[Bug c++/80265] __builtin_{memcmp,memchr,strlen} are not usable in constexpr functions

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80265

--- Comment #20 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Author: redi
Date: Tue Sep 12 16:27:01 2017
New Revision: 252030

URL: https://gcc.gnu.org/viewcvs?rev=252030&root=gcc&view=rev
Log:
Finish implementing P0426R1 "Constexpr for std::char_traits" for C++17

As discussed in PR c++/80265 ("__builtin_{memcmp,memchr,strlen} are
not usable in constexpr functions"), use __builtin_constant_p to tell
whether we can defer to a constexpr algorithm.

I used __always_inline__ just to be thorough.  It isn't really really
necessary as far as I could determine.

Changes like these:

         if (__n == 0)
           return 0;
 -      return wmemcmp(__s1, __s2, __n);
 +      else
 +        return wmemcmp(__s1, __s2, __n);

are necessary otherwise G++ complains that we're calling a
non-constexpr function, which looks like a a manifestation of PR67026
to me.

libstdc++-v3:
2017-06-12  Pedro Alves  <palves@redhat.com>

        * doc/xml/manual/status_cxx2017.xml: Update C++17 constexpr
        char_traits status.
        * doc/html/*: Regenerate.

        * include/bits/char_traits.h (_GLIBCXX_ALWAYS_INLINE): Define if
        not already defined.
        (__cpp_lib_constexpr_char_traits): Uncomment.
        (__constant_string_p, __constant_char_array_p): New.
        (std::char_traits<char>, std::char_traits<wchar_t>): Add
        _GLIBCXX17_CONSTEXPR on compare, length and find and use
        __constant_string_p, __constant_char_array_p and
        __builtin_constant_p to defer to __gnu_cxx::char_traits at compile
        time.

        * testsuite/21_strings/char_traits/requirements/
        constexpr_functions_c++17.cc: Uncomment
        __cpp_lib_constexpr_char_traits tests.  Uncomment
        test_compare<char>, test_length<char>, test_find<char>,
        test_compare<wchar_t>, test_length<wchar_t> and test_find<wchar_t>
        static_assert tests.

Modified:
    branches/gcc-7-branch/libstdc++-v3/ChangeLog
    branches/gcc-7-branch/libstdc++-v3/doc/html/manual/status.html
    branches/gcc-7-branch/libstdc++-v3/doc/xml/manual/status_cxx2017.xml
    branches/gcc-7-branch/libstdc++-v3/include/bits/char_traits.h
   
branches/gcc-7-branch/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++17.cc