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

[Max Winkler via cfe-commits]

https://github.com/MaxEW707 updated https://github.com/llvm/llvm-project/pull/84651

>From 28cbaad3c16e985306c7b977cb7d9e8e89c39a07 Mon Sep 17 00:00:00 2001
From: MaxEW707 <82551778+MaxEW707 at users.noreply.github.com>
Date: Sat, 9 Mar 2024 15:40:52 -0500
Subject: [PATCH 1/2] Fix `CanSkipVTablePointerInitialization` for dynamic
 classes with an anonymous union

---
 clang/lib/CodeGen/CGClass.cpp                 |  2 +-
 .../skip-vtable-pointer-initialization.cpp    | 63 +++++++++++++++++++
 2 files changed, 64 insertions(+), 1 deletion(-)

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() {
+}
+
+}

>From fd190fb64877636c94cbaf73d1977018e592a622 Mon Sep 17 00:00:00 2001
From: MaxEW707 <max.enrico.winkler at gmail.com>
Date: Mon, 1 Apr 2024 20:36:54 -0400
Subject: [PATCH 2/2] remove RUN line in unit test

---
 clang/test/CodeGenCXX/skip-vtable-pointer-initialization.cpp | 1 -
 1 file changed, 1 deletion(-)

diff --git a/clang/test/CodeGenCXX/skip-vtable-pointer-initialization.cpp b/clang/test/CodeGenCXX/skip-vtable-pointer-initialization.cpp
index d99a45869fdb54..9c5388fd64a502 100644
--- a/clang/test/CodeGenCXX/skip-vtable-pointer-initialization.cpp
+++ b/clang/test/CodeGenCXX/skip-vtable-pointer-initialization.cpp
@@ -1,5 +1,4 @@
 // 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