[PATCH] libstdc++: Small extended float support tweaks

[Jakub Jelinek via Gcc-patches]

Hi!

The following patch isn't for immediate commit, as it has several
dependencies, in particular:
https://gcc.gnu.org/pipermail/gcc-patches/2022-October/603665.html
https://gcc.gnu.org/pipermail/gcc-patches/2022-October/604080.html
https://gcc.gnu.org/pipermail/libstdc++/2022-October/054849.html
On top of those, this patch
1) enables the std::float128_t overloads for x86 with glibc 2.26+
2) makes std::nextafter(std::float16_t, std::float16_t) and
   std::nextafter(std::bfloat16_t, std::bfloat16_t) constexpr
3) adds (small) testsuite coverage for that

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk
if/when the above dependencies are in?

2022-10-21  Jakub Jelinek  <ja...@redhat.com>

        * config/os/gnu-linux/os_defines.h (_GLIBCXX_HAVE_FLOAT128_MATH):
        Uncomment.
        * include/c_global/cmath (nextafter(_Float16, _Float16)): Make it 
constexpr.
        If std::__is_constant_evaluated() call __builtin_nextafterf16.
        (nextafter(__gnu_cxx::__bfloat16_t, __gnu_cxx::__bfloat16_t): Similarly
        but call __builtin_nextafterf16b.
        * testsuite/26_numerics/headers/cmath/nextafter_c++23.cc (test): Add
        static assertions to test constexpr nextafter.

--- libstdc++-v3/config/os/gnu-linux/os_defines.h.jj    2022-10-18 
11:35:55.514865483 +0200
+++ libstdc++-v3/config/os/gnu-linux/os_defines.h       2022-10-20 
16:57:59.715681664 +0200
@@ -57,7 +57,7 @@
        || (defined(__powerpc__) && defined(_ARCH_PWR8) \
            && defined(__LITTLE_ENDIAN__) && (_CALL_ELF == 2) \
            && defined(__FLOAT128__)))
-//# define _GLIBCXX_HAVE_FLOAT128_MATH 1
+# define _GLIBCXX_HAVE_FLOAT128_MATH 1
 #endif
 
 #if __GLIBC_PREREQ(2, 27)
--- libstdc++-v3/include/c_global/cmath.jj      2022-10-19 11:23:51.484488161 
+0200
+++ libstdc++-v3/include/c_global/cmath 2022-10-20 17:03:56.760805581 +0200
@@ -2755,9 +2755,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   nearbyint(_Float16 __x)
   { return _Float16(__builtin_nearbyintf(__x)); }
 
-  inline _Float16
+  constexpr _Float16
   nextafter(_Float16 __x, _Float16 __y)
   {
+    if (std::__is_constant_evaluated())
+      return __builtin_nextafterf16(__x, __y);
 #ifdef __INT16_TYPE__
     using __float16_int_type = __INT16_TYPE__;
 #else
@@ -3471,9 +3473,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   nearbyint(__gnu_cxx::__bfloat16_t __x)
   { return __gnu_cxx::__bfloat16_t(__builtin_nearbyintf(__x)); }
 
-  inline __gnu_cxx::__bfloat16_t
+  constexpr __gnu_cxx::__bfloat16_t
   nextafter(__gnu_cxx::__bfloat16_t __x, __gnu_cxx::__bfloat16_t __y)
   {
+    if (std::__is_constant_evaluated())
+      return __builtin_nextafterf16b(__x, __y);
 #ifdef __INT16_TYPE__
     using __bfloat16_int_type = __INT16_TYPE__;
 #else
--- libstdc++-v3/testsuite/26_numerics/headers/cmath/nextafter_c++23.cc.jj      
2022-10-20 16:57:29.940088318 +0200
+++ libstdc++-v3/testsuite/26_numerics/headers/cmath/nextafter_c++23.cc 
2022-10-20 17:19:40.141923257 +0200
@@ -100,6 +100,8 @@ test ()
   VERIFY( std::fpclassify(t36) == FP_NAN );
   T t37 = std::nextafter(T(-0.0), T());
   VERIFY( t37 == T() && !std::signbit(t37) );
+  static_assert(std::nextafter(T(1.0), T(2.0)) > T(1.0));
+  static_assert(std::nextafter(std::nextafter(T(1.0), T(5.0)), T(0.0)) == 
T(1.0));
 }
 
 int

        Jakub