[clang] [TBAA] Don't emit pointer tbaa for unnamed structs or unions. (PR #116596)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang-codegen

@llvm/pr-subscribers-clang

Author: Florian Hahn (fhahn)

<details>
<summary>Changes</summary>

For unnamed structs or unions, C's compatible types rule applies. Two compatible types in different compilation units can have different mangled names, meaning the metadata emitted below would incorrectly mark them as no-alias. Use AnyPtr for such types in both C and C++, as C and C++ types may be visible when doing LTO.

---
Full diff: https://github.com/llvm/llvm-project/pull/116596.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CodeGenTBAA.cpp (+9) 
- (modified) clang/test/CodeGen/tbaa-pointers.c (+1-4) 


``````````diff
diff --git a/clang/lib/CodeGen/CodeGenTBAA.cpp b/clang/lib/CodeGen/CodeGenTBAA.cpp
index c31579e8323174..95d9f893a2c7d1 100644
--- a/clang/lib/CodeGen/CodeGenTBAA.cpp
+++ b/clang/lib/CodeGen/CodeGenTBAA.cpp
@@ -230,6 +230,15 @@ llvm::MDNode *CodeGenTBAA::getTypeInfoHelper(const Type *Ty) {
               ->getString();
       TyName = Name;
     } else {
+      // For unnamed structs or unions C's compatible types rule applies. Two
+      // compatible types in different compilation units can have different
+      // mangled names, meaning the metadata emitted below would incorrectly
+      // mark them as no-alias. Use AnyPtr for such types in both C and C++, as
+      // C and C++ types may be visible when doing LTO.
+      const auto *RT = Ty->getAs<RecordType>();
+      if (RT && !RT->getDecl()->getDeclName())
+        return AnyPtr;
+
       // For non-builtin types use the mangled name of the canonical type.
       llvm::raw_svector_ostream TyOut(TyName);
       MangleCtx->mangleCanonicalTypeName(QualType(Ty, 0), TyOut);
diff --git a/clang/test/CodeGen/tbaa-pointers.c b/clang/test/CodeGen/tbaa-pointers.c
index 9417a0e2f09e8c..068459f4dce118 100644
--- a/clang/test/CodeGen/tbaa-pointers.c
+++ b/clang/test/CodeGen/tbaa-pointers.c
@@ -190,8 +190,6 @@ typedef struct {
   int i1;
 } TypedefS;
 
-// FIXME: The !tbaa tag for unnamed structs doesn't account for compatible
-// types in C.
 void unamed_struct_typedef(TypedefS *ptr) {
 // COMMON-LABEL: define void @unamed_struct_typedef(
 // COMMON-SAME: ptr noundef [[PTRA:%.+]])
@@ -238,5 +236,4 @@ void unamed_struct_typedef(TypedefS *ptr) {
 // DEFAULT: [[S2_TY]]  = !{!"S2", [[ANY_POINTER]], i64 0}
 // COMMON:  [[INT_TAG]] = !{[[INT_TY:!.+]], [[INT_TY]], i64 0}
 // COMMON:  [[INT_TY]] = !{!"int", [[CHAR]], i64 0}
-// ENABLED: [[P1TYPEDEF]] = !{[[P1TYPEDEF_TY:!.+]],  [[P1TYPEDEF_TY]], i64 0}
-// ENABLED: [[P1TYPEDEF_TY]] = !{!"p1 _ZTS8TypedefS", [[ANY_POINTER]], i64 0}
+// ENABLED: [[P1TYPEDEF]] = !{[[ANY_POINTER]],  [[ANY_POINTER]], i64 0}

``````````

</details>


https://github.com/llvm/llvm-project/pull/116596