[cxx-abi-dev] abi_tag mangling

[Jason Merrill]

On Mon, Aug 1, 2016 at 2:27 PM, Jason Merrill <jason at redhat.com> wrote:
> On Mon, Jun 27, 2016 at 4:35 PM, Richard Smith <richardsmith at google.com> wrote:
>> On 14 June 2016 at 14:09, Jason Merrill <jason at redhat.com> wrote:
>>> On Mon, Jun 6, 2016 at 5:29 PM, Richard Smith <richardsmith at google.com> wrote:
>>> > On 3 June 2016 at 15:25, Jason Merrill <jason at redhat.com> wrote:
>>> >> On Jun 2, 2016, at 3:01 PM, Richard Smith <richardsmith at google.com> wrote:
>>> >>
>>> >> > If a type is not part of the regular mangling, and it contains an ABI tag in
>>> >> > its mangling but not as part of its type (for instance, a dependent
>>> >> > expression in a template argument references an entity with an ABI tag),
>>> >> > does that appear in the resulting mangling?
>>> >>
>>> >> > (That is, is an implementation required to effectively mangle the return
>>> >> > type and throw it away, or is some walk over the top-level type required
>>> >> > instead?)
>>> >>
>>> >> Hmm, good question.  The current G++ implementation scans dependent
>>> >> expressions in a (member function of a class) template, and considers
>>> >> those in both the signature and the return type of a non-template
>>> >> member function (which is not part of its signature), leading to a
>>> >> somewhat curious result for this example:
>>> >>
>>> >> struct [[gnu::abi_tag ("foo")]] A
>>> >> {
>>> >>   template <class T> static T f();
>>> >> };
>>> >>
>>> >> template <class T> struct B
>>> >> {
>>> >>   static decltype(A::f<T>()) g(decltype(A::f<T>()));
>>> >>   static decltype(A::f<T>()) h();
>>> >> };
>>> >>
>>> >> int main()
>>> >> {
>>> >>   B<int>::g(0); // _ZN1BIiE1gEi -- the tag does not appear in the mangled name
>>> >>   B<int>::h();  // _ZN1BIiE1hB3fooEv
>>> >> }
>>> >>
>>> >> Here G++ sees that A (and thus tag "foo") appears in the signature of
>>> >> B<T>::g, so it doesn't tag B<T>::g, even though the dependent
>>> >> expression isn't actually part of the eventual mangling of B<int>::g
>>> >> because for non-template member functions we mangle the instantiated
>>> >> type rather than the temploid type.  The tag doesn't appear in the
>>> >> signature of B<T>::h, so h gets tagged.
>>> >
>>> > Is that how this is supposed to work, or is it a bug? It would seem more
>>> > reasonable to say the tag set added after to the declaration's name is the
>>> > tag set from scanning the original unsubstituted declaration, minus the tags
>>> > produced by mangling (and I suspect that probably doesn't change the
>>> > mangling for any cases in the wild).
>>>
>>> It does seem like a bug.  For non-template member functions, since the
>>> signature we mangle is the fully-instantiated signature, it probably
>>> also makes sense to use that signature in determining tags, so that
>>> neither of the above functions would mention a tag:
>>>
>>> struct [[gnu::abi_tag ("foo")]] A
>>> {
>>>   template <class T> static T f();
>>>   template <class T> static A g();
>>> };
>>>
>>> template <class T> struct B
>>> {
>>>   static decltype(A::f<T>()) fa(decltype(A::f<T>()));
>>>   static decltype(A::f<T>()) fv();
>>>   static decltype(A::g<T>()) ga(decltype(A::g<T>()));
>>>   static decltype(A::g<T>()) gv();
>>> };
>>>
>>> int main()
>>> {
>>>   B<int>::fa(0);   // _ZN1BIiE2faEi
>>>   B<int>::fv();    // _ZN1BIiE2fvEv
>>>   B<int>::ga(A()); // _ZN1BIiE2gaE1AB3foo
>>>   B<int>::gv();    // _ZN1BIiE2gvB3fooEv
>>> }
>>>
>>> Thoughts?
>>
>> Seems reasonable to me. This runs the risk of the ABI variance existing only
>> in the SFINAE condition, rather than in the signature, but that's probably
>> not an issue in practice.
>
> It shouldn't be an issue at all, since SFINAE only applies to member
> templates, for which we already mangle the return type, not the above
> member functions of a class template.

>> Would the upshot then be that the tags are always
>> determined as if by mangling the declared return type (after substitution)
>> and checking which tags would appear?
>
> Yes.

So, here's the total proposed addition to the current ABI document.

https://github.com/MentorEmbedded/cxx-abi/compare/gh-pages...jicama:gh-pages

> Did you have a reaction to Dmitry's mail?

Jason