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

[David Blaikie via cfe-commits]

On Mon, Dec 11, 2017 at 5:38 PM John McCall <rjmccall at gmail.com> wrote:

> On Mon, Dec 11, 2017 at 6:19 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>> 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.
>>
>
> Incidentally, your comments are not showing up on Phabricator for some
> reason.
>

Yeah, Phab doesn't do a great job looking on the mailing list for
interesting replies - I forget, maybe it only catches bottom post or top
post but not infix replies or something...


>
> The term "trivially movable" suggests itself, with two caveats:
>   - What we're talking about is trivial *destructive* movability, i.e.
> that the combination of moving the value to a new object and not destroying
> the old object can be done trivially, which is not quite the same as
> trivial movability in the normal C++ sense, which I guess could be a
> property that someone theoretically might care about (if the type is
> trivially destructed, but it isn't copyable for semantic reasons?).
>   - Trivial destructive movability is a really common property, and it's
> something that a compiler would really like to optimize based on even in
> cases where trivial_abi can't be adopted for binary-compatibility reasons.
> Stealing the term for the stronger property that the type is trivially
> destructively movable *and its ABI should reflect that in a specific way*
> would be unfortunate.
>

Fair point.


> "trivially_movable" is a long attribute anyway, and
> "trivially_destructively_movable" is even worse.
>
> Maybe that second point is telling us that this isn't purely descriptive —
> it's inherently talking about the ABI, not just the semantics of the type.
> I might be talking myself into accepting trivial_abi if we don't end up
> with a better suggestion.
>

*nod* I think if you want to slice this that way (ensuring there's space to
support describing a similar property for use only in non-ABI-breaking
contexts as well) it seems like ABI is the term to use somewhere in the
name.


> Random thing that occurred to me: is it actually reasonable to enforce
> trivial_abi correctness in a non-template context?  Templates aren't the
> only case where a user could reasonably want to add trivial_abi and just
> have it be suppressed if it's wrong.  Imagine if some stdlib made
> std::string trivial_abi; someone might reasonably want to make my
> named_holder example above trivial_abi as well, with the expectation that
> it would only have an effect on targets where std::string was trivial_abi.
> At the very least, I'm concerned that we might be opening ourselves up to a
> need to add supporting features, like a way to be conditionally trivial_abi
> based on context.
>

Fair point, much like the quirky but useful behavior of "= default". Good
point about non-dependent contexts still being relevant to this subjective
behavior...

I was already leaning towards this being a warning rather than an error -
this situation leans me moreso that way & possibly suppressing the warning
when the types are split between system and non-system headers (if the
attribute's on a type in a non-system header, but the type that's blocking
it from being active is in a system header, don't warn).


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