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GNU C Library master sources branch master updated. glibc-2.26.9000-586-g386e1c2

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- Log -----------------------------------------------------------------;a=commitdiff;h=386e1c26ac473d6863133ab9cbe3bbda16c15816

commit 386e1c26ac473d6863133ab9cbe3bbda16c15816
Author: Romain Naour <>
Date:   Mon Oct 16 23:21:56 2017 +0200

    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 <>
    Cc: Joseph Myers <>

diff --git a/ChangeLog b/ChangeLog
index 64f5ca7..e3f0faa 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2017-10-17  Romain Naour  <>  (tiny change)
+	[BZ #22296]
+	* math/math.h: Let signbit use the builtin in C++ mode with gcc
+	< 6.x
 2017-10-17  Adhemerval Zanella  <>
 	* scripts/ (Context.add_all_configs):
diff --git a/math/math.h b/math/math.h
index faa2481..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


Summary of changes:
 ChangeLog   |    6 ++++++
 math/math.h |    9 +++++++++
 2 files changed, 15 insertions(+), 0 deletions(-)

GNU C Library master sources

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