[cfe-dev] [RFC] Attribute that can be used to instruct clang to pass and return non-trivial structs directly

David Blaikie via cfe-dev cfe-dev at lists.llvm.org
Thu Nov 16 10:09:42 PST 2017

On Wed, Nov 15, 2017 at 8:09 PM John McCall <rjmccall at apple.com> wrote:

> On Nov 15, 2017, at 10:45 PM, David Blaikie <dblaikie at gmail.com> wrote:
> On Wed, Nov 15, 2017 at 11:56 AM Richard Smith <richard at metafoo.co.uk>
> wrote:
>> On 15 November 2017 at 11:34, John McCall via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>> On Nov 15, 2017, at 11:13 AM, David Blaikie <dblaikie at gmail.com> wrote:
>>> On Tue, Nov 14, 2017 at 12:13 PM John McCall via cfe-dev <
>>> cfe-dev at lists.llvm.org> wrote:
>>>> I don't think we'd thought about documenting this (additionally) in
>>>> terms of precise edits to the standard, but that's not a bad idea.
>>> Wasn't there a Clang policy about language extensions that required they
>>> be at least proposed for standardization? (I can't seem to find that
>>> anymore, but I think Doug proposed it/wrote it up at some point)
>>> Maybe attributes don't fall under this policy? Not sure.
>>> That's a very good question.  I remember us talking about that, but I
>>> don't think it ever turned into a firm policy.
>> It's in a somewhat non-obvious place on the website:
>> http://clang.llvm.org/get_involved.html
> Ah, thanks! I kept getting search results listing that page & figured it
> was a false positive... I should've looked more closely.
> Sounds like (4) is the only sticky one - and I guess this would be partly
> the C++ standards committee and the Itanium ABI group. (perhaps this is
> worth a discussion/proposal on the latter - to avoid anything like the
> abi_tag difficulties (which I only have a vague sense of)?)
> I'm not quite sure what connection you're drawing here.  In this case, I
> think (4) is satisfied because Apple is promising to maintain the feature,
> as we will have internal clients that will rely on it — although it sounds
> like enough other people are interested in it that that really won't be a
> problem.

Ah, sorry, I didn't mean to refer to (4) in your list, but (4) in the list
on "Contributing Extensions to Clang" here:

Which says fairly unambiguously: "the extension itself must have an active
proposal and proponent within that committee and have a reasonable chance
of acceptance. ... This criterion does not apply to all extensions, since
some extensions fall outside of the realm of the standards bodies."

It sounds like this bullet point could just be softened a little somehow to
accommodate this situation?

Or maybe it does apply & we should make a point of bringing it up to the
Itanium ABI group?

(I think since the only surface area in C++ is an attribute, and C++ has
syntax entirely designed for extensibility here, there's no immediate need
to bring it up there - it'd barely be meaningful to standardize it since
it's such an implementation detail, I'd guess... - though I suppose the
contract is useful "this type doesn't care about where its bits are in
memory" is probably a generically useful property to discuss in the C++
standard, but easy to move from custom attribute to standard attribute, etc)

> I think what you may be suggesting is that, if we're indeed going to
> revise Clang's policy about features, we should include a codicil that
> encourages people to seek standardization of any new Clang vendor
> extensions when that's reasonably possible.  In practice, the committees
> will not standardize the Clang feature, they'll standardize their own
> hopefully-similar feature, but it's still useful to seek standardization.
> This is closely related to Richard's point about trying to ensure that new
> features don't drive fragmentation.
> Mostly I'm just want to make sure we hold ourselves to a similar standard
> than we expect from everyone else - rather than using the rules as a way to
> keep people out while not necessarily meeting the same bar ourselves. &
> this seems like a good chance to reflect on the rules, see if they do/still
> fit, etc. Sounds like they mostly do.
> Don't mean to bog anything down in bureaucracy or anything.
> No, you're absolutely right to bring it up.
> Should I draft a revision to the policy?  Any other initial commentary
> before I do?
> John.
> - Dave
>>> I think the important points about language features are:
>>> 1. We don't want to take a feature that we don't like the design of
>>> unless we're forced to by a language standard.  We're allowed to be
>>> opinionated about language design!  Required, even.
>>> 2. We don't want to take a feature that's poorly-specified (again,
>>> unless we're forced to by a language standard :)).  The specification
>>> doesn't have to be expressed in terms of precise edits to a standard —
>>> among other things, this would often be really annoying, since a lot of
>>> features are intended to apply in both C and C++, and they may have
>>> implications for other extensions like ObjC/OpenMP/whatever  — but it
>>> should be at a point where such edits are reasonably extrapolable.  I
>>> wouldn't say that it needs to be something that we can imagine an actual
>>> standards committee taking, since there are a lot of reasons a committee
>>> might reject a feature that don't necessarily imply a lack of quality;
>>> also, this would be rather inconsistent of us, since we've certainly taken
>>> features in the past that I'm not sure have much chance of standardization.
>>> 3. We want to be very cautious about accepting new language syntax
>>> because it could infringe on future language evolution.  This is one place
>>> where attribute-only features have a substantial advantage.
>>> 4. We want major language features to be maintained.  The concern here
>>> grows with the amount of code contributed and how tightly it needs to be
>>> integrated with the rest of the compiler.  This is one of those area where
>>> life is not really fair, because we can't realistically assume that any
>>> single contributor is going to be able to commit to maintaining a feature
>>> the same way that an organization can.  For example, I personally have a
>>> long history of contributing to Clang, and I think the language designs
>>> I've contributed have been relatively good — but if I proposed a language
>>> feature on my own behalf, without any commitment from Apple or anyone else
>>> to continue maintaining that contribution if e.g. I got hit by a bus, I'm
>>> not sure it would be reasonable for the project to accept my proposal.
>>> And implicit in all of these is that the feature ought to be
>>> "open-source" — if you're going to propose a novel, non-standard feature,
>>> you need to be willing to accept feedback about both the specification and
>>> its basic design, and it really shouldn't depend on anything proprietary
>>> like a closed-source runtime library.  We're allowed as a project to be
>>> opinionated about this sort of thing, too.
>>> But I think if we like the feature, and we like its specification, and
>>> we don't think it infringes on language evolution, and we have strong
>>> reason to think it's going to be maintained, we don't need to hew tightly
>>> to a "no new features" mandate.
>> That seems essentially reasonable to me.  We also need to be cognizant of
>> the possibility of fracturing the developer community if our extensions
>> fundamentally change the way that code is written. (Which is not the same
>> as saying we can't have such extensions, just that they need to be
>> especially welll-considered, and we should have a very good reason if we're
>> not attempting to standardize them. Clang's header modules support falls
>> somewhat into this category.)
>> Attribute-based features don't get an automatic pass, but by their nature
>> they're much more likely to meet these criteria.
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