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Let signbit use the builtin in C++ mode with gcc < 6.x (bug 22296)

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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>
This commit is contained in:
Romain Naour 2017-10-16 23:21:56 +02:00 committed by Gabriel F. T. Gomes
parent 3d26560135
commit 386e1c26ac
2 changed files with 15 additions and 0 deletions
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6
ChangeLog
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@ -1,3 +1,9 @@
2017-10-17 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
2017-10-17 Adhemerval Zanella <adhemerval.zanella@linaro.org>
* scripts/build-many-glibcs.py (Context.add_all_configs):

9
math/math.h
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@ -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