[PATCH] Avoid referencing a vtable when not required

[Nico Weber]

(fixes PR20221)


On Tue, Jul 8, 2014 at 6:19 PM, Reid Kleckner <rnk at google.com> wrote:

> Hi rsmith,
>
> This fixes compilation errors about incomplete types used with WebKit's
> RefPtr template.  Simply an out of line constructor should not
> instantiate all inline and defaulted virtual methods.
>
> http://reviews.llvm.org/D4429
>
> Files:
>   lib/Sema/SemaExpr.cpp
>   test/SemaTemplate/virtual-member-functions.cpp
>
> Index: lib/Sema/SemaExpr.cpp
> ===================================================================
> --- lib/Sema/SemaExpr.cpp
> +++ lib/Sema/SemaExpr.cpp
> @@ -11445,8 +11445,6 @@
>      } else if (Constructor->getInheritedConstructor()) {
>        DefineInheritingConstructor(Loc, Constructor);
>      }
> -
> -    MarkVTableUsed(Loc, Constructor->getParent());
>    } else if (CXXDestructorDecl *Destructor =
>                   dyn_cast<CXXDestructorDecl>(Func)) {
>      Destructor = cast<CXXDestructorDecl>(Destructor->getFirstDecl());
> Index: test/SemaTemplate/virtual-member-functions.cpp
> ===================================================================
> --- test/SemaTemplate/virtual-member-functions.cpp
> +++ test/SemaTemplate/virtual-member-functions.cpp
> @@ -44,16 +44,16 @@
>
>  template<typename T>
>  struct HasOutOfLineKey {
> -  HasOutOfLineKey() { }
> +  HasOutOfLineKey() { } // expected-note{{in instantiation of member
> function 'HasOutOfLineKey<int>::f' requested here}}
>    virtual T *f(float *fp);
>  };
>
>  template<typename T>
>  T *HasOutOfLineKey<T>::f(float *fp) {
>    return fp; // expected-error{{cannot initialize return object of type
> 'int *' with an lvalue of type 'float *'}}
>  }
>
> -HasOutOfLineKey<int> out_of_line; // expected-note{{in instantiation of
> member function 'HasOutOfLineKey<int>::f' requested here}}
> +HasOutOfLineKey<int> out_of_line; // expected-note{{in instantiation of
> member function 'HasOutOfLineKey<int>::HasOutOfLineKey' requested here}}
>
>  namespace std {
>    class type_info;
> @@ -102,3 +102,53 @@
>    Y* f(X<void>* x) { return dynamic_cast<Y*>(x); } // expected-note {{in
> instantiation of member function 'DynamicCast::X<void>::foo' requested
> here}}
>    Y* f2(X<void>* x) { return dynamic_cast<Y*>(x); }
>  }
> +
> +namespace avoid_using_vtable {
> +// We shouldn't emit the vtable for this code, in any ABI.  If we emit the
> +// vtable, we emit an implicit virtual dtor, which calls ~RefPtr, which
> requires
> +// a complete type for DeclaredOnly.
> +//
> +// Previously we would reference the vtable in the MS C++ ABI, even
> though we
> +// don't need to emit either the ctor or the dtor.  In the Itanium C++
> ABI, the
> +// 'trace' method is the key function, so even though we use the vtable,
> we
> +// don't emit it.
> +
> +template <typename T>
> +struct RefPtr {
> +  T *m_ptr;
> +  ~RefPtr() { m_ptr->deref(); }
> +};
> +struct DeclaredOnly;
> +struct Base {
> +  virtual ~Base();
> +};
> +
> +struct AvoidVTable : Base {
> +  RefPtr<DeclaredOnly> m_insertionStyle;
> +  virtual void trace();
> +  AvoidVTable();
> +};
> +// Don't call the dtor, because that will emit an implicit dtor, and
> require a
> +// complete type for DeclaredOnly.
> +void foo() { new AvoidVTable; }
> +}
> +
> +namespace vtable_uses_incomplete {
> +// Opposite of the previous test that avoids a vtable, this one tests
> that we
> +// use the vtable when the ctor is defined inline.
> +template <typename T>
> +struct RefPtr {
> +  T *m_ptr;
> +  ~RefPtr() { m_ptr->deref(); }  // expected-error {{member access into
> incomplete type 'vtable_uses_incomplete::DeclaredOnly'}}
> +};
> +struct DeclaredOnly; // expected-note {{forward declaration of
> 'vtable_uses_incomplete::DeclaredOnly'}}
> +struct Base {
> +  virtual ~Base();
> +};
> +
> +struct UsesVTable : Base {
> +  RefPtr<DeclaredOnly> m_insertionStyle;
> +  virtual void trace();
> +  UsesVTable() {} // expected-note {{in instantiation of member function
> 'vtable_uses_incomplete::RefPtr<vtable_uses_incomplete::DeclaredOnly>::~RefPtr'
> requested here}}
> +};
> +}
>
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