[llvm-dev] RFC: Getting ProfileSummaryInfo and BlockFrequencyInfo from various types of passes under the new pass manager

[Hiroshi Yamauchi via llvm-dev]

Here's a sketch of the proposed approach for just one pass (but imagine
more)

https://reviews.llvm.org/D58845

On Fri, Mar 1, 2019 at 12:54 PM Fedor Sergeev via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 2/28/19 12:47 AM, Hiroshi Yamauchi via llvm-dev wrote:
>
> Hi all,
>
> To implement more profile-guided optimizations, we’d like to use
> ProfileSummaryInfo (PSI) and BlockFrequencyInfo (BFI) from more passes of
> various types, under the new pass manager.
>
> The following is what we came up with. Would appreciate feedback. Thanks.
>
> Issue
>
> It’s not obvious (to me) how to best do this, given that we cannot request
> an outer-scope analysis result from an inner-scope pass through analysis
> managers [1] and that we might unnecessarily running some analyses unless
> we conditionally build pass pipelines for PGO cases.
>
> Indeed, this is an intentional restriction in new pass manager, which is
> more or less a reflection of a fundamental property of outer-inner IRUnit
> relationship
> and transformations/analyses run on those units. The main intent for
> having those inner IRUnits (e.g. Loops) is to run local transformations and
> save compile time
> on being local to a particular small piece of IR. Loop Pass manager allows
> you to run a whole pipeline of different transformations still locally,
> amplifying the save.
> As soon as you run function-level analysis from within the loop pipeline
> you essentially break this pipelining.
> Say, as you run your loop transformation it modifies the loop (and the
> function) and potentially invalidates the analysis,
> so you have to rerun your analysis again and again. Hence instead of
> saving on compile time it ends up increasing it.
>

Exactly.


> I have hit this issue somewhat recently with dependency of loop passes on
> BranchProbabilityInfo.
> (some loop passes, like IRCE can use it for profitability analysis).
>
The only solution that appears to be reasonable there is to teach all the
> loops passes that need to be pipelined
> to preserve BPI (or any other module/function-level analyses) similar to
> how they preserve DominatorTree and
> other "LoopStandard" analyses.
>

Is this implemented - do the loop passes preserve BPI?

In buildFunctionSimplificationPipeline (where LoopFullUnrollPass is added
as in the sketch), LateLoopOptimizationsEPCallbacks
and LoopOptimizerEndEPCallbacks seem to allow some arbitrary loop passes to
be inserted into the pipelines (via flags)?

I wonder how hard it'd be to teach all the relevant loop passes to preserve
BFI (or BPI)..


> It seems that for different types of passes to be able to get PSI and BFI,
> we’d need to ensure PSI is cached for a non-module pass, and PSI, BFI and
> the ModuleAnalysisManager proxy are cached for a loop pass in the pass
> pipelines. This may mean potentially needing to insert BFI/PSI in front of
> many passes [2]. It seems not obvious how to conditionally insert BFI for
> PGO pipelines because there isn’t always a good flag to detect PGO cases
> [3] or we tend to build pass pipelines before examining the code (or
> without propagating enough info down) [4].
>
> Proposed approach
>
> - Cache PSI right after the profile summary in the IR is written in the
> pass pipeline [5]. This would avoid the need to insert RequiredAnalysisPass
> for PSI before each non-module pass that needs it. PSI can be technically
> invalidated but unlikely. If it does, we insert another RequiredAnalysisPass
>  [6].
>
> - Conditionally insert RequireAnalysisPass for BFI, if PGO, right before
> each loop pass that needs it. This doesn't seem avoidable because BFI can
> be invalidated whenever the CFG changes. We detect PGO based on the command
> line flags and/or whether the module has the profile summary info (we may
> need to pass the module to more functions.)
>
> - Add a new proxy ModuleAnalysisManagerLoopProxy for a loop pass to be
> able to get to the ModuleAnalysisManager in one step and PSI through it.
>
> Alternative approaches
>
> Dropping BFI and use PSI only
> We could consider not using BFI and solely relying on PSI and
> function-level profiles only (as opposed to block-level), but profile
> precision would suffer.
>
> Computing BFI in-place
> We could consider computing BFI “in-place” by directly running BFI outside
> of the pass manager [7]. This would let us avoid using the analysis manager
> constraints but it would still involve running an outer-scope analysis from
> an inner-scope pass and potentially cause problems in terms of pass
> pipelining and concurrency. Moreover, a potential downside of running
> analyses in-place is that it won’t take advantage of cached analysis
> results provided by the pass manager.
>
> Adding inner-scope versions of PSI and BFI
> We could consider adding a function-level and loop-level PSI and
> loop-level BFI, which internally act like their outer-scope versions but
> provide inner-scope results only. This way, we could always call getResult
> for PSI and BFI. However, this would still involve running an outer-scope
> analysis from an inner-scope pass.
>
> Caching the FAM and the MAM proxies
> We could consider caching the FunctionalAnalysisManager and the
> ModuleAnalysisManager proxies once early on instead of adding a new proxy.
> But it seems to not likely work well because the analysis cache key type
> includes the function or the module and some pass may add a new function
> for which the proxy wouldn’t be cached. We’d need to write and insert a
> pass in select locations to just fill the cache. Adding the new proxy would
> take care of these with a three-line change.
>
> Conditional BFI
> We could consider adding a conditional BFI analysis that is a wrapper
> around BFI and computes BFI only if profiles are available (either checking
> the module has profile summary or depend on the PSI.) With this, we
> wouldn’t need to conditionally build pass pipelines and may work for the
> new pass manager. But a similar wouldn’t work for the old pass manager
> because we cannot conditionally depend on an analysis under it.
>
> There is LazyBlockFrequencyInfo.
> Not sure how well it fits this idea.
>

Good point. LazyBlockFrequencyInfo seems usable with the old pass manager
(save unnecessary BFI/BPI) and would work for function passes. I think
the restriction
still applies - a loop pass cannot still request (outer-scope) BFI, lazy or
not, new or old (pass manager). Another assumption is that it'd be cheap
and safe to unconditionally depend on PSI or check the module's profile
summary.


> regards,
>   Fedor.
>
>
>
> [1] We cannot call AnalysisManager::getResult for an outer scope but only
> getCachedResult. Probably because of potential pipelining or concurrency
> issues.
> [2] For example, potentially breaking up multiple pipelined loop passes
> and insert RequireAnalysisPass<BlockFrequencyAnalysis> in front of each of
> them.
> [3] For example, -fprofile-instr-use and -fprofile-sample-use aren’t
> present in ThinLTO post link builds.
> [4] For example, we could check whether the module has the profile summary
> metadata annotated when building pass pipelines but we don’t always pass
> the module down to the place where we build pass pipelines.
> [5] By inserting RequireAnalysisPass<ProfileSummaryInfo> after the
> PGOInstrumentationUse and the SampleProfileLoaderPass passes (and around
> the PGOIndirectCallPromotion pass for the Thin LTO post link pipeline.)
> [6] For example, the context-sensitive PGO.
> [7] Directly calling its constructor along with the dependent analyses
> results, eg. the jump threading pass.
>
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