[PGO] Add llvm.loop.estimated_trip_count metadata #152775
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This patch implements the `llvm.loop.estimated_trip_count` metadata discussed in [[RFC] Fix Loop Transformations to Preserve Block Frequencies](https://discourse.llvm.org/t/rfc-fix-loop-transformations-to-preserve-block-frequencies/85785). As [suggested in the RFC comments](https://discourse.llvm.org/t/rfc-fix-loop-transformations-to-preserve-block-frequencies/85785/4), it adds the new metadata to all loops at the time of profile ingestion and estimates each trip count from the loop's `branch_weights` metadata. As [suggested in the PR#128785 review](#128785 (comment)), it does so via a `PGOEstimateTripCountsPass` pass, which creates the new metadata for the loop but omits the value if it cannot estimate a trip count due to the loop's form. An important observation not previously discussed is that `PGOEstimateTripCountsPass` *often* cannot estimate a loop's trip count but later passes can transform the loop in a way that makes it possible. Currently, such passes do not necessarily update the metadata, but eventually that should be fixed. Until then, if the new metadata has no value, `llvm::getLoopEstimatedTripCount` disregards it and tries again to estimate the trip count from the loop's `branch_weights` metadata.
Somehow, on some of my builds, `llvm::` prefixes are dropped from some symbol names in the printed past list.
For estimated trip count metdata.
Suggested at <#148758 (comment)>.
jdenny-ornl
requested review from
MatzeB,
arsenm,
nikic,
fhahn,
mtrofin and
jdoerfert
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This resurrects PR #148758. I'll address previous issues shortly. |
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✅ With the latest revision this PR passed the C/C++ code formatter. |
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I believe I have addressed most issues raised in PR #148758. For now, this PR shows what the pass pipeline tests (e.g., I have not addressed the general issue raised there about whether pgo-estimated-trip-counts is actually worthwhile and whether we should constrain it or disable it by default. What do people think now after seeing the above changes? |
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This patch implements the
llvm.loop.estimated_trip_countmetadata discussed in [RFC] Fix Loop Transformations to Preserve Block Frequencies. As suggested in the RFC comments, it adds the new metadata to all loops at the time of profile ingestion and estimates each trip count from the loop'sbranch_weightsmetadata. As suggested in the PR #128785 review, it does so via a newPGOEstimateTripCountsPasspass, which creates the new metadata for each loop but omits the value if it cannot estimate a trip count due to the loop's form.An important observation is that
PGOEstimateTripCountsPassoften cannot estimate a loop's trip count, but later passes can sometimes transform the loop in a way that makes it possible. Currently, such passes do not necessarily update the metadata, but eventually that should be fixed. Until then, if the new metadata has no value,llvm::getLoopEstimatedTripCountdisregards it and tries again to estimate the trip count from the loop's currentbranch_weightsmetadata.