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Check for all known bits on ret in InstCombine

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From a combination of @llvm.assume calls (and perhaps through other means, such
as range metadata), it is possible that all bits of a return value might be
known. Previously, InstCombine did not check for this (which is understandable
given assumptions of constant propagation), but means that we'd miss simple
cases where assumptions are involved.

llvm-svn: 217346
This commit is contained in:
Hal Finkel 2014-09-07 21:28:34 +00:00
parent 9ede5a39dd
commit 5d195fd587
3 changed files with 31 additions and 0 deletions
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1
lib/Transforms/InstCombine/InstCombine.h
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@ -232,6 +232,7 @@ public:
Instruction *visitStoreInst(StoreInst &SI);
Instruction *visitBranchInst(BranchInst &BI);
Instruction *visitSwitchInst(SwitchInst &SI);
Instruction *visitReturnInst(ReturnInst &RI);
Instruction *visitInsertValueInst(InsertValueInst &IV);
Instruction *visitInsertElementInst(InsertElementInst &IE);
Instruction *visitExtractElementInst(ExtractElementInst &EI);

18
lib/Transforms/InstCombine/InstructionCombining.cpp
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@ -2004,7 +2004,25 @@ Instruction *InstCombiner::visitFree(CallInst &FI) {
return nullptr;
}
Instruction *InstCombiner::visitReturnInst(ReturnInst &RI) {
if (RI.getNumOperands() == 0) // ret void
return nullptr;
Value *ResultOp = RI.getOperand(0);
Type *VTy = ResultOp->getType();
if (!VTy->isIntegerTy())
return nullptr;
// There might be assume intrinsics dominating this return that completely
// determine the value. If so, constant fold it.
unsigned BitWidth = VTy->getPrimitiveSizeInBits();
APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
computeKnownBits(ResultOp, KnownZero, KnownOne, 0, &RI);
if ((KnownZero|KnownOne).isAllOnesValue())
RI.setOperand(0, Constant::getIntegerValue(VTy, KnownOne));
return nullptr;
}
Instruction *InstCombiner::visitBranchInst(BranchInst &BI) {
// Change br (not X), label True, label False to: br X, label False, True

12
test/Transforms/InstCombine/assume.ll
View File

@ -43,6 +43,18 @@ entry:
; Function Attrs: nounwind
declare void @llvm.assume(i1) #1
define i32 @simple(i32 %a) #1 {
entry:
; CHECK-LABEL: @simple
; CHECK: call void @llvm.assume
; CHECK: ret i32 4
%cmp = icmp eq i32 %a, 4
tail call void @llvm.assume(i1 %cmp)
ret i32 %a
}
; Function Attrs: nounwind uwtable
define i32 @can1(i1 %a, i1 %b, i1 %c) {
entry: