Update floating-point feature test macro handling for C2X

ISO C2X has made some changes to the handling of feature test macros
related to features from the floating-point TSes, and to exactly what
such features are present in what headers, that require corresponding
changes in glibc.

* For the few features that were controlled by
  __STDC_WANT_IEC_60559_BFP_EXT__ (and the corresponding DFP macro) in
  C2X, there is now instead a new feature test macro
  __STDC_WANT_IEC_60559_EXT__ covering both binary and decimal FP.
  This controls CR_DECIMAL_DIG in <float.h> (provided by GCC; I
  implemented support for the new feature test macro for GCC 11) and
  the totalorder and payload functions in <math.h>.  C2X no longer
  says anything about __STDC_WANT_IEC_60559_BFP_EXT__ (so it's
  appropriate for that macro to continue to enable exactly the
  features from TS 18661-1).

* The SNAN macros for each floating-point type have moved to <float.h>
  (and been renamed in the process).  Thus, the copies in <math.h>
  should only be defined for __STDC_WANT_IEC_60559_BFP_EXT__, not for
  C2X.

* The fmaxmag and fminmag functions have been removed (replaced by new
  functions for the new min/max operations in IEEE 754-2019).  Thus
  those should also only be declared for
  __STDC_WANT_IEC_60559_BFP_EXT__.

* The _FloatN / _FloatNx handling for the last two points in glibc is
  trickier, since __STDC_WANT_IEC_60559_TYPES_EXT__ is still in C2X
  (the integration of TS 18661-3 as an Annex, that is, which hasn't
  yet been merged into the C standard git repository but has been
  accepted by WG14), so C2X with that macro should not declare some
  things that are declared for older standards with that macro.  The
  approach taken here is to provide the declarations (when
  __STDC_WANT_IEC_60559_TYPES_EXT__ is enabled) only when (defined
  __USE_GNU || !__GLIBC_USE (ISOC2X)), so if C2X features are enabled
  then those declarations (that are only in TS 18661-3 and not in C2X)
  will only be provided if _GNU_SOURCE is defined as well.  Thus
  _GNU_SOURCE remains a superset of the TS features as well as of C2X.

Some other somewhat related changes in C2X are not addressed here.
There's an open proposal not to include the fmin and fmax functions
for the _FloatN / _FloatNx types, given the new min/max operations,
which could be handled like the previous point if adopted.  And the
fromfp functions have been changed to return a result in floating type
rather than intmax_t / uintmax_t; my inclination there is to treat
that like that change of totalorder type (new symbol versions etc. for
the ABI change; old versions become compat symbols and are no longer
supported as an API).

Tested for x86_64 and x86.

1 files changed, 26 insertions, 2 deletions

diff --git a/bits/libc-header-start.h b/bits/libc-header-start.h
index 296e1ce..47f3910 100644
--- a/bits/libc-header-start.h
+++ b/bits/libc-header-start.h
@@ -44,8 +44,26 @@
 
 /* ISO/IEC TS 18661-1:2014 defines the __STDC_WANT_IEC_60559_BFP_EXT__
    macro.  Most but not all symbols enabled by that macro in TS
-   18661-1 are enabled unconditionally in C2X; the symbols in Annex F
-   still require that macro in C2X.  */
+   18661-1 are enabled unconditionally in C2X.  In C2X, the symbols in
+   Annex F still require a new feature test macro
+   __STDC_WANT_IEC_60559_EXT__ instead (C2X does not define
+   __STDC_WANT_IEC_60559_BFP_EXT__), while a few features from TS
+   18661-1 are not included in C2X (and thus should depend on
+   __STDC_WANT_IEC_60559_BFP_EXT__ even when C2X features are
+   enabled).
+
+   __GLIBC_USE (IEC_60559_BFP_EXT) controls those features from TS
+   18661-1 not included in C2X.
+
+   __GLIBC_USE (IEC_60559_BFP_EXT_C2X) controls those features from TS
+   18661-1 that are also included in C2X (with no feature test macro
+   required in C2X).
+
+   __GLIBC_USE (IEC_60559_EXT) controls those features from TS 18661-1
+   that are included in C2X but conditional on
+   __STDC_WANT_IEC_60559_EXT__.  (There are currently no features
+   conditional on __STDC_WANT_IEC_60559_EXT__ that are not in TS
+   18661-1.)  */
 #undef __GLIBC_USE_IEC_60559_BFP_EXT
 #if defined __USE_GNU || defined __STDC_WANT_IEC_60559_BFP_EXT__
 # define __GLIBC_USE_IEC_60559_BFP_EXT 1
@@ -58,6 +76,12 @@
 #else
 # define __GLIBC_USE_IEC_60559_BFP_EXT_C2X 0
 #endif
+#undef __GLIBC_USE_IEC_60559_EXT
+#if __GLIBC_USE (IEC_60559_BFP_EXT) || defined __STDC_WANT_IEC_60559_EXT__
+# define __GLIBC_USE_IEC_60559_EXT 1
+#else
+# define __GLIBC_USE_IEC_60559_EXT 0
+#endif
 
 /* ISO/IEC TS 18661-4:2015 defines the
    __STDC_WANT_IEC_60559_FUNCS_EXT__ macro.  Other than the reduction