[llvm] [InstCombine] Check no wrap flags before folding icmp of GEPs with same indices (PR #121628)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

Alive2: https://alive2.llvm.org/ce/z/Dr3Sbe
Closes https://github.com/llvm/llvm-project/issues/121581.

---
Full diff: https://github.com/llvm/llvm-project/pull/121628.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+4-2) 
- (modified) llvm/test/Transforms/InstCombine/icmp-gep.ll (+48) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index d6fdade25559fe..8b23583c510637 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -747,6 +747,8 @@ Instruction *InstCombinerImpl::foldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
                         ConstantExpr::getPointerBitCastOrAddrSpaceCast(
                             cast<Constant>(RHS), Base->getType()));
   } else if (GEPOperator *GEPRHS = dyn_cast<GEPOperator>(RHS)) {
+    GEPNoWrapFlags NW = GEPLHS->getNoWrapFlags() & GEPRHS->getNoWrapFlags();
+
     // If the base pointers are different, but the indices are the same, just
     // compare the base pointer.
     if (PtrBase != GEPRHS->getOperand(0)) {
@@ -764,7 +766,8 @@ Instruction *InstCombinerImpl::foldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
 
       // If all indices are the same, just compare the base pointers.
       Type *BaseType = GEPLHS->getOperand(0)->getType();
-      if (IndicesTheSame && CmpInst::makeCmpResultType(BaseType) == I.getType())
+      if (IndicesTheSame &&
+          CmpInst::makeCmpResultType(BaseType) == I.getType() && CanFold(NW))
         return new ICmpInst(Cond, GEPLHS->getOperand(0), GEPRHS->getOperand(0));
 
       // If we're comparing GEPs with two base pointers that only differ in type
@@ -804,7 +807,6 @@ Instruction *InstCombinerImpl::foldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
       return transformToIndexedCompare(GEPLHS, RHS, Cond, DL, *this);
     }
 
-    GEPNoWrapFlags NW = GEPLHS->getNoWrapFlags() & GEPRHS->getNoWrapFlags();
     if (GEPLHS->getNumOperands() == GEPRHS->getNumOperands() &&
         GEPLHS->getSourceElementType() == GEPRHS->getSourceElementType()) {
       // If the GEPs only differ by one index, compare it.
diff --git a/llvm/test/Transforms/InstCombine/icmp-gep.ll b/llvm/test/Transforms/InstCombine/icmp-gep.ll
index f9b90c224d8327..7f8f1ae73948d1 100644
--- a/llvm/test/Transforms/InstCombine/icmp-gep.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-gep.ll
@@ -709,3 +709,51 @@ define i1 @pointer_icmp_aligned_with_offset_negative(ptr align 8 %a, ptr align 8
   %cmp = icmp eq ptr %gep, %a2
   ret i1 %cmp
 }
+
+define i1 @gep_diff_base_same_indices(ptr %x, ptr %y, i64 %z) {
+; CHECK-LABEL: @gep_diff_base_same_indices(
+; CHECK-NEXT:    [[X:%.*]] = getelementptr i8, ptr [[X1:%.*]], i64 [[Z:%.*]]
+; CHECK-NEXT:    [[Y:%.*]] = getelementptr i8, ptr [[Y1:%.*]], i64 [[Z]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult ptr [[X]], [[Y]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %gep1 = getelementptr i8, ptr %x, i64 %z
+  %gep2 = getelementptr i8, ptr %y, i64 %z
+  %cmp = icmp ult ptr %gep1, %gep2
+  ret i1 %cmp
+}
+
+define i1 @gep_diff_base_same_indices_nuw(ptr %x, ptr %y, i64 %z) {
+; CHECK-LABEL: @gep_diff_base_same_indices_nuw(
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult ptr [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %gep1 = getelementptr nuw i8, ptr %x, i64 %z
+  %gep2 = getelementptr nuw i8, ptr %y, i64 %z
+  %cmp = icmp ult ptr %gep1, %gep2
+  ret i1 %cmp
+}
+
+define i1 @gep_diff_base_same_indices_nusw(ptr %x, ptr %y, i64 %z) {
+; CHECK-LABEL: @gep_diff_base_same_indices_nusw(
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult ptr [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %gep1 = getelementptr nusw i8, ptr %x, i64 %z
+  %gep2 = getelementptr nusw i8, ptr %y, i64 %z
+  %cmp = icmp ult ptr %gep1, %gep2
+  ret i1 %cmp
+}
+
+define i1 @gep_diff_base_same_indices_nuw_nusw(ptr %x, ptr %y, i64 %z) {
+; CHECK-LABEL: @gep_diff_base_same_indices_nuw_nusw(
+; CHECK-NEXT:    [[X:%.*]] = getelementptr nuw i8, ptr [[X1:%.*]], i64 [[Z:%.*]]
+; CHECK-NEXT:    [[Y:%.*]] = getelementptr nusw i8, ptr [[Y1:%.*]], i64 [[Z]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult ptr [[X]], [[Y]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %gep1 = getelementptr nuw i8, ptr %x, i64 %z
+  %gep2 = getelementptr nusw i8, ptr %y, i64 %z
+  %cmp = icmp ult ptr %gep1, %gep2
+  ret i1 %cmp
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/121628