[PATCH] Performance improvement for log()

[Steven Munroe <munroesj at linux dot vnet dot ibm dot com>]

While looking a customers found a number of problems with generic
implementation of log() and __ieee754_log().

First the macros that do extended precision (double double) arithmetic
use a ABS macro from gmp-impl.h

#define ABS(x) (x >= 0 ? x : -x)

This is unfortunate since PowerPC has a hardware fabs instruction but
GCC does not recognize the pattern and generates fcmpu/bc/fmr sequences
(instead of fabs instructions). This patch redefines ABS(x) as
__builtin_fabs(x).

Also the call out to __isnan() in the posix wrapper code (w_log()) was
showing up hot in the profiles. So I added --with-cpu=[power4|power6]
implementations that effectively in-line the isnan() test within
__log().

Attachment: ppc-log-20080303.txt
Description: Text document