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InstSimplify: Fold equality comparisons between non-inbounds GEPs.

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Overflow doesn't affect the correctness of equalities. Computing this is cheap,
we just reuse the computation for the inbounds case and try to peel of more
non-inbounds GEPs. This pattern is unlikely to ever appear in code generated by
Clang, but SCEV occasionally produces it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191200 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer 2013-09-23 14:16:38 +00:00
parent 9a1aaeb012
commit baca5334db
2 changed files with 24 additions and 2 deletions
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17
lib/Analysis/InstructionSimplify.cpp
View File

@ -668,7 +668,8 @@ Value *llvm::SimplifyAddInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
/// follow non-inbounds geps. This allows it to remain usable for icmp ult/etc.
/// folding.
static Constant *stripAndComputeConstantOffsets(const DataLayout *TD,
Value *&V) {
Value *&V,
bool AllowNonInbounds = false) {
assert(V->getType()->getScalarType()->isPointerTy());
// Without DataLayout, just be conservative for now. Theoretically, more could
@ -685,7 +686,8 @@ static Constant *stripAndComputeConstantOffsets(const DataLayout *TD,
Visited.insert(V);
do {
if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
if (!GEP->isInBounds() || !GEP->accumulateConstantOffset(*TD, Offset))
if ((!AllowNonInbounds && !GEP->isInBounds()) ||
!GEP->accumulateConstantOffset(*TD, Offset))
break;
V = GEP->getPointerOperand();
} else if (Operator::getOpcode(V) == Instruction::BitCast) {
@ -1837,6 +1839,17 @@ static Constant *computePointerICmp(const DataLayout *TD,
return ConstantInt::get(GetCompareTy(LHS),
!CmpInst::isTrueWhenEqual(Pred));
}
// Even if an non-inbounds GEP occurs along the path we can still optimize
// equality comparisons concerning the result. We avoid walking the whole
// chain again by starting where the last calls to
// stripAndComputeConstantOffsets left off and accumulate the offsets.
Constant *LHSNoBound = stripAndComputeConstantOffsets(TD, LHS, true);
Constant *RHSNoBound = stripAndComputeConstantOffsets(TD, RHS, true);
if (LHS == RHS)
return ConstantExpr::getICmp(Pred,
ConstantExpr::getAdd(LHSOffset, LHSNoBound),
ConstantExpr::getAdd(RHSOffset, RHSNoBound));
}
// Otherwise, fail.

9
test/Transforms/InstSimplify/compare.ll
View File

@ -717,3 +717,12 @@ define i1 @alloca_gep(i64 %a, i64 %b) {
ret i1 %cmp
; CHECK-NEXT: ret i1 false
}
define i1 @non_inbounds_gep_compare(i64* %a) {
; CHECK-LABEL: @non_inbounds_gep_compare(
; Equality compares with non-inbounds GEPs can be folded.
%x = getelementptr i64* %a, i64 42
%cmp = icmp eq i64* %a, %x
ret i1 %cmp
; CHECK-NEXT: ret i1 false
}