[PATCH] D41039: Add support for attribute "trivial_abi"

[David Blaikie via cfe-commits]

On Mon, Dec 11, 2017 at 3:16 PM John McCall via Phabricator <
reviews at reviews.llvm.org> wrote:

> rjmccall added a comment.
>
> In https://reviews.llvm.org/D41039#951648, @ahatanak wrote:
>
> > I had a discussion with Duncan today and he pointed out that perhaps we
> shouldn't allow users to annotate a struct with "trivial_abi" if one of its
> subobjects is non-trivial and is not annotated with "trivial_abi" since
> that gives users too much power.
> >
> > Should we error out or drop "trivial_abi" from struct Outer when the
> following code is compiled?
> >
> >   struct Inner1 {
> >     ~Inner1(); // non-trivial
> >     int x;
> >   };
> >
> >   struct __attribute__((trivial_abi)) Outer {
> >     ~Outer();
> >     Inner1 x;
> >   };
> >
> >
> > The current patch doesn't error out or drop the attribute, but the patch
> would probably be much simpler if we didn't allow it.
>
>
> I think it makes sense to emit an error if there is provably a
> non-trivial-ABI component.  However, for class temploids I think that
> diagnostic should only fire on the definition, not on instantiations; for
> example:
>
>   template <class T> struct __attribute__((trivial_abi)) holder {
>      T value;
>      ~holder() {}
>   };
>   holder<std::string> hs; // this instantiation should be legal despite
> the fact that holder<std::string> cannot be trivial-ABI.
>
> But we should still be able to emit the diagnostic in template
> definitions, e.g.:
>
>   template <class T> struct __attribute__((trivial_abi)) named_holder {
>      std::string name; // there are no instantiations of this template
> that could ever be trivial-ABI
>      T value;
>      ~named_holder() {}
>   };
>
> The wording should be something akin to the standard template rule that a
> template is ill-formed if it has no valid instantiations, no diagnostic
> required.
>
> I would definitely like to open the conversation about the name of the
> attribute.  I don't think we've used "abi" in an existing attribute name;
> usually it's more descriptive.  And "trivial" is a weighty word in the
> standard.  I'm not sure I have a great counter-proposal off the top of my
> head, though.
>

Agreed on both counts (would love a better name, don't have any stand-out
candidates off the top of my head).

I feel like a more descriptive term about the property of the object would
make me happier - something like "address_independent_identity"
(s/identity/value/?) though, yeah, that's not spectacular by any stretch.


>
>
> https://reviews.llvm.org/D41039
>
>
>
>
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