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Add a wrapper function to set branch weights metadata. 

Summary:
This wrapper checks if there is at least one non-zero weight before
setting the metadata.

Reviewers: davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39872

llvm-svn: 317845
This commit is contained in:
Easwaran Raman 2017-11-09 22:52:20 +00:00
parent 8e92025718
commit 0a0913def2
2 changed files with 73 additions and 28 deletions
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62
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -776,6 +776,31 @@ static bool ValuesOverlap(std::vector<ValueEqualityComparisonCase> &C1,
return false;
}
// Set branch weights on SwitchInst. This sets the metadata if there is at
// least one non-zero weight.
static void setBranchWeights(SwitchInst *SI, ArrayRef<uint32_t> Weights) {
// Check that there is at least one non-zero weight. Otherwise, pass
// nullptr to setMetadata which will erase the existing metadata.
MDNode *N = nullptr;
if (llvm::any_of(Weights, [](uint32_t W) { return W != 0; }))
N = MDBuilder(SI->getParent()->getContext()).createBranchWeights(Weights);
SI->setMetadata(LLVMContext::MD_prof, N);
}
// Similar to the above, but for branch and select instructions that take
// exactly 2 weights.
static void setBranchWeights(Instruction *I, uint32_t TrueWeight,
uint32_t FalseWeight) {
assert(isa<BranchInst>(I) || isa<SelectInst>(I));
// Check that there is at least one non-zero weight. Otherwise, pass
// nullptr to setMetadata which will erase the existing metadata.
MDNode *N = nullptr;
if (TrueWeight || FalseWeight)
N = MDBuilder(I->getParent()->getContext())
.createBranchWeights(TrueWeight, FalseWeight);
I->setMetadata(LLVMContext::MD_prof, N);
}
/// If TI is known to be a terminator instruction and its block is known to
/// only have a single predecessor block, check to see if that predecessor is
/// also a value comparison with the same value, and if that comparison
@ -865,9 +890,7 @@ bool SimplifyCFGOpt::SimplifyEqualityComparisonWithOnlyPredecessor(
}
}
if (HasWeight && Weights.size() >= 2)
SI->setMetadata(LLVMContext::MD_prof,
MDBuilder(SI->getParent()->getContext())
.createBranchWeights(Weights));
setBranchWeights(SI, Weights);
DEBUG(dbgs() << "Leaving: " << *TI << "\n");
return true;
@ -1172,9 +1195,7 @@ bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(TerminatorInst *TI,
SmallVector<uint32_t, 8> MDWeights(Weights.begin(), Weights.end());
NewSI->setMetadata(
LLVMContext::MD_prof,
MDBuilder(BB->getContext()).createBranchWeights(MDWeights));
setBranchWeights(NewSI, MDWeights);
}
EraseTerminatorInstAndDCECond(PTI);
@ -2738,9 +2759,7 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, unsigned BonusInstThreshold) {
SmallVector<uint32_t, 8> MDWeights(NewWeights.begin(),
NewWeights.end());
PBI->setMetadata(
LLVMContext::MD_prof,
MDBuilder(BI->getContext()).createBranchWeights(MDWeights));
setBranchWeights(PBI, MDWeights[0], MDWeights[1]);
} else
PBI->setMetadata(LLVMContext::MD_prof, nullptr);
} else {
@ -3309,9 +3328,7 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
// Halve the weights if any of them cannot fit in an uint32_t
FitWeights(NewWeights);
PBI->setMetadata(LLVMContext::MD_prof,
MDBuilder(BI->getContext())
.createBranchWeights(NewWeights[0], NewWeights[1]));
setBranchWeights(PBI, NewWeights[0], NewWeights[1]);
}
// OtherDest may have phi nodes. If so, add an entry from PBI's
@ -3349,9 +3366,7 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
FitWeights(NewWeights);
NV->setMetadata(LLVMContext::MD_prof,
MDBuilder(BI->getContext())
.createBranchWeights(NewWeights[0], NewWeights[1]));
setBranchWeights(NV, NewWeights[0], NewWeights[1]);
}
}
}
@ -3406,9 +3421,7 @@ static bool SimplifyTerminatorOnSelect(TerminatorInst *OldTerm, Value *Cond,
// Create a conditional branch sharing the condition of the select.
BranchInst *NewBI = Builder.CreateCondBr(Cond, TrueBB, FalseBB);
if (TrueWeight != FalseWeight)
NewBI->setMetadata(LLVMContext::MD_prof,
MDBuilder(OldTerm->getContext())
.createBranchWeights(TrueWeight, FalseWeight));
setBranchWeights(NewBI, TrueWeight, FalseWeight);
}
} else if (KeepEdge1 && (KeepEdge2 || TrueBB == FalseBB)) {
// Neither of the selected blocks were successors, so this
@ -3594,9 +3607,7 @@ static bool tryToSimplifyUncondBranchWithICmpInIt(
Weights.push_back(Weights[0]);
SmallVector<uint32_t, 8> MDWeights(Weights.begin(), Weights.end());
SI->setMetadata(
LLVMContext::MD_prof,
MDBuilder(SI->getContext()).createBranchWeights(MDWeights));
setBranchWeights(SI, MDWeights);
}
}
SI->addCase(Cst, NewBB);
@ -4323,10 +4334,7 @@ static bool TurnSwitchRangeIntoICmp(SwitchInst *SI, IRBuilder<> &Builder) {
TrueWeight /= 2;
FalseWeight /= 2;
}
NewBI->setMetadata(LLVMContext::MD_prof,
MDBuilder(SI->getContext())
.createBranchWeights((uint32_t)TrueWeight,
(uint32_t)FalseWeight));
setBranchWeights(NewBI, TrueWeight, FalseWeight);
}
}
@ -4423,9 +4431,7 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, AssumptionCache *AC,
}
if (HasWeight && Weights.size() >= 2) {
SmallVector<uint32_t, 8> MDWeights(Weights.begin(), Weights.end());
SI->setMetadata(LLVMContext::MD_prof,
MDBuilder(SI->getParent()->getContext())
.createBranchWeights(MDWeights));
setBranchWeights(SI, MDWeights);
}
return !DeadCases.empty();

39
llvm/test/Transforms/SimplifyCFG/suppress-zero-branch-weights.ll Normal file
View File

@ -0,0 +1,39 @@
; RUN: opt -S -simplifycfg < %s | FileCheck %s
; We're sign extending an 8-bit value.
; The switch condition must be in the range [-128, 127], so any cases outside of that range must be dead.
; Only the first case has a non-zero weight, but that gets eliminated. Note
; that this shouldn't have been the case in the first place, but the test here
; ensures that all-zero branch weights are not attached causing problems downstream.
define i1 @repeated_signbits(i8 %condition) {
; CHECK-LABEL: @repeated_signbits(
; CHECK: switch i32
; CHECK-DAG: i32 -128, label %a
; CHECK-DAG: i32 -1, label %a
; CHECK-DAG: i32 0, label %a
; CHECK-DAG: i32 127, label %a
; CHECK-NEXT: ]
;
entry:
%sext = sext i8 %condition to i32
switch i32 %sext, label %default [
i32 -2147483648, label %a
i32 -129, label %a
i32 -128, label %a
i32 -1, label %a
i32 0, label %a
i32 127, label %a
i32 128, label %a
i32 2147483647, label %a
], !prof !1
a:
ret i1 1
default:
ret i1 0
}
!1 = !{!"branch_weights", i32 1, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0}