Provide a C++ version of issignaling that does not use __MATH_TG

The macro __MATH_TG contains the logic to select between long double and
_Float128, when these types are ABI-distinct.  This logic relies on
__builtin_types_compatible_p, which is not available in C++ mode.

On the other hand, C++ function overloading provides the means to
distinguish between the floating-point types.  The overloading
resolution will match the correct parameter regardless of type
qualifiers, i.e.: const and volatile.

Tested for powerpc64le, s390x, and x86_64.

	* math/math.h [defined __cplusplus] (issignaling): Provide a C++
	definition for issignaling that does not rely on __MATH_TG,
	since __MATH_TG uses __builtin_types_compatible_p, which is only
	available in C mode.
	(CFLAGS-test-math-issignaling.cc): New variable.
	* math/Makefile [CXX] (tests): Add test-math-issignaling.
	* math/test-math-issignaling.cc: New test for C++ implementation
	of type-generic issignaling.
	* sysdeps/powerpc/powerpc64le/Makefile [subdir == math]
	(CXXFLAGS-test-math-issignaling.cc): Add -mfloat128 to the build
	options of test-math-issignaling on powerpc64le.

1 files changed, 18 insertions, 1 deletions

diff --git a/math/math.h b/math/math.h
index 275534be11..b5d6c43fcf 100644
--- a/math/math.h
+++ b/math/math.h
@@ -474,7 +474,24 @@ enum
 # include <bits/iscanonical.h>
 
 /* Return nonzero value if X is a signaling NaN.  */
-# define issignaling(x) __MATH_TG ((x), __issignaling, (x))
+# ifndef __cplusplus
+#  define issignaling(x) __MATH_TG ((x), __issignaling, (x))
+# else
+   /* In C++ mode, __MATH_TG cannot be used, because it relies on
+      __builtin_types_compatible_p, which is a C-only builtin.  On the
+      other hand, overloading provides the means to distinguish between
+      the floating-point types.  The overloading resolution will match
+      the correct parameter (regardless of type qualifiers (i.e.: const
+      and volatile).  */
+extern "C++" {
+inline int issignaling (float __val) { return __issignalingf (__val); }
+inline int issignaling (double __val) { return __issignaling (__val); }
+inline int issignaling (long double __val) { return __issignalingl (__val); }
+#  if __HAVE_DISTINCT_FLOAT128
+inline int issignaling (_Float128 __val) { return __issignalingf128 (__val); }
+#  endif
+} /* extern C++ */
+# endif
 
 /* Return nonzero value if X is subnormal.  */
 # define issubnormal(x) (fpclassify (x) == FP_SUBNORMAL)