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[LoopUnroll] Fix assert fail on zeroed branch weights #165938

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Merged
merged 1 commit into from
Nov 3, 2025
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[LoopUnroll] Fix assert fail on zeroed branch weights #165938

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5 changes: 4 additions & 1 deletion llvm/lib/Transforms/Utils/LoopUtils.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -992,9 +992,12 @@ BranchProbability llvm::getBranchProbability(BranchInst *B,
uint64_t Weight0, Weight1;
if (!extractBranchWeights(*B, Weight0, Weight1))
return BranchProbability::getUnknown();
uint64_t Denominator = Weight0 + Weight1;
if (Denominator == 0)
return BranchProbability::getUnknown();
if (!ForFirstTarget)
std::swap(Weight0, Weight1);
return BranchProbability::getBranchProbability(Weight0, Weight0 + Weight1);
return BranchProbability::getBranchProbability(Weight0, Denominator);
}

bool llvm::setBranchProbability(BranchInst *B, BranchProbability P,
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30 changes: 30 additions & 0 deletions llvm/test/Transforms/LoopUnroll/zeroed-branch-weights.ll
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@@ -0,0 +1,30 @@
; Check that zeroed branch weights do not crash or otherwise break basic
; LoopUnroll behavior when it tries to compute a probability from them.

; RUN: opt < %s -S -unroll-count=2 -passes='loop-unroll' 2>&1 | FileCheck %s

define void @test() {
entry:
br label %loop

loop:
br i1 false, label %end, label %loop, !prof !0

end:
ret void
}

!0 = !{!"branch_weights", i32 0, i32 0}

; CHECK: define void @test() {
; CHECK: entry:
; CHECK: br label %loop
; CHECK: loop:
; CHECK: br i1 false, label %end, label %loop.1, !prof !0
; CHECK: loop.1:
; CHECK: br i1 false, label %end, label %loop, !prof !0, !llvm.loop !1
; CHECK-NOT: loop.2
; CHECK: end:
; CHECK: ret void
; CHECK: }
; CHECK: !0 = !{!"branch_weights", i32 0, i32 0}
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@wlei-llvm wlei-llvm Nov 3, 2025

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I guess we alternatively can avoid to create all zero branch_weights metadata by setting the ElideAllZero parameter in setBranchWeights , in this case we can save some metadata memory.