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[SimplifyCFG] Fix weight calculation for simplifySwitchOfPowersOfTwo #165956

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[SimplifyCFG] Fix weight calculation for simplifySwitchOfPowersOfTwo #165956

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17 changes: 11 additions & 6 deletions llvm/lib/Transforms/Utils/SimplifyCFG.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7648,13 +7648,18 @@ static bool simplifySwitchOfPowersOfTwo(SwitchInst *SI, IRBuilder<> &Builder,
NewWeights[1] = Weights[0] / 2;
NewWeights[0] = OrigDenominator - NewWeights[1];
setFittedBranchWeights(*BI, NewWeights, /*IsExpected=*/false);

// For the original switch, we reduce the weight of the default by the
// amount by which the previous branch contributes to getting to default,
// and then make sure the remaining weights have the same relative ratio
// wrt eachother.
// The probability of executing the default block stays constant. It was
// p_d = Weights[0] / OrigDenominator
// we rewrite as W/D
// We want to find the probability of the default branch of the switch
// statement. Let's call it X. We have W/D = W/2D + X * (1-W/2D)
// i.e. the original probability is the probability we go to the default
// branch from the BI branch, or we take the default branch on the SI.
// Meaning X = W / (2D - W), or (W/2) / (D - W/2)
// This matches using W/2 for the default branch probability numerator and
// D-W/2 as the denominator.
Weights[0] = NewWeights[1];
uint64_t CasesDenominator = OrigDenominator - Weights[0];
Weights[0] /= 2;
for (auto &W : drop_begin(Weights))
W = NewWeights[0] * static_cast<double>(W) / CasesDenominator;

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2 changes: 1 addition & 1 deletion llvm/test/Transforms/SimplifyCFG/X86/switch-of-powers-of-two.ll
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Expand Up @@ -141,5 +141,5 @@ return:
;.
; CHECK: [[PROF0]] = !{!"function_entry_count", i32 10}
; CHECK: [[PROF1]] = !{!"branch_weights", i32 58, i32 5}
; CHECK: [[PROF2]] = !{!"branch_weights", i32 56, i32 5}
; CHECK: [[PROF2]] = !{!"branch_weights", i32 53, i32 5}
;.
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