jwakely added inline comments.
================
Comment at: clang/include/clang/Basic/TokenKinds.def:528
+TYPE_TRAIT_1(__is_nothrow_copy_constructible, IsNothrowCopyConstructible,
KEYCXX)
+TYPE_TRAIT_1(__is_trivially_copy_constructible, IsTriviallyCopyConstructible,
KEYCXX)
TYPE_TRAIT_2(__reference_binds_to_temporary, ReferenceBindsToTemporary, KEYCXX)
----------------
cjdb wrote:
> ldionne wrote:
> > cjdb wrote:
> > > erichkeane wrote:
> > > > royjacobson wrote:
> > > > > erichkeane wrote:
> > > > > > cjdb wrote:
> > > > > > > erichkeane wrote:
> > > > > > > > So this one is a whole 'thing'. The Clang definition of
> > > > > > > > 'trivially copy constructible' is a few DRs behind. We should
> > > > > > > > probably discuss this with libcxx to make sure use of this
> > > > > > > > wouldn't be broken.
> > > > > > > I'd prefer to get those DRs in before finalising D135238 and
> > > > > > > subsequent ones. Do you know the DR numbers I should be looking
> > > > > > > at, or should I just poke npaperbot?
> > > > > > Not off the top of my head, Aaron and I both poked at it at one
> > > > > > point trying to get trivially constructible right at one point, but
> > > > > > I think we both gave up due to the ABI/versioning concerns.
> > > > > Maybe DR1734? Although it's about the trivially copyable trait, not
> > > > > trivially copy constructible.
> > > > >
> > > > Yeah, I think that was the DR, that number sounds familiar.
> > > The `__is_trivially_*` traits were, in part, what inspired the Great
> > > Split of D116208. I could remove them for now and revisit once I rip my
> > > hair out over these DRs, if that would substantially improve the chances
> > > of these commits landing (other commentary notwithstanding).
> > I am not sure I see a problem with the "triviality" part of this -- we
> > already use a compiler builtin for `std::is_trivially_constructible`, so I
> > would expect either this patch is fine, or we already have a latent bug in
> > libc++.
> >
> > I think I can echo @philnik's comment about this not necessarily providing
> > the biggest benefit since our implementation of
> > `std::is_trivially_copy_constructible` is a fairly trivial wrapper on top
> > of `__is_trivially_constructible`, but I wouldn't object to the patch on
> > that basis. I think it would probably be possible to instead provide a set
> > of basis builtin operations that we can then build all of the library type
> > traits on top of -- that would provide the highest bang-for-our-buck ratio.
> >
> > At the same time, there's something kind of enticing in the consistency of
> > defining every single type trait as a builtin, without exception. If that's
> > the end goal, I think that would also be neat and we'd likely regroup all
> > of our type traits under a single header, since each of them would
> > literally be a one liner.
> >
> > There's also the question of whether GCC provides these builtins -- if they
> > don't and if they don't have plans to, then we'd actually need to add
> > complexity in libc++ to support both, which we would be unlikely to do
> > given that there's probably not a huge compile-time performance benefit.
> >
> > TLDR, I think the two questions that can help gauge how much interest there
> > will be from libc++ to use this are:
> >
> > 1. Is the plan to provide *all* the type traits as builtins?
> > 2. Will GCC implement them?
> >
> > That being said, libc++ might not be the only potential user of these
> > builtins, so I wouldn't necessarily make it a hard requirement to satisfy
> > us.
> >
> > I think I can echo @philnik's comment about this not necessarily providing
> > the biggest benefit since our implementation of
> > `std::is_trivially_copy_constructible` is a fairly trivial wrapper on top
> > of `__is_trivially_constructible`, but I wouldn't object to the patch on
> > that basis.
>
> I haven't had time to do anything properly in the way of benchmarking, but
> after looking at @philnik's quoted code, I see that I'd naively addressed
> `__is_constructible(T, T const&)`, forgetting that `__add_lvalue_reference`
> would've fixed that issue.
>
> > 1. Is the plan to provide *all* the type traits as builtins?
>
> Yes, with the possible exception of `enable_if` and `add_const` etc. (see
> D116203 for why the qualifier ones aren't already in). The hardest ones will
> probably be `common_type`, `common_reference`, `*invocable*`, and
> `*swappable*`. The former two depend on technology that doesn't exist in
> Clang yet, and the latter two are likely hard due there not being prior art.
>
> > 2. Will GCC implement them?
>
> @jwakely do you know if there can be cross-compiler synergy here?
>
>
Which traits are you asking about, just the
__is_{,trivially,nothrow}_copy_constructible ones? Or all type traits?
Either way, I think the answer is no. We already use
__is_{,trivially,nothrow}_constructible for the copy-constructible traits, and
it works fine. I'm not aware of any problems with it. So I don't see any
benefit in adding these ones. As already mentioned, it creates *more* code to
maintain when using these traits, because we still have to support older
versions of other compilers that don't have all the new traits yet.
We've added traits for a few more things recently (`__remove_cv`,
`__remove_cvref`, `__remove_reference`) but they can only be used in limited
ways (you can't use them directly to implement `std::remove_cv_t` for example,
because they don't mangle to the same thing, which is observable). So I'm
sceptical about the benefits of implementing *every* trait as a built-in. There
are some which are bottlenecks and worth doing, and some which aren't.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D135238/new/
https://reviews.llvm.org/D135238
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