[llvm] [InstCombine] Fix GEPNoWrapFlags propagation in `foldGEPOfPhi` (PR #121572)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

Closes https://github.com/llvm/llvm-project/issues/121459.


---
Full diff: https://github.com/llvm/llvm-project/pull/121572.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstructionCombining.cpp (+5) 
- (modified) llvm/test/Transforms/InstCombine/opaque-ptr.ll (+58) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 934156f04f7fdd..f63de1f0d410e2 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2782,6 +2782,7 @@ static Instruction *foldGEPOfPhi(GetElementPtrInst &GEP, PHINode *PN,
   // loop iteration).
   if (Op1 == &GEP)
     return nullptr;
+  GEPNoWrapFlags NW = Op1->getNoWrapFlags();
 
   int DI = -1;
 
@@ -2838,6 +2839,8 @@ static Instruction *foldGEPOfPhi(GetElementPtrInst &GEP, PHINode *PN,
         }
       }
     }
+
+    NW &= Op2->getNoWrapFlags();
   }
 
   // If not all GEPs are identical we'll have to create a new PHI node.
@@ -2847,6 +2850,8 @@ static Instruction *foldGEPOfPhi(GetElementPtrInst &GEP, PHINode *PN,
     return nullptr;
 
   auto *NewGEP = cast<GetElementPtrInst>(Op1->clone());
+  NewGEP->setNoWrapFlags(NW);
+
   if (DI == -1) {
     // All the GEPs feeding the PHI are identical. Clone one down into our
     // BB so that it can be merged with the current GEP.
diff --git a/llvm/test/Transforms/InstCombine/opaque-ptr.ll b/llvm/test/Transforms/InstCombine/opaque-ptr.ll
index bac51c82f36dda..b05274658e812e 100644
--- a/llvm/test/Transforms/InstCombine/opaque-ptr.ll
+++ b/llvm/test/Transforms/InstCombine/opaque-ptr.ll
@@ -654,6 +654,64 @@ join:
   ret ptr %gep
 }
 
+define ptr @gep_of_phi_of_gep_flags1(i1 %c, ptr %p) {
+; CHECK-LABEL: @gep_of_phi_of_gep_flags1(
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[IF:%.*]], label [[ELSE:%.*]]
+; CHECK:       if:
+; CHECK-NEXT:    br label [[JOIN:%.*]]
+; CHECK:       else:
+; CHECK-NEXT:    br label [[JOIN]]
+; CHECK:       join:
+; CHECK-NEXT:    [[TMP1:%.*]] = phi i64 [ 4, [[IF]] ], [ 8, [[ELSE]] ]
+; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr i8, ptr [[P:%.*]], i64 [[TMP1]]
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, ptr [[TMP2]], i64 4
+; CHECK-NEXT:    ret ptr [[GEP]]
+;
+  br i1 %c, label %if, label %else
+
+if:
+  %gep1 = getelementptr inbounds i32, ptr %p, i64 1
+  br label %join
+
+else:
+  %gep2 = getelementptr i32, ptr %p, i64 2
+  br label %join
+
+join:
+  %phi = phi ptr [ %gep1, %if ], [ %gep2, %else ]
+  %gep = getelementptr i32, ptr %phi, i64 1
+  ret ptr %gep
+}
+
+define ptr @gep_of_phi_of_gep_flags2(i1 %c, ptr %p) {
+; CHECK-LABEL: @gep_of_phi_of_gep_flags2(
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[IF:%.*]], label [[ELSE:%.*]]
+; CHECK:       if:
+; CHECK-NEXT:    br label [[JOIN:%.*]]
+; CHECK:       else:
+; CHECK-NEXT:    br label [[JOIN]]
+; CHECK:       join:
+; CHECK-NEXT:    [[TMP1:%.*]] = phi i64 [ 4, [[IF]] ], [ 8, [[ELSE]] ]
+; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr nuw i8, ptr [[P:%.*]], i64 [[TMP1]]
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, ptr [[TMP2]], i64 4
+; CHECK-NEXT:    ret ptr [[GEP]]
+;
+  br i1 %c, label %if, label %else
+
+if:
+  %gep1 = getelementptr nuw i32, ptr %p, i64 1
+  br label %join
+
+else:
+  %gep2 = getelementptr nuw i32, ptr %p, i64 2
+  br label %join
+
+join:
+  %phi = phi ptr [ %gep1, %if ], [ %gep2, %else ]
+  %gep = getelementptr i32, ptr %phi, i64 1
+  ret ptr %gep
+}
+
 define ptr @gep_of_phi_of_gep_different_type(i1 %c, ptr %p) {
 ; CHECK-LABEL: @gep_of_phi_of_gep_different_type(
 ; CHECK-NEXT:    br i1 [[C:%.*]], label [[IF:%.*]], label [[ELSE:%.*]]

``````````

</details>


https://github.com/llvm/llvm-project/pull/121572