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fix a bug noticed by inspection: when instcombine sinks loads through

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phis, it didn't preserve the alignment of the load.  This is a missed
optimization of the alignment is high and a miscompilation when the
alignment is low.

llvm-svn: 85736
This commit is contained in:
Chris Lattner 2009-11-01 19:50:13 +00:00
parent 2db49d0a22
commit b3fe1009a5
2 changed files with 45 additions and 4 deletions
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28
lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -10744,7 +10744,15 @@ Instruction *InstCombiner::FoldPHIArgOpIntoPHI(PHINode &PN) {
// code size and simplifying code.
Constant *ConstantOp = 0;
const Type *CastSrcTy = 0;
// When processing loads, we need to propagate two bits of information to the
// sunk load: whether it is volatile, and what its alignment is. We currently
// don't sink loads when some have their alignment specified and some don't.
// visitLoadInst will propagate an alignment onto the load when TD is around,
// and if TD isn't around, we can't handle the mixed case.
bool isVolatile = false;
unsigned LoadAlignment = 0;
if (isa<CastInst>(FirstInst)) {
CastSrcTy = FirstInst->getOperand(0)->getType();
} else if (isa<BinaryOperator>(FirstInst) || isa<CmpInst>(FirstInst)) {
@ -10754,7 +10762,10 @@ Instruction *InstCombiner::FoldPHIArgOpIntoPHI(PHINode &PN) {
if (ConstantOp == 0)
return FoldPHIArgBinOpIntoPHI(PN);
} else if (LoadInst *LI = dyn_cast<LoadInst>(FirstInst)) {
isVolatile = LI->isVolatile();
LoadAlignment = LI->getAlignment();
// We can't sink the load if the loaded value could be modified between the
// load and the PHI.
if (LI->getParent() != PN.getIncomingBlock(0) ||
@ -10791,6 +10802,13 @@ Instruction *InstCombiner::FoldPHIArgOpIntoPHI(PHINode &PN) {
!isSafeAndProfitableToSinkLoad(LI))
return 0;
// If some of the loads have an alignment specified but not all of them,
// we can't do the transformation.
if ((LoadAlignment != 0) != (LI->getAlignment() != 0))
return 0;
LoadAlignment = std::max(LoadAlignment, LI->getAlignment());
// If the PHI is of volatile loads and the load block has multiple
// successors, sinking it would remove a load of the volatile value from
// the path through the other successor.
@ -10832,10 +10850,12 @@ Instruction *InstCombiner::FoldPHIArgOpIntoPHI(PHINode &PN) {
}
// Insert and return the new operation.
if (CastInst* FirstCI = dyn_cast<CastInst>(FirstInst))
if (CastInst *FirstCI = dyn_cast<CastInst>(FirstInst))
return CastInst::Create(FirstCI->getOpcode(), PhiVal, PN.getType());
if (BinaryOperator *BinOp = dyn_cast<BinaryOperator>(FirstInst))
return BinaryOperator::Create(BinOp->getOpcode(), PhiVal, ConstantOp);
if (CmpInst *CIOp = dyn_cast<CmpInst>(FirstInst))
return CmpInst::Create(CIOp->getOpcode(), CIOp->getPredicate(),
PhiVal, ConstantOp);
@ -10847,8 +10867,8 @@ Instruction *InstCombiner::FoldPHIArgOpIntoPHI(PHINode &PN) {
if (isVolatile)
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
cast<LoadInst>(PN.getIncomingValue(i))->setVolatile(false);
return new LoadInst(PhiVal, "", isVolatile);
return new LoadInst(PhiVal, "", isVolatile, LoadAlignment);
}
/// DeadPHICycle - Return true if this PHI node is only used by a PHI node cycle
@ -11304,7 +11324,7 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
}
Instruction *InstCombiner::visitAllocaInst(AllocaInst &AI) {
// Convert: malloc Ty, C - where C is a constant != 1 into: malloc [C x Ty], 1
// Convert: alloca Ty, C - where C is a constant != 1 into: alloca [C x Ty], 1
if (AI.isArrayAllocation()) { // Check C != 1
if (const ConstantInt *C = dyn_cast<ConstantInt>(AI.getArraySize())) {
const Type *NewTy =

21
test/Transforms/InstCombine/phi.ll
View File

@ -141,4 +141,25 @@ BB2:
; CHECK-NEXT: ret i32* %B
}
define i32 @test9(i32* %A, i32* %B) {
entry:
%c = icmp eq i32* %A, null
br i1 %c, label %bb1, label %bb
bb:
%C = load i32* %B, align 1
br label %bb2
bb1:
%D = load i32* %A, align 1
br label %bb2
bb2:
%E = phi i32 [ %C, %bb ], [ %D, %bb1 ]
ret i32 %E
; CHECK: bb2:
; CHECK-NEXT: phi i32* [ %B, %bb ], [ %A, %bb1 ]
; CHECK-NEXT: %E = load i32* %{{[^,]*}}, align 1
; CHECK-NEXT: ret i32 %E
}