[InstCombine] Preserve nuw on sub of geps (PR44419)
Fix https://bugs.llvm.org/show_bug.cgi?id=44419 by preserving the
nuw on sub of geps. We only do this if the offset has a multiplication
as the final operation, as we can't be sure the operations is nuw
in the other cases without more thorough analysis.
Differential Revision: https://reviews.llvm.org/D72048
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 36d9b80..ec976a9 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1585,7 +1585,7 @@
/// &A[10] - &A[0]: we should compile this to "10". LHS/RHS are the pointer
/// operands to the ptrtoint instructions for the LHS/RHS of the subtract.
Value *InstCombiner::OptimizePointerDifference(Value *LHS, Value *RHS,
- Type *Ty) {
+ Type *Ty, bool IsNUW) {
// If LHS is a gep based on RHS or RHS is a gep based on LHS, we can optimize
// this.
bool Swapped = false;
@@ -1653,6 +1653,15 @@
// Emit the offset of the GEP and an intptr_t.
Value *Result = EmitGEPOffset(GEP1);
+ // If this is a single inbounds GEP and the original sub was nuw,
+ // then the final multiplication is also nuw. We match an extra add zero
+ // here, because that's what EmitGEPOffset() generates.
+ Instruction *I;
+ if (IsNUW && !GEP2 && !Swapped && GEP1->isInBounds() &&
+ match(Result, m_Add(m_Instruction(I), m_Zero())) &&
+ I->getOpcode() == Instruction::Mul)
+ I->setHasNoUnsignedWrap();
+
// If we had a constant expression GEP on the other side offsetting the
// pointer, subtract it from the offset we have.
if (GEP2) {
@@ -2051,13 +2060,15 @@
Value *LHSOp, *RHSOp;
if (match(Op0, m_PtrToInt(m_Value(LHSOp))) &&
match(Op1, m_PtrToInt(m_Value(RHSOp))))
- if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType()))
+ if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType(),
+ I.hasNoUnsignedWrap()))
return replaceInstUsesWith(I, Res);
// trunc(p)-trunc(q) -> trunc(p-q)
if (match(Op0, m_Trunc(m_PtrToInt(m_Value(LHSOp)))) &&
match(Op1, m_Trunc(m_PtrToInt(m_Value(RHSOp)))))
- if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType()))
+ if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType(),
+ /* IsNUW */ false))
return replaceInstUsesWith(I, Res);
// Canonicalize a shifty way to code absolute value to the common pattern.