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, 79 insertions, 0 deletions

diff --git a/math/test-math-iszero.cc b/math/test-math-iszero.cc
index 027e972654..5c07261626 100644
--- a/math/test-math-iszero.cc
+++ b/math/test-math-iszero.cc
@@ -22,6 +22,13 @@
 
 #include <limits>
 
+/* Support for _Float128 in std::numeric_limits is limited.
+   Include ieee754_float128.h and use the bitfields in the union
+   ieee854_float128.ieee_nan to build corner-case inputs.  */
+#if __HAVE_DISTINCT_FLOAT128
+# include <ieee754_float128.h>
+#endif
+
 static bool errors;
 
 static void
@@ -72,12 +79,84 @@ check_type ()
          std::numeric_limits<T>::has_denorm == std::denorm_absent);
 }
 
+#if __HAVE_DISTINCT_FLOAT128
+static void
+check_float128 ()
+{
+  ieee854_float128 q;
+
+  q.d = 0.0Q;
+  CHECK (iszero (q.d), 1);
+  q.d = -0.0Q;
+  CHECK (iszero (q.d), 1);
+  q.d = 1.0Q;
+  CHECK (iszero (q.d), 0);
+  q.d = -1.0Q;
+  CHECK (iszero (q.d), 0);
+
+  /* Normal min.  */
+  q.ieee.negative = 0;
+  q.ieee.exponent = 0x0001;
+  q.ieee.mantissa0 = 0x0000;
+  q.ieee.mantissa1 = 0x00000000;
+  q.ieee.mantissa2 = 0x00000000;
+  q.ieee.mantissa3 = 0x00000000;
+  CHECK (iszero (q.d), 0);
+  q.ieee.negative = 1;
+  CHECK (iszero (q.d), 0);
+
+  /* Normal max.  */
+  q.ieee.negative = 0;
+  q.ieee.exponent = 0x7FFE;
+  q.ieee.mantissa0 = 0xFFFF;
+  q.ieee.mantissa1 = 0xFFFFFFFF;
+  q.ieee.mantissa2 = 0xFFFFFFFF;
+  q.ieee.mantissa3 = 0xFFFFFFFF;
+  CHECK (iszero (q.d), 0);
+  q.ieee.negative = 1;
+  CHECK (iszero (q.d), 0);
+
+  /* Infinity.  */
+  q.ieee.negative = 0;
+  q.ieee.exponent = 0x7FFF;
+  q.ieee.mantissa0 = 0x0000;
+  q.ieee.mantissa1 = 0x00000000;
+  q.ieee.mantissa2 = 0x00000000;
+  q.ieee.mantissa3 = 0x00000000;
+  CHECK (iszero (q.d), 0);
+
+  /* Quiet NaN.  */
+  q.ieee_nan.quiet_nan = 1;
+  q.ieee_nan.mantissa0 = 0x0000;
+  CHECK (iszero (q.d), 0);
+
+  /* Signaling NaN.  */
+  q.ieee_nan.quiet_nan = 0;
+  q.ieee_nan.mantissa0 = 0x4000;
+  CHECK (iszero (q.d), 0);
+
+  /* Denormal min.  */
+  q.ieee.negative = 0;
+  q.ieee.exponent = 0x0000;
+  q.ieee.mantissa0 = 0x0000;
+  q.ieee.mantissa1 = 0x00000000;
+  q.ieee.mantissa2 = 0x00000000;
+  q.ieee.mantissa3 = 0x00000001;
+  CHECK (iszero (q.d), 0);
+  q.ieee.negative = 1;
+  CHECK (iszero (q.d), 0);
+}
+#endif
+
 static int
 do_test (void)
 {
   check_type<float> ();
   check_type<double> ();
   check_type<long double> ();
+#if __HAVE_DISTINCT_FLOAT128
+  check_float128 ();
+#endif
   return errors;
 }