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[SimplifyCFG] Don't speculatively execute BB if it's predictably not taken

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If the branch isn't `unpredictable`, and it is predicted to *not* branch
to the block we are considering speculatively executing,
then it seems counter-productive to execute the code that is predicted not to be executed.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D106650
This commit is contained in:
Roman Lebedev 2021-07-26 02:30:31 +03:00
parent ddc2eed16c
commit cc6967667d
2 changed files with 17 additions and 2 deletions
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14
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -2372,6 +2372,20 @@ bool SimplifyCFGOpt::SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
}
assert(EndBB == BI->getSuccessor(!Invert) && "No edge from to end block");
// If the branch is non-unpredictable, and is predicted to *not* branch to
// the `then` block, then avoid speculating it.
if (!BI->getMetadata(LLVMContext::MD_unpredictable)) {
uint64_t TWeight, FWeight;
if (BI->extractProfMetadata(TWeight, FWeight) && (TWeight + FWeight) != 0) {
uint64_t EndWeight = Invert ? TWeight : FWeight;
BranchProbability BIEndProb =
BranchProbability::getBranchProbability(EndWeight, TWeight + FWeight);
BranchProbability Likely = TTI.getPredictableBranchThreshold();
if (BIEndProb >= Likely)
return false;
}
}
// Keep a count of how many times instructions are used within ThenBB when
// they are candidates for sinking into ThenBB. Specifically:
// - They are defined in BB, and

5
test/Transforms/SimplifyCFG/speculatively-execute-block-profmd.ll
View File

@ -83,11 +83,12 @@ define i32 @predictably_nontaken(i1 %c, i32 %a, i32 %b) {
; CHECK: dispatch:
; CHECK-NEXT: call void @sideeffect1()
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: br i1 [[CMP]], label [[END]], label [[COND_TRUE:%.*]], !prof [[PROF0]]
; CHECK: cond.true:
; CHECK-NEXT: [[VAL:%.*]] = add i32 [[A]], [[B]]
; CHECK-NEXT: [[SPEC_SELECT:%.*]] = select i1 [[CMP]], i32 0, i32 [[VAL]], !prof [[PROF0]]
; CHECK-NEXT: br label [[END]]
; CHECK: end:
; CHECK-NEXT: [[RES:%.*]] = phi i32 [ -1, [[ENTRY:%.*]] ], [ [[SPEC_SELECT]], [[DISPATCH]] ]
; CHECK-NEXT: [[RES:%.*]] = phi i32 [ -1, [[ENTRY:%.*]] ], [ 0, [[DISPATCH]] ], [ [[VAL]], [[COND_TRUE]] ]
; CHECK-NEXT: call void @sideeffect2()
; CHECK-NEXT: ret i32 [[RES]]
;