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authorRomain Naour <romain.naour@gmail.com>2017-10-16 23:21:56 +0200
committerGabriel F. T. Gomes <gabriel@inconstante.eti.br>2017-10-17 12:06:44 -0200
commit386e1c26ac473d6863133ab9cbe3bbda16c15816 (patch)
tree7c645ee0d6c11960740cd05edbcd840db4eae6ea /math
parent3d265601358974c9be8264e53dadd8f9aea5690c (diff)
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Let signbit use the builtin in C++ mode with gcc < 6.x (bug 22296)
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, libstdc++ 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>
Diffstat (limited to 'math')
-rw-r--r--math/math.h9
1 files changed, 9 insertions, 0 deletions
diff --git a/math/math.h b/math/math.h
index faa2481731..5ad8156555 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