[PATCH v2] Let signbit use the builtin in C++ mode with gcc < 6.x (bug 22146)

[Romain Naour <romain dot naour at gmail dot com>]

When using gcc < 6.x, signbit does not use the type-generic
__builtin_signbit builtin, instead it uses __MATH_TG.
However, when library support for float128 is available, __MATH_TG uses
__builtin_types_compatible_p, which is not available in C++ mode.

On the other hand, libstdc++ undefines (in cmath) many macros from
math.h, including signbit, so that it can provide its own functions.
However, during its configure tests, libstdc++ just tests for the
availability of the macros (it does not undefine them, nor does it
provide its own functions).

Finally, when libstdc++ is configured with optimization for size
enabled, its configure tests include math.h and get the definition of
signbit that uses __MATH_TG (and __builtin_types_compatible_p).
Since libstdc++ does not undefine the macros during its configure
tests, they fail.

This patch lets signbit use the builtin in C++ mode when gcc < 6.x is
used. This allows the configure test in libstdc++ to work.

Tested for x86_64.

	[BZ #22296]
	math/math.h: Let signbit use the builtin in C++ mode with gcc
	< 6.x

Cc: Gabriel F. T. Gomes <gftg@linux.vnet.ibm.com>
Cc: Joseph Myers <joseph@codesourcery.com>
---
v2
Rework the patch following the review:
https://sourceware.org/ml/libc-alpha/2017-09/msg00787.html
---
 ChangeLog   | 5 +++++
 math/math.h | 9 +++++++++
 2 files changed, 14 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index f3b5e8c..de87072 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2017-10-16  Romain Naour <romain.naour@gmail.com>  (tiny change)
+
+	[BZ #22296]
+	math/math.h: Let signbit use the builtin in C++ mode with gcc
+	< 6.x
 2017-10-16  Florian Weimer  <fweimer@redhat.com>
 
 	* version.h (VERSION): Switch to ".9000" as the development
diff --git a/math/math.h b/math/math.h
index faa24817..5ad8156 100644
--- a/math/math.h
+++ b/math/math.h
@@ -448,6 +448,15 @@ enum
 /* Return nonzero value if sign of X is negative.  */
 # if __GNUC_PREREQ (6,0)
 #  define signbit(x) __builtin_signbit (x)
+# elif defined __cplusplus
+  /* In C++ mode, __MATH_TG cannot be used, because it relies on
+     __builtin_types_compatible_p, which is a C-only builtin.
+     The check for __cplusplus allows the use of the builtin instead of
+     __MATH_TG. This is provided for libstdc++, only to let its configure
+     test work. No further use of this definition of signbit is expected
+     in C++ mode, since libstdc++ provides its own version of signbit
+     in cmath (which undefines signbit). */
+#  define signbit(x) __builtin_signbitl (x)
 # elif __GNUC_PREREQ (4,0)
 #  define signbit(x) __MATH_TG ((x), __builtin_signbit, (x))
 # else
-- 
2.9.5