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.
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