[clang] [clang] Check inline defs when emitting speculative vtable (PR #100785)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-codegen

Author: Fabian Parzefall (FPar)

<details>
<summary>Changes</summary>

Clang should only emit an available_externally vtable when there are no unused virtual inline functions. Currently, if such such a function is declared without inline inside the class, but is defined inline outside the class, Clang might emit the vtable as available_externally. This happens because Clang only considers the declarations of vtable entries, but not the definitions. This patch addresses this by inspecting the definitions in addition to the declarations.

---
Full diff: https://github.com/llvm/llvm-project/pull/100785.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/ItaniumCXXABI.cpp (+4-1) 
- (modified) clang/test/CodeGenCXX/vtable-available-externally.cpp (+18-7) 


``````````diff
diff --git a/clang/lib/CodeGen/ItaniumCXXABI.cpp b/clang/lib/CodeGen/ItaniumCXXABI.cpp
index cd76f8406e7b7..c940089c93361 100644
--- a/clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ b/clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -442,7 +442,10 @@ class ItaniumCXXABI : public CodeGen::CGCXXABI {
          continue;
 
        const CXXMethodDecl *Method = VtableComponent.getFunctionDecl();
-       if (!Method->getCanonicalDecl()->isInlined())
+       const FunctionDecl *FD = Method->getDefinition();
+       const bool IsInlined =
+           Method->getCanonicalDecl()->isInlined() || (FD && FD->isInlined());
+       if (!IsInlined)
          continue;
 
        StringRef Name = CGM.getMangledName(VtableComponent.getGlobalDecl());
diff --git a/clang/test/CodeGenCXX/vtable-available-externally.cpp b/clang/test/CodeGenCXX/vtable-available-externally.cpp
index a57eb39edfe10..ab105260bc75a 100644
--- a/clang/test/CodeGenCXX/vtable-available-externally.cpp
+++ b/clang/test/CodeGenCXX/vtable-available-externally.cpp
@@ -250,28 +250,39 @@ struct C : A {
   virtual void car();
 };
 
+// Inline definition outside body, so we can't emit vtable available_externally
+// (see previous).
+// CHECK-TEST10-DAG: @_ZTVN6Test101FE = external unnamed_addr constant
+struct F : A {
+  void foo();
+  virtual void cat();         // inline outside body
+};
+inline void F::cat() {}
+
 // no key function, vtable will be generated everywhere it will be used
 // CHECK-TEST10-DAG: @_ZTVN6Test101EE = linkonce_odr unnamed_addr constant
 // CHECK-FORCE-EMIT-DAG: @_ZTVN6Test101EE = linkonce_odr unnamed_addr constant
 
 struct E : A {};
 
-void g(A& a) {
+void h(A& a) {
   a.foo();
   a.bar();
 }
 
-void f() {
+void g() {
   A a;
-  g(a);
+  h(a);
   B b;
-  g(b);
+  h(b);
   C c;
-  g(c);
+  h(c);
   D d;
-  g(d);
+  h(d);
   E e;
-  g(e);
+  h(e);
+  F f;
+  h(f);
 }
 
 }  // Test10

``````````

</details>


https://github.com/llvm/llvm-project/pull/100785