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

Sean Silva via llvm-dev llvm-dev at lists.llvm.org
Thu Jul 14 00:51:36 PDT 2016

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

The more I think about it, the more I'm convinced that the real "hard"
problem that the new PM is exposing us to is having the ability for any
pass to ask for any analysis on any IRUnitT (and any specific IRUnit of
that IRUnitT) and have the result stored somewhere and then invalidated.
This means that "getAnalysisUsage" is not just a list of passes, but much
more complicated and is essentially a set of arbitrary pairs "(analysis,
IRUnit)" (and the associated potential tangle of dependencies between the
state cached on these tuples). With the old PM, you essentially are looking
at a problem of scheduling the lifetime of analyses of the same IRUnit
intermingled with transformation passes on that same IRUnit, so you only
have the "analysis" part of the tuple above, making things much simpler
(and handling dependencies is much simpler too). We've obviously outgrown
this model with examples like LAA, AssumptionCacheTracker, etc. that hack
around this in the old PM. We may want to have a fresh re-examination of
what problems we are exactly trying to solve.

For me, my main concern now is what changes need to be made in order to
feel confident running a pipeline in the new PM without assertions+ASan.

Sorry for the long post, just brain-dumping before heading home.

-- Sean Silva

> -- Sean Silva
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