sean Silva" <chisophugis at gmail.com>,
"Xinliang David Li" <
>> davidxl at google.com>, "llvm-dev" <llvm-dev at
lists.llvm.org>, "Davide
>> Italiano" <dccitaliano at gmail.com>, "Sanjoy Das"
<
>> sanjoy at playingwithpointers.com>, "Pete Cooper"
<peter_cooper at apple.com>,
>> "Chandler Carruth" <chandlerc at gmail.com>
>> *Sent: *Thursday, July 14, 2016 9:37:38 PM
>> *Subject: *Re: [PM] I think that the new PM needs to learn about
>> inter-analysis dependencies...
>>
>>
>>
>> Sent from my iPhone
>>
>> On 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.
>>
>>
>> I share the same feeling.
>>
>> 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.
>>
>>
>> Because of the hard vs soft dependency, I'm not sure if this has to
be
>> implicit or if it'd be better made explicit though, i.e. with a
separate
>> API call.
>>
>> Can you explain? You still need to invalidate for soft dependencies.
>>
>
> You don't necessarily need too. For example, a pass may manually modify
> LoopInfo to keep it up to date but not update DominatorTree.
> On the other hand, invalidation in the case of soft dependencies is still
> correct, so we may decide to not worry about the hard/soft distinction
> unless there are real compile time savings to be gotten by adding special
> handling for soft dependencies.
> I.e. the PreservedAnalyses set is about what is preserved, not about what
> is invalidated. For a PreservedAnalyses set PA, the set that needs to be
> invalidated is something like:
> union(transitiveHardDeps(x) for x in ~PA) & ~PA
>
The `& ~PA` is spurious here. (I think when I started writing this I was
thinking in terms of transitively following soft deps too or something like
that)

Now that I think about it, we will probably want (for debugging /
development) some sort of debug output along the lines of "your
transformation pass marked analysis Foo as preserved, but it was still
invalidated because analysis Bar was not preserved".

-- Sean Silva

>
> I.e. in response to Mehdi's question, the soft case can be phrased as
an
> explicit opt-out of dependency tracking.
>
> -- Sean Silva
>
>
>>
>>
>>  -Hal
>>
>>
>> --
>> Mehdi
>>
>>
>>  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.
>>
>> 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
>>
>>
>>
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>
>
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