[clang] [Clang][CodeGen] Respect -fwrapv-pointer when emitting struct GEPs (PR #134269)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

This patch turns off inbounds/nuw flags for member accesses when `-fwrapv-pointer` is set.
Note: There are many calls to `CGBuilderTy::CreateStructGEP`, and most of them are safe. So I think it is probably overkill to handle this in `CreateStructGEP`.

Closes https://github.com/llvm/llvm-project/issues/132449.

It is required by https://github.com/llvm/llvm-project/pull/130734.


---
Full diff: https://github.com/llvm/llvm-project/pull/134269.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CGExpr.cpp (+9-2) 
- (modified) clang/test/CodeGen/pointer-overflow.c (+21) 


``````````diff
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 3d3a111f0514a..d32591ed896d0 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -4922,6 +4922,9 @@ static Address emitAddrOfFieldStorage(CodeGenFunction &CGF, Address base,
   unsigned idx =
     CGF.CGM.getTypes().getCGRecordLayout(rec).getLLVMFieldNo(field);
 
+  if (CGF.getLangOpts().PointerOverflowDefined)
+    return CGF.Builder.CreateConstGEP2_32(base, 0, idx, field->getName());
+
   return CGF.Builder.CreateStructGEP(base, idx, field->getName());
 }
 
@@ -4979,9 +4982,13 @@ LValue CodeGenFunction::EmitLValueForField(LValue base,
     if (!UseVolatile) {
       if (!IsInPreservedAIRegion &&
           (!getDebugInfo() || !rec->hasAttr<BPFPreserveAccessIndexAttr>())) {
-        if (Idx != 0)
+        if (Idx != 0) {
           // For structs, we GEP to the field that the record layout suggests.
-          Addr = Builder.CreateStructGEP(Addr, Idx, field->getName());
+          if (getLangOpts().PointerOverflowDefined)
+            Addr = Builder.CreateConstGEP2_32(Addr, 0, Idx, field->getName());
+          else
+            Addr = Builder.CreateStructGEP(Addr, Idx, field->getName());
+        }
       } else {
         llvm::DIType *DbgInfo = getDebugInfo()->getOrCreateRecordType(
             getContext().getRecordType(rec), rec->getLocation());
diff --git a/clang/test/CodeGen/pointer-overflow.c b/clang/test/CodeGen/pointer-overflow.c
index 9c7821b841980..70e3c855bff90 100644
--- a/clang/test/CodeGen/pointer-overflow.c
+++ b/clang/test/CodeGen/pointer-overflow.c
@@ -10,3 +10,24 @@ void test(void) {
   // DEFAULT: getelementptr inbounds nuw i32, ptr
   // FWRAPV-POINTER: getelementptr i32, ptr
 }
+
+struct S {
+  int a;
+  int b;
+  int c: 10;
+  int d: 10;
+};
+
+int test_struct(struct S* s) {
+  // -fwrapv-pointer should turn off inbounds nuw for struct GEP's
+  return s->b;
+  // DEFAULT: getelementptr inbounds nuw %struct.S, ptr
+  // FWRAPV-POINTER: getelementptr %struct.S, ptr
+}
+
+int test_struct_bitfield(struct S* s) {
+  // -fwrapv-pointer should turn off inbounds nuw for struct GEP's
+  return s->d;
+  // DEFAULT: getelementptr inbounds nuw %struct.S, ptr
+  // FWRAPV-POINTER: getelementptr %struct.S, ptr
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/134269