Adding __float128 (i.e TS 18661-3) [v2]

[Joseph Myers]

On Fri, 10 Jun 2016, Paul E. Murphy wrote:

> TS 18661-3 defines a number of new macros which should be
> added to float.h.  This is a compiler header, we should
> avoid injecting any of these macros into a user
> compilation.

Once my patch <https://gcc.gnu.org/ml/gcc-patches/2016-06/msg01481.html> 
is in GCC, users can get those macros from <float.h> by using GCC 7 or 
later (and defining __STDC_WANT_IEC_60559_TYPES_EXT__ before including 
<float.h>).

I think it's fairly clear what glibc should do for internal use of such 
macros: have an internal-only wrapper include/float.h along the lines of:

#ifndef _ISOMAC
# define __STDC_WANT_IEC_60559_TYPES_EXT__
#endif
#include_next <float.h>
#ifndef _ISOMAC
# include <features.h>
# if !__GNUC_PREREQ (7, 0)
/* Define whatever macros such as FLT128_MAX are needed when building
   glibc, if on a platform where those macros are needed.  */
# endif
#endif

(and the final part would be removed once GCC 7 or later can be presumed 
for building glibc).

(Similarly, everything else in the glibc float128 support should be 
written on the basis that _Float128 type names, f128 constant suffixes 
etc. are the preferred form to use, with __float128, q suffixes etc. being 
fallbacks for older compilers.)

-- 
Joseph S. Myers
joseph@codesourcery.com