[InstCombine] Refine nuw propagation in OptimizePointerDifference

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -2164,10 +2164,13 @@ Value *InstCombinerImpl::OptimizePointerDifference(Value *LHS, Value *RHS,
 
   // If this is a single inbounds GEP and the original sub was nuw,
   // then the final multiplication is also nuw.
-  if (auto *I = dyn_cast<Instruction>(Result))
+  if (auto *I = dyn_cast<OverflowingBinaryOperator>(Result))
     if (IsNUW && match(Offset2, m_Zero()) && Base.LHSNW.isInBounds() &&
-        I->getOpcode() == Instruction::Mul)
-      I->setHasNoUnsignedWrap();
+        I->hasNoSignedWrap() && !I->hasNoUnsignedWrap() &&
+        ((I->getOpcode() == Instruction::Mul &&
+          match(I->getOperand(1), m_NonNegative())) ||
+         I->getOpcode() == Instruction::Shl))
+      cast<Instruction>(I)->setHasNoUnsignedWrap();
 
   // If we have a 2nd GEP of the same base pointer, subtract the offsets.
   // If both GEPs are inbounds, then the subtract does not have signed overflow.

llvm/test/Transforms/InstCombine/sub-gep.ll

@@ -1092,7 +1092,7 @@ define <2 x i64> @splat_geps_multiple(ptr %base, i64 %idx0, <2 x i64> %idx1, <2
 
 define i64 @nuw_ptrdiff_shl_nsw(ptr %base, i64 %idx) {
 ; CHECK-LABEL: @nuw_ptrdiff_shl_nsw(
-; CHECK-NEXT:    [[OFFSET:%.*]] = shl nsw i64 [[IDX:%.*]], 3
+; CHECK-NEXT:    [[OFFSET:%.*]] = shl nuw nsw i64 [[IDX:%.*]], 3
 ; CHECK-NEXT:    ret i64 [[OFFSET]]
 ;
   %offset = shl nsw i64 %idx, 3
@@ -1131,7 +1131,7 @@ define i64 @nuw_ptrdiff_mul_nsw_nneg_scale(ptr %base, i64 %idx) {
 
 define i64 @nuw_ptrdiff_mul_nsw_unknown_scale(ptr %base, i64 %idx, i64 %scale) {
 ; CHECK-LABEL: @nuw_ptrdiff_mul_nsw_unknown_scale(
-; CHECK-NEXT:    [[OFFSET:%.*]] = mul nuw nsw i64 [[IDX:%.*]], [[SCALE:%.*]]
+; CHECK-NEXT:    [[OFFSET:%.*]] = mul nsw i64 [[IDX:%.*]], [[SCALE:%.*]]
 ; CHECK-NEXT:    ret i64 [[OFFSET]]
 ;
   %offset = mul nsw i64 %idx, %scale