Provide a C++ version of iszero that does not use __MATH_TG (bug 21930)

When signaling nans are enabled (with -fsignaling-nans), the C++ version
of iszero uses the fpclassify macro, which is defined with __MATH_TG.
However, when support for float128 is available, __MATH_TG uses the
builtin __builtin_types_compatible_p, which is only available in C mode.

This patch refactors the C++ version of iszero so that it uses function
overloading to select between the floating-point types, instead of
relying on fpclassify and __MATH_TG.

Tested for powerpc64le, s390x, x86_64, and with build-many-glibcs.py.

	[BZ #21930]
	* math/math.h [defined __cplusplus && defined __SUPPORT_SNAN__]
	(iszero): New C++ implementation that does not use
	fpclassify/__MATH_TG/__builtin_types_compatible_p, when
	signaling nans are enabled, since __builtin_types_compatible_p
	is a C-only feature.
	* math/test-math-iszero.cc: When __HAVE_DISTINCT_FLOAT128 is
	defined, include ieee754_float128.h for access to the union and
	member ieee854_float128.ieee.
	[__HAVE_DISTINCT_FLOAT128] (do_test): Call check_float128.
	[__HAVE_DISTINCT_FLOAT128] (check_float128): New function.
	* sysdeps/powerpc/powerpc64le/Makefile [subdir == math]
	(CXXFLAGS-test-math-iszero.cc): Add -mfloat128 to the build
	options of test-math-zero on powerpc64le.

1 files changed, 29 insertions, 4 deletions

diff --git a/math/math.h b/math/math.h
index 9162bee50c..f3eb677a40 100644
--- a/math/math.h
+++ b/math/math.h
@@ -513,15 +513,40 @@ inline int issignaling (_Float128 __val) { return __issignalingf128 (__val); }
 #  endif
 # else	/* __cplusplus */
 extern "C++" {
+#  ifdef __SUPPORT_SNAN__
+inline int
+iszero (float __val)
+{
+  return __fpclassifyf (__val) == FP_ZERO;
+}
+inline int
+iszero (double __val)
+{
+  return __fpclassify (__val) == FP_ZERO;
+}
+inline int
+iszero (long double __val)
+{
+#   ifdef __NO_LONG_DOUBLE_MATH
+  return __fpclassify (__val) == FP_ZERO;
+#   else
+  return __fpclassifyl (__val) == FP_ZERO;
+#   endif
+}
+#   if __HAVE_DISTINCT_FLOAT128
+inline int
+iszero (_Float128 __val)
+{
+  return __fpclassifyf128 (__val) == FP_ZERO;
+}
+#   endif
+#  else
 template <class __T> inline bool
 iszero (__T __val)
 {
-#  ifdef __SUPPORT_SNAN__
-  return fpclassify (__val) == FP_ZERO;
-#  else
   return __val == 0;
-#  endif
 }
+#  endif
 } /* extern C++ */
 # endif	/* __cplusplus */
 #endif /* Use IEC_60559_BFP_EXT.  */