[LoopPeel] Fix branch weights' effect on block frequencies #128785
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✅ With the latest revision this PR passed the C/C++ code formatter. |
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For example: ``` declare void @f(i32) define void @test(i32 %n) { entry: br label %do.body do.body: %i = phi i32 [ 0, %entry ], [ %inc, %do.body ] %inc = add i32 %i, 1 call void @f(i32 %i) %c = icmp sge i32 %inc, %n br i1 %c, label %do.end, label %do.body, !prof !0 do.end: ret void } !0 = !{!"branch_weights", i32 1, i32 9} ``` Given those branch weights, once any loop iteration is actually reached, the probability of the loop exiting at the iteration's end is 1/(1+9). That is, the loop is likely to exit every 10 iterations and thus has an estimated trip count of 10. `opt -passes='print<block-freq>'` shows that 10 is indeed the frequency of the loop body: ``` Printing analysis results of BFI for function 'test': block-frequency-info: test - entry: float = 1.0, int = 1801439852625920 - do.body: float = 10.0, int = 18014398509481984 - do.end: float = 1.0, int = 1801439852625920 ``` Key Observation: The frequency of reaching any particular iteration is less than for the previous iteration because the previous iteration has a non-zero probability of exiting the loop. This observation holds even though every loop iteration, once actually reached, has exactly the same probability of exiting and thus exactly the same branch weights. Now we use `opt -unroll-force-peel-count=2 -passes=loop-unroll` to peel 2 iterations and insert them before the remaining loop. We expect the key observation above not to change, but it does under the implementation without this patch. The block frequency becomes 1.0 for the first iteration, 0.9 for the second, and 6.4 for the main loop body. Again, a decreasing frequency is expected, but it decreases too much: the total frequency of the original loop body becomes 8.3. The new branch weights reveal the problem: ``` !0 = !{!"branch_weights", i32 1, i32 9} !1 = !{!"branch_weights", i32 1, i32 8} !2 = !{!"branch_weights", i32 1, i32 7} ``` The exit probability is now 1/10 for the first peeled iteration, 1/9 for the second, and 1/8 for the remaining loop iterations. It seems this behavior is trying to ensure a decreasing block frequency. However, as in the key observation above for the original loop, that happens correctly without decreasing the branch weights across iterations. This patch changes the peeling implementation not to decrease the branch weights across loop iterations so that the frequency for every iteration is the same as it was in the original loop. The total frequency of the loop body, summed across all its occurrences, thus remains 10 after peeling. Unfortunately, that change means a later analysis cannot accurately estimate the trip count of the remaining loop while examining the remaining loop in isolation without considering the probability of actually reaching it. For that purpose, this patch stores the new trip count as separate metadata named `llvm.loop.estimated_trip_count` and extends `llvm::getLoopEstimatedTripCount` to prefer it, if present, over branch weights. An alternative fix is for `llvm::getLoopEstimatedTripCount` to subtract the `llvm.loop.peeled.count` metadata from the trip count estimated by a loop's branch weights. However, there might be other loop transformations that still corrupt block frequencies in a similar manner and require a similar fix. `llvm.loop.estimated_trip_count` is intended to provide a general way to store estimated trip counts when branch weights cannot directly store them. This patch introduces several fixme comments that need to be addressed before it can land.
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arsenm
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Apr 30, 2025
arsenm
reviewed
Apr 30, 2025
Extending beyond the limitations of `getExpectedExitLoopLatchBranch` is a possible improvement for the future not an urgent fixme. No one has pointed out code that computes estimated trip counts without using `llvm::getLoopEstimatedTripCount`.
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judging from reactiong on the RFC it seems others fine with the new metadata. Then no objections from my side. |
That's right.
Existing uses of the estimated trip count are listed under point 1 of this RFC comment. As discussed there and afterward, it's not clear to me what to do with estimated trip counts right now, so I think that needs a separate RFC. For now, the new metadata just gets it out of our way for fixing block frequencies. |
mtrofin
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Jun 16, 2025
The update adds this PR's new metadata.
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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.
That's PR llvm#128785, which is now a parent PR. First, remove a todo that's now documented more generally than LoopPeel in plenty of other places. Second, update LoopPeel's setLoopEstimatedTripCount call to avoid a now redundant argument that eventually won't be supported.
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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 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.
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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](llvm/llvm-project#128785 (comment)), 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.
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[LoopPeel] Fix branch weights' effect on block frequencies
This patch implements the LoopPeel changes discussed in [RFC] Fix Loop Transformations to Preserve Block Frequencies.
In summary, a loop's latch block can have branch weight metadata that encodes an estimated trip count that is derived from application profile data. Initially, the loop body's block frequencies agree with the estimated trip count, as expected. However, sometimes loop transformations adjust those branch weights in a way that correctly maintains the estimated trip count but that corrupts the block frequencies. This patch addresses that problem in LoopPeel, which it changes to:
llvm.loop.estimated_trip_countmetadata, introduced by PR [PGO] Addllvm.loop.estimated_trip_countmetadata #148758.This patch introduces a fixme comment in LoopPeel.cpp that should be discussed before it lands.