use of fma

[Paul Zimmermann <Paul.Zimmermann@inria.fr>]

       Hi,

I noticed that on recent x86_64 with fma (fused-multiply-add) in hardware,
as claimed by /proc/cpuinfo, glibc does not use fma by default.

The reason seems to be that with the default flags (-O2 -g), gcc does not
define __FP_FAST_FMA:

tomate$ gcc -O2 -g -dM -E -xc /dev/null | grep -q __FP_FAST_FMA
tomate$ echo $?
1

but it is defined with -march=native:

tomate$ gcc -O2 -g -march=native -dM -E -xc /dev/null | grep -q __FP_FAST_FMA
tomate$ echo $?
0

However, on PowerPC __FP_FAST_FMA is defined without -march=native:

pzimmermann@drac-12:~$ gcc -O2 -g -dM -E -xc /dev/null | grep -q __FP_FAST_FMA
pzimmermann@drac-12:~$ echo $?
0

Would it make sense to add -march=native to CFLAGS, or to add an option
like --enable-fma to configure?

Paul