llvm-project

f7b65011 - [PGO] Add `llvm.loop.estimated_trip_count` metadata (#148758)

[PGO] Add `llvm.loop.estimated_trip_count` metadata (#148758) 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](https://github.com/llvm/llvm-project/pull/128785#discussion_r2151091036), it does so via a new `PGOEstimateTripCountsPass` pass, 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 not previously discussed is that `PGOEstimateTripCountsPass` *often* 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::getLoopEstimatedTripCount` disregards it and tries again to estimate the trip count from the loop's current `branch_weights` metadata.