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[LoopRotate] Set loop back edge weight to not less than exit weight #86496

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Merged
merged 4 commits into from
Mar 28, 2024
Merged

[LoopRotate] Set loop back edge weight to not less than exit weight #86496

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31d4ce6
[LoopRotate] Add test to track update for inaccurate branch weight
HaohaiWen Mar 25, 2024
2b3c4a2
[LoopRotate] Set loop back edge weight to not less than exit weight
HaohaiWen Mar 25, 2024
db9a078
Address comment
HaohaiWen Mar 28, 2024
a146e92
Merge remote-tracking branch 'origin/main' into looprotate
HaohaiWen Mar 28, 2024
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10 changes: 10 additions & 0 deletions llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -347,9 +347,19 @@ static void updateBranchWeights(BranchInst &PreHeaderBI, BranchInst &LoopBI,
// probabilities as if there are only 0-trip and 1-trip cases.
ExitWeight0 = OrigLoopExitWeight - OrigLoopBackedgeWeight;
}
} else {
// Theoretically, if the loop body must be executed at least once, the
// backedge count must be not less than exit count. However the branch
// weight collected by sampling-based PGO may be not very accurate due to
// sampling. Therefore this workaround is required here to avoid underflow
// of unsigned in following update of branch weight.
if (OrigLoopExitWeight > OrigLoopBackedgeWeight)
OrigLoopBackedgeWeight = OrigLoopExitWeight;
}
assert(OrigLoopExitWeight >= ExitWeight0 && "Bad branch weight");
ExitWeight1 = OrigLoopExitWeight - ExitWeight0;
EnterWeight = ExitWeight1;
assert(OrigLoopBackedgeWeight >= EnterWeight && "Bad branch weight");
LoopBackWeight = OrigLoopBackedgeWeight - EnterWeight;
} else if (OrigLoopExitWeight == 0) {
if (OrigLoopBackedgeWeight == 0) {
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4 changes: 2 additions & 2 deletions llvm/test/Transforms/LoopRotate/update-branch-weights.ll
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Expand Up @@ -240,7 +240,7 @@ loop_exit:

; BFI_AFTER-LABEL: block-frequency-info: func6_inaccurate_branch_weight
; BFI_AFTER: - entry: {{.*}} count = 1024
; BFI_AFTER: - loop_body: {{.*}} count = 4294967296
; BFI_AFTER: - loop_body: {{.*}} count = 1024
; BFI_AFTER: - loop_exit: {{.*}} count = 1024

; IR-LABEL: define void @func6_inaccurate_branch_weight(
Expand Down Expand Up @@ -292,4 +292,4 @@ loop_exit:
; IR: [[PROF_FUNC3_0]] = !{!"branch_weights", i32 0, i32 1}
; IR: [[PROF_FUNC4_0]] = !{!"branch_weights", i32 1, i32 0}
; IR: [[PROF_FUNC5_0]] = !{!"branch_weights", i32 0, i32 0}
; IR: [[PROF_FUNC6_0]] = !{!"branch_weights", i32 -1, i32 1024}
; IR: [[PROF_FUNC6_0]] = !{!"branch_weights", i32 0, i32 1024}