Re: [PATCH 04/11 v3] libstdc++: Make use of __builtin_bit_cast

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Jun 24, 2021 at 03:11:01PM +0100, Jonathan Wakely wrote:
> On Thu, 24 Jun 2021 at 15:08, Jakub Jelinek wrote:
> >
> > On Thu, Jun 24, 2021 at 04:01:34PM +0200, Matthias Kretz wrote:
> > > --- a/libstdc++-v3/include/experimental/bits/simd.h
> > > +++ b/libstdc++-v3/include/experimental/bits/simd.h
> > > @@ -1598,7 +1598,9 @@ template <typename _To, typename _From>
> > >    _GLIBCXX_SIMD_INTRINSIC constexpr _To
> > >    __bit_cast(const _From __x)
> > >    {
> > > -    // TODO: implement with / replace by __builtin_bit_cast ASAP
> > > +#if __has_builtin(__builtin_bit_cast)
> >
> > Shouldn't that use #if _GLIBCXX_HAS_BUILTIN(__builtin_bit_cast) in
> > c++config to define a new macro and use that macro here?
> > Though it is true that c++config already uses
> > #if __has_builtin(__builtin_is_constant_evaluated)
> > and so would fail miserably for compilers that don't support __has_builtin
> 
> GCC was the last of our supported compilers to implement
> __has_builtin, so for GCC trunk we can assume that it's always
> supported.

We don't support mixing GCC and libstdc++ versions, so I'm not worried
about GCC.  At least according to godbolt, already clang 3.0 supports it
which is 10 years old, so probably fine too, but ICC 19.0/19.1 still doesn't
support it, only ICC 2021 does.  And ICC 19.1 seems to be released in
October 2020.

So, wouldn't it be better not to #undef _GLIBCXX_HAS_BUILTIN, move its
definition a little bit earlier and use it also for
__builtin_is_constant_evaluated?

	Jakub