[Buildroot] [PATCH] package/glibc: fix signbit build issue with gcc < 6.x

[Romain Naour]

As reported in [1], mesa3d package fail to build with a x86_64 toolchain
using gcc 5.5.0 and glibc 2.26.

Add a new patch providing signbit() functions for C++ that does not use
__MATH_TG macro.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=22296

Signed-off-by: Romain Naour <romain.naour at gmail.com>
---
 ...-version-of-signbit-that-does-not-use-__M.patch | 65 ++++++++++++++++++++++
 1 file changed, 65 insertions(+)
 create mode 100644 package/glibc/0007-Provide-a-C-version-of-signbit-that-does-not-use-__M.patch

diff --git a/package/glibc/0007-Provide-a-C-version-of-signbit-that-does-not-use-__M.patch b/package/glibc/0007-Provide-a-C-version-of-signbit-that-does-not-use-__M.patch
new file mode 100644
index 0000000..ea7a4e7
--- /dev/null
+++ b/package/glibc/0007-Provide-a-C-version-of-signbit-that-does-not-use-__M.patch
@@ -0,0 +1,65 @@
+From 2a735f52ec321f06b2af4b4265160d54260e5c44 Mon Sep 17 00:00:00 2001
+From: Romain Naour <romain.naour at gmail.com>
+Date: Sun, 15 Oct 2017 20:04:24 +0200
+Subject: [PATCH] Provide a C++ version of signbit that does not use __MATH_TG
+ (bug 22296)
+
+The macro __MATH_TG contains the logic to select between long double and
+_Float128, when these types are ABI-distinct.  This logic relies on
+__builtin_types_compatible_p, which is not available in C++ mode.
+
+On the other hand, C++ function overloading provides the means to
+distinguish between the floating-point types.  The overloading
+resolution will match the correct parameter regardless of type
+qualifiers, i.e.: const and volatile.
+
+This is the same fix as for issignaling [1] but the issue appear
+only with gcc < 6.x. Since GCC 6.0, __builtin_signbit is type-generic,
+so signbit() is defined without __MATH_TG.
+
+Tested for x86_64.
+
+	* math/math.h [defined __cplusplus] (signbit): Provide a C++
+	definition for signbit that does not rely on __MATH_TG,
+	since __MATH_TG uses __builtin_types_compatible_p, which is only
+	available in C mode.
+
+[1] a16e8bc08edca84d507715c66d6cddbbc7ed3b62
+
+Signed-off-by: Romain Naour <romain.naour at gmail.com>
+---
+patch status: submited upstream
+https://sourceware.org/ml/libc-alpha/2017-10/msg00654.html
+---
+ math/math.h | 15 +++++++++++++++
+ 1 file changed, 15 insertions(+)
+
+diff --git a/math/math.h b/math/math.h
+index 2b216c6..0013ad9 100644
+--- a/math/math.h
++++ b/math/math.h
+@@ -418,6 +418,21 @@ 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.  On the
++      other hand, overloading provides the means to distinguish between
++      the floating-point types.  The overloading resolution will match
++      the correct parameter (regardless of type qualifiers (i.e.: const
++      and volatile).  */
++extern "C++" {
++inline int signbit (float __val) { return __signbitf (__val); }
++inline int signbit (double __val) { return __signbit (__val); }
++inline int signbit (long double __val) { return __signbitl (__val); }
++#  if __HAVE_DISTINCT_FLOAT128
++inline int signbit (_Float128 __val) { return __signbitf128 (__val); }
++#  endif
++} /* extern C++ */
+ # elif __GNUC_PREREQ (4,0)
+ #  define signbit(x) __MATH_TG ((x), __builtin_signbit, (x))
+ # else
+-- 
+2.9.5
+
-- 
2.9.5