[TBAA] Don't emit pointer tbaa for unnamed structs or unions.

[fhahn]

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.

[llvmbot]

@llvm/pr-subscribers-clang-codegen

@llvm/pr-subscribers-clang

Author: Florian Hahn (fhahn)

Changes

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 --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}

[rjmccall]

I'd like this to note that

• using AnyPtr is over-conservative here, and we could make this summarize the members as per the C compatibility rule in the future and
• this also covers anonymous structs and unions, which have a different compatibility rule, but it doesn't matter because you can never have a pointer to an anonymous struct or union.

[fhahn]

Thanks, added the clarifications as suggested!

[rjmccall]

This getAs<> is now redundant with the isRecordType() above; please do the getAs<> above and test RT there, and then you can assume you have a record here.

[fhahn]

Done, thanks!