[newlib-cygwin] Disable powf/log2?f/exp2?f optimization for single-precision Arm FPU

[Corinna Vinschen <corinna@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=7d09d0e26191c1ca66c25dcfe0e62328ce87276f

commit 7d09d0e26191c1ca66c25dcfe0e62328ce87276f
Author: Thomas Preudhomme <thomas.preudhomme@foss.arm.com>
Date:   Tue Jan 23 10:43:54 2018 +0000

    Disable powf/log2?f/exp2?f optimization for single-precision Arm FPU
    
    New optimized powf, logf, log2f, expf and exp2f yield worse performance
    on Arm targets with only single precision instructions because the
    double precision arithmetic is then implemented via softfloat routines.
    This patch uses the old implementation when double precision
    instructions are not available on Arm targets.
    
    Testing: Built newlib with GCC's rmprofile Arm multilibs and compared
    before/after -> only the above functions are changed and calls to them
    (name change from logf to __ieee754_logf and similar). Testing the
    changed function on a panel of values yields the same result before the
    original patches to improve them and after this one. Double checking the
    performance by looping the same panel of values being tested on Arm
    Cortex-M4 does show the performance regression is fixed.

Diff:
---
 newlib/libc/include/machine/ieeefp.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/newlib/libc/include/machine/ieeefp.h b/newlib/libc/include/machine/ieeefp.h
index b1b4466..9fbef84 100644
--- a/newlib/libc/include/machine/ieeefp.h
+++ b/newlib/libc/include/machine/ieeefp.h
@@ -78,7 +78,9 @@
 # else
 #  define __IEEE_BIG_ENDIAN
 # endif
-# define __OBSOLETE_MATH_DEFAULT 0
+# if __ARM_FP & 0x8
+#  define __OBSOLETE_MATH_DEFAULT 0
+# endif
 #else
 # define __IEEE_BIG_ENDIAN
 # ifdef __ARMEL__