Re: Enable SSE math on i386 with -Ofast

[Jan Hubicka <hubicka at ucw dot cz>]

> On Fri, 4 Oct 2013, Jan Hubicka wrote:
> 
> > Hi,
> > this patch makes -Ofast to also imply -mfpmath=sse.  It is important win on
> > SPECfP (2000 and 2006). Even though for exmaple the following
> > float a(float b)
> > {
> >    return b+10;
> > }
> > 
> > results in somewhat ridiculous
> > a:
> > .LFB0:  
> >         .cfi_startproc
> >         subl    $4, %esp
> >         .cfi_def_cfa_offset 8
> >         movss   .LC0, %xmm0
> >         addss   8(%esp), %xmm0
> >         movss   %xmm0, (%esp)
> >         flds    (%esp)
> >         addl    $4, %esp
> >         .cfi_def_cfa_offset 4
> >         ret
> > 
> > I wonder if we can get rid at least of the redundant stack alignment on ESP...
> > 
> > Bootstrapped/regtested x86_64-linux, will commit it on weekend if there are no
> > complains.  I wonder if -ffast-math should do the same - it is documented as enabling
> > explicit set of options, bu that can be changed I guess.
> 
> I wonder if we can restrict -mfpmath=sse to local functions where we can

We can, but why? Parameters are passed in memory that is equaly bad for 387 and
SSE.  Only return values are passed in registers, that is not that expensive to
have one extra reload per function except for functions containing almost
nothing that should be inlined if they are local.  In meantime I (partially,
since megrez stopped producing 32bit spec2k6 results) benchmarked
-mfpmath=sse,387 and it does not seem to be a loss anymore.  So perhaps we can
give it a try?

> change the ABI ... (do we change the local functions ABI with 
> -mfpmath=sse?)

We don't.  It is probably quite easy to default to sse_regparm and change return value type.
I will look into it.

Honza