[Polly] Introduce PhaseManager and remove LPM support #125442
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Subject: Re: [llvm/llvm-project] [Polly] Introduce PhaseManager and remove LPM support (PR #125442)
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@efriedma-quic<https://github.com/efriedma-quic> requested your review on: #125442<#125442> [Polly] Introduce PhaseManager and remove LPM support.
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FYI, @rahulana-quic @tobiasgrosser For some reason I cannot add you as reviewers, but feel free to start a review. |
…ersbur/polly_PhaseManager
…nager' into users/meinersbur/polly_PhaseManager
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Hi @Meinersbur |
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OK, please signal your LGTM when you are ready |
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Thanks for your patience @Meinersbur. |
kartcq
reviewed
Apr 9, 2025
Meinersbur
changed the base branch from
users/meinersbur/polly_NPM-ScopInliner
to
users/meinersbur/polly_CodePreparation
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Instead of relying on any pass manager to schedule Polly's passes, add Polly's own pipeline manager which is seen as a monolithic pass in LLVM's pass manager. Polly's former passes are now phases of the new PhaseManager component.
Relying on LLVM's pass manager (the legacy as well as the New Pass Manager1) to manage Polly's phases never was a good fit that the PhaseManager resolves:
Polly passes were modifying analysis results, in particular RegionInfo and ScopInfo. This means that there was not just one unique and "definite" analysis result, the actual result depended on which analyses ran prior, and the pass manager was not allowed to throw away cached analyses or prior SCoP optimizations would have been forgotten. The LLVM pass manger's persistance of analysis results is not contractual but designed for caching.
Polly depends on a particular execution order of passes and regions (e.g. regression tests, invalidation of consecutive SCoPs). LLVM's pass manager does not guarantee any excecution order.
Polly does not completely preserve DominatorTree, RegionInfo, LoopInfo, or ScalarEvolution, but only as-needed for Polly's own uses. Because the ScopDetection object stores references to those analyses, it still had to lie to the pass manager that they would be preserved, or the pass manager would have released and recomputed the invalidated analysis objects that ScopDetection/ScopInfo was still referencing. To ensure that no non-Polly pass would see these not-completely-preserved analyses, all analyses still had to be thrown away after the ScopPassManager, respectively with a BarrierNoopPass in case of the LPM.
The NPM's PassInstrumentation wraps the IR unit into an
llvm::Anyobject, but implementatons such as PrintIRInstrumentation call llvm_unreachable on encountering an unknown IR unit, such as SCoPs, with no extension points to add support. Hence LLVM crashes when dumping IR between SCoP passes (such as-print-before-changedwith Polly being active).The new PhaseManager uses some command line options that previously belonged to Polly's legacy passes, such as
-polly-print-detect(so the option will continue to work). Hence the LPM support is incompatible with the new approach and support for it is removed.PRs from this patch series:
Footnotes
When Chandler Carruth announced working on the NPM, I talked to him about supporting Polly's use cases. His response was "passes must adhere to the constraints of the pass manager". ↩