[clang] [Clang][CodeGen] Fix `CanSkipVTablePointerInitialization` for dynamic classes with a trivial anonymous union (PR #84651)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Max Winkler (MaxEW707)

<details>
<summary>Changes</summary>

Hit this when trying upgrade an old project of mine. I couldn't find a corresponding existing issue for this when spelunking the open issues here on github.
Thankfully I can work-around it today with the `[[clang::no_destroy]]` attribute for my use case. However it should still be properly fixed.

### Issue and History ###

https://godbolt.org/z/EYnhce8MK for reference.
All subsequent text below refers to the example in the godbolt above.

Anonymous unions never have their destructor invoked automatically. Therefore we can skip vtable initialization of the destructor of a dynamic class if that destructor effectively does no work.

This worked previously as the following check would be hit and return true for the trivial anonymous union, https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/CGClass.cpp#L1348, resulting in the code skipping vtable initialization.

This was broken here https://github.com/llvm/llvm-project/commit/982bbf404eba2d968afda5c674d4821652159c53 in relation to comments made on this review here https://reviews.llvm.org/D10508.

### Fixes ###

The check the code is doing is correct however the return value is inverted. We want to return true here since a field with anonymous union never has its destructor invoked and thus effectively has a trivial destructor body from the perspective of requiring vtable init in the parent dynamic class.

Also added some extra missing unit tests to test for this use case.


---
Full diff: https://github.com/llvm/llvm-project/pull/84651.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CGClass.cpp (+1-1) 
- (modified) clang/test/CodeGenCXX/skip-vtable-pointer-initialization.cpp (+63) 


``````````diff
diff --git a/clang/lib/CodeGen/CGClass.cpp b/clang/lib/CodeGen/CGClass.cpp
index d18f186ce5b415..ca3ac7142af9c0 100644
--- a/clang/lib/CodeGen/CGClass.cpp
+++ b/clang/lib/CodeGen/CGClass.cpp
@@ -1395,7 +1395,7 @@ FieldHasTrivialDestructorBody(ASTContext &Context,
 
   // The destructor for an implicit anonymous union member is never invoked.
   if (FieldClassDecl->isUnion() && FieldClassDecl->isAnonymousStructOrUnion())
-    return false;
+    return true;
 
   return HasTrivialDestructorBody(Context, FieldClassDecl, FieldClassDecl);
 }
diff --git a/clang/test/CodeGenCXX/skip-vtable-pointer-initialization.cpp b/clang/test/CodeGenCXX/skip-vtable-pointer-initialization.cpp
index eb8f21b57aa7b6..d99a45869fdb54 100644
--- a/clang/test/CodeGenCXX/skip-vtable-pointer-initialization.cpp
+++ b/clang/test/CodeGenCXX/skip-vtable-pointer-initialization.cpp
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 %s -triple=x86_64-apple-darwin10 -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 %s -triple=x86_64-pc-linux-gnu -emit-llvm -o - | FileCheck %s
 
 // See Test9 for test description.
 // CHECK: @_ZTTN5Test91BE = linkonce_odr unnamed_addr constant
@@ -198,3 +199,65 @@ struct C : virtual B {
 C::~C() {}
 
 }
+
+namespace Test10 {
+
+// Check that we don't initialize the vtable pointer in A::~A(), since the class has an anonymous union which
+// never has its destructor invoked.
+struct A {
+    virtual void f();
+    ~A();
+
+    union
+    {
+        int i;
+        unsigned u;
+    };
+};
+
+// CHECK-LABEL: define{{.*}} void @_ZN6Test101AD2Ev
+// CHECK-NOT: store ptr getelementptr inbounds ({ [3 x ptr] }, ptr @_ZTVN6Test101AE, i32 0, inrange i32 0, i32 2), ptr
+A::~A() {
+}
+
+}
+
+namespace Test11 {
+
+// Check that we don't initialize the vtable pointer in A::~A(), even if the base class has a non trivial destructor.
+struct Field {
+    ~Field();
+};
+
+struct A : public Field {
+    virtual void f();
+    ~A();
+};
+
+// CHECK-LABEL: define{{.*}} void @_ZN6Test111AD2Ev
+// CHECK-NOT: store ptr getelementptr inbounds ({ [3 x ptr] }, ptr @_ZTVN6Test111AE, i32 0, inrange i32 0, i32 2), ptr
+A::~A() {
+}
+
+}
+
+namespace Test12 {
+
+// Check that we don't initialize the vtable pointer in A::~A(), since the class has an anonymous struct with trivial fields.
+struct A {
+    virtual void f();
+    ~A();
+
+    struct
+    {
+        int i;
+        unsigned u;
+    };
+};
+
+// CHECK-LABEL: define{{.*}} void @_ZN6Test121AD2Ev
+// CHECK-NOT: store ptr getelementptr inbounds ({ [3 x ptr] }, ptr @_ZTVN6Test121AE, i32 0, inrange i32 0, i32 2), ptr
+A::~A() {
+}
+
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/84651