Re: [PATCH] Finish implementing P0426R1 "Constexpr for std::char_traits" for C++17

[Jonathan Wakely <jwakely at redhat dot com>]

On 23/04/17 19:33 +0100, Pedro Alves wrote:
> On 04/23/2017 06:54 PM, Pedro Alves wrote:
> > Hi!
> >
> > As I had suggested in PR c++/80265, here's a patch that uses
> > __builtin_constant_p to tell whether we can defer to a constexpr
> > algorithm, which avoids having to wait for compiler support.
> >
> > Unfortunately I ran out of cycles today to run a full bootstrap/regtest
> > cycle, but constexpr_functions_c++17.cc passes cleanly on
> > x86_64 GNU/Linux at least.
> >
> > If this looks like a reasonable approach, I can maybe try running
> > full tests on the gcc compile farm next week.
> >
> > WDYT?
>
> Reading the patch via the list made me spot something
> that was incorrect.
>
> > -      static char_type*
> > +      static _GLIBCXX17_CONSTEXPR char_type*
> >        assign(char_type* __s, size_t __n, char_type __a)
> >        {
> > +#if __cplusplus > 201402
> > +	if (__constant_string_p(__s)
>
> This should probably have been __constant_char_array_p, but now
> that I look again at the paper, I see I got myself confused -- this
> assign overload is not meant to be constexpr in the first place.
>
> So here's an updated patch that drops that bit.  (Same testing
> as before.)

Pedro, this is OK for trunk now we're in stage 1. Please go ahead and
commit it - thanks.

It's probably safe for gcc-7-branch too, but let's leave it on trunk
for a while first.