https://github.com/aleks-tmb updated https://github.com/llvm/llvm-project/pull/70094
>From 4265cc4a05d939a29e57a116e696fa1eb032c249 Mon Sep 17 00:00:00 2001 From: Aleksandr Popov <apo...@azul.com> Date: Tue, 24 Oct 2023 16:47:50 +0000 Subject: [PATCH] [LoopPeeling] Fix weights updating of peeled off branches In https://reviews.llvm.org/D64235 a new algorithm has been introduced for updating the branch weights of latch blocks and their copies. It increases the probability of going to the exit block for each next peel iteration, calculating weights by (F - I * E, E), where: - F is a weight of the edge from latch to header. - E is a weight of the edge from latch to exit. - I is a number of peeling iteration. E.g: Let's say the latch branch weights are (100,300) and the estimated trip count is 4. If we peel off all 4 iterations the weights of the copied branches will be: 0: (100,300) 1: (100,200) 2: (100,100) 3: (100,1) https://godbolt.org/z/93KnoEsT6 So we make the original loop almost unreachable from the 3rd peeled copy according to the profile data. But that's only true if the profiling data is accurate. Underestimated trip count can lead to a performance issues with the register allocator, which may decide to spill intervals inside the loop assuming it's unreachable. Since we don't know how accurate the profiling data is, it seems better to set neutral 1/1 weights on the last peeled latch branch. After this change, the weights in the example above will look like this: 0: (100,300) 1: (100,200) 2: (100,100) 3: (100,100) --- llvm/lib/Transforms/Utils/LoopPeel.cpp | 10 +++++++--- llvm/test/Transforms/LoopUnroll/peel-loop-pgo-deopt.ll | 5 ++--- llvm/test/Transforms/LoopUnroll/peel-loop-pgo.ll | 5 ++--- 3 files changed, 11 insertions(+), 9 deletions(-) diff --git a/llvm/lib/Transforms/Utils/LoopPeel.cpp b/llvm/lib/Transforms/Utils/LoopPeel.cpp index 31f065b691f864c..30c525dd82bbd97 100644 --- a/llvm/lib/Transforms/Utils/LoopPeel.cpp +++ b/llvm/lib/Transforms/Utils/LoopPeel.cpp @@ -636,9 +636,13 @@ static void updateBranchWeights(Instruction *Term, WeightInfo &Info) { MDB.createBranchWeights(Info.Weights)); for (auto [Idx, SubWeight] : enumerate(Info.SubWeights)) if (SubWeight != 0) - Info.Weights[Idx] = Info.Weights[Idx] > SubWeight - ? Info.Weights[Idx] - SubWeight - : 1; + // Don't set the probability of taking the edge from latch to loop header + // to less than 1, as this could significantly reduce the loop's hotness, + // which would be incorrect in the case of underestimating the trip count. + Info.Weights[Idx] = + Info.Weights[Idx] > SubWeight + ? std::max(Info.Weights[Idx] - SubWeight, SubWeight) + : SubWeight; } /// Initialize the weights for all exiting blocks. diff --git a/llvm/test/Transforms/LoopUnroll/peel-loop-pgo-deopt.ll b/llvm/test/Transforms/LoopUnroll/peel-loop-pgo-deopt.ll index a0dc216133fb7d5..d91cb5bab382782 100644 --- a/llvm/test/Transforms/LoopUnroll/peel-loop-pgo-deopt.ll +++ b/llvm/test/Transforms/LoopUnroll/peel-loop-pgo-deopt.ll @@ -21,7 +21,7 @@ ; CHECK: br i1 %{{.*}}, label %[[NEXT2:.*]], label %for.cond.for.end_crit_edge, !prof !18 ; CHECK: [[NEXT2]]: ; CHECK: br i1 %c, label %{{.*}}, label %side_exit.loopexit, !prof !15 -; CHECK: br i1 %{{.*}}, label %for.body, label %{{.*}}, !prof !19 +; CHECK: br i1 %{{.*}}, label %for.body, label %{{.*}}, !prof !18 define i32 @basic(ptr %p, i32 %k, i1 %c) #0 !prof !15 { entry: @@ -85,6 +85,5 @@ attributes #1 = { nounwind optsize } ; This is a weights of latch and its copies. ;CHECK: !16 = !{!"branch_weights", i32 3001, i32 1001} ;CHECK: !17 = !{!"branch_weights", i32 2000, i32 1001} -;CHECK: !18 = !{!"branch_weights", i32 999, i32 1001} -;CHECK: !19 = !{!"branch_weights", i32 1, i32 1001} +;CHECK: !18 = !{!"branch_weights", i32 1001, i32 1001} diff --git a/llvm/test/Transforms/LoopUnroll/peel-loop-pgo.ll b/llvm/test/Transforms/LoopUnroll/peel-loop-pgo.ll index cadb6739dbc3fbb..15dce234baee91d 100644 --- a/llvm/test/Transforms/LoopUnroll/peel-loop-pgo.ll +++ b/llvm/test/Transforms/LoopUnroll/peel-loop-pgo.ll @@ -24,7 +24,7 @@ ; CHECK: [[NEXT1]]: ; CHECK: br i1 %{{.*}}, label %[[NEXT2:.*]], label %for.cond.for.end_crit_edge, !prof !17 ; CHECK: [[NEXT2]]: -; CHECK: br i1 %{{.*}}, label %for.body, label %{{.*}}, !prof !18 +; CHECK: br i1 %{{.*}}, label %for.body, label %{{.*}}, !prof !17 define void @basic(ptr %p, i32 %k) #0 !prof !15 { entry: @@ -105,6 +105,5 @@ attributes #1 = { nounwind optsize } ;CHECK: !15 = !{!"branch_weights", i32 3001, i32 1001} ;CHECK: !16 = !{!"branch_weights", i32 2000, i32 1001} -;CHECK: !17 = !{!"branch_weights", i32 999, i32 1001} -;CHECK: !18 = !{!"branch_weights", i32 1, i32 1001} +;CHECK: !17 = !{!"branch_weights", i32 1001, i32 1001} _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
