[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

2 files changed, 16 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 36d9b80eea2..ec976a971e3 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1585,7 +1585,7 @@ Instruction *InstCombiner::visitFAdd(BinaryOperator &I) {
 ///  &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 @@ Value *InstCombiner::OptimizePointerDifference(Value *LHS, Value *RHS,
   // 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 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
   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.
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index 524f9313d8c..d793a9f641d 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -369,7 +369,8 @@ public:
   Instruction *visitFNeg(UnaryOperator &I);
   Instruction *visitAdd(BinaryOperator &I);
   Instruction *visitFAdd(BinaryOperator &I);
-  Value *OptimizePointerDifference(Value *LHS, Value *RHS, Type *Ty);
+  Value *OptimizePointerDifference(
+      Value *LHS, Value *RHS, Type *Ty, bool isNUW);
   Instruction *visitSub(BinaryOperator &I);
   Instruction *visitFSub(BinaryOperator &I);
   Instruction *visitMul(BinaryOperator &I);