[PATCH v2] Let fpclassify use the builtin when optimizing for size in C++ mode (bug 22146)

["Gabriel F. T. Gomes" <gabriel at inconstante dot eti dot br>]

Changes since v1:

  - Change in patch subject (was:
    https://sourceware.org/ml/libc-alpha/2017-09/msg00785.html)
  - Do not provide fpclassify as a function.  fpclassify must be a
    macro, so that libstdc++ can undefine it and provide its own
    implementation as a function.
  - Instead, only let fpclassify use the builtin when in C++ mode (and
    even if optimization for size is on).

-- 8< --
When optimization for size is on (-Os), fpclassify does not use the
type-generic __builtin_fpclassify 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 fpclassify, 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
fpclassify 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 fpclassify use the builtin in C++ mode, even when
optimization for size is on.  This allows the configure test in
libstdc++ to work.

Tested for powerpc64le and x86_64.

	[BZ #22146]
	math/math.h: Let fpclassify use the builtin in C++ mode, even
	when optimazing for size.
---
 math/math.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/math/math.h b/math/math.h
index 6c2ad97fb8..c6c289d5d2 100644
--- a/math/math.h
+++ b/math/math.h
@@ -432,7 +432,13 @@ enum
 
 /* Return number of classification appropriate for X.  */
 # if __GNUC_PREREQ (4,4) && !defined __SUPPORT_SNAN__			      \
-     && !defined __OPTIMIZE_SIZE__
+     && (!defined __OPTIMIZE_SIZE__ || defined __cplusplus)
+     /* The check for __cplusplus allows the use of the builtin, even
+	when optimization for size is on.  This is provided for
+	libstdc++, only to let its configure test work when it is built
+	with -Os.  No further use of this definition of fpclassify is
+	expected in C++ mode, since libstdc++ provides its own version
+	of fpclassify in cmath (which undefines fpclassify).  */
 #  define fpclassify(x) __builtin_fpclassify (FP_NAN, FP_INFINITE,	      \
      FP_NORMAL, FP_SUBNORMAL, FP_ZERO, x)
 # else
-- 
2.13.5