Re: use of fma

[Florian Weimer <fweimer@redhat.com>]

* Paul Zimmermann:

> 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

I assume that this is powerpc64le, which has a POWER8 baseline.  The
switch to little endian neatly eliminated requirements for backwards
compatibility.

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

This is already covered by the existing mechanisms for compiler flag
changes.  The problem is that not all x86 CPUs currently in production
support FMA.  We aren't even at the stage yet where we could discuss
phasing out support for old CPUs.  So building everything to require FMA
by default would break things for users.

We already have some FMA-using function variants selected by IFUNC
resolvers.  Search for “ifunc-fma” in the source tree for examples.
More could be added if beneficial.

If you have FMA-capable hardware and want to build glibc to take
advantage of it unconditionally (without IFUNCs), use GCC 11 and
-march=x86-64-v3.  It should be compatible with all such CPUs, and the
build will also use additional CPU features not present in the x86-64
baseline specification.

Thanks,
Florian