------- Comment #7 from meissner at linux dot vnet dot ibm dot com 2010-03-22
22:24 -------
Subject: Re: Powerpc generates worse code for
-mvsx on gromacs even though there are no VSX instructions used
On Mon, Mar 22, 2010 at 10:20:21PM -0000, vmakarov at redhat dot com wrote:
>
>
> ------- Comment #6 from vmakarov at redhat dot com 2010-03-22 22:20 -------
> (In reply to comment #4)
> > FWIW, I seem to get considerably worse code from mainline than you -- for
> > -O3
> > -ffast-math -mcpu=power7 -mvsx -maltivec I get 140 stfs and 192 lfs insns
> > (compared to 117 & 139 respectively that you reported).
> >
>
> I suspect the differnce is because Mike calculated only stfs/lfs and you
> stfs(x)/lfs(x). But may be I am wrong.
I only calculated the stores and loads to the stack, i.e.
egrep (stfs|lfs).*\(1\)
since I was just looking for the spills.
> > Just for fun, I ran the same code through the a ppc compiler with the LRS
> > code
> > from reload-v2 and get 133:178 stfs/lsf insns, so that code clearly is
> > helping,
> > but it's not enough to offset the badness shown by IRA.
> >
> >
> > I couldn't reconcile how -fno-ira-share-spill-slots would be changing the
> > number of load/store insns, so I poked at that a bit.
>
> Yes, I cannot understand that too.
Note, while -fno-ira-share-spill-slots as fewer spills, I just measured the
results on the machine, and the time spent is pretty much the same.
> > -fno-ira-share-spill-slots twiddles whether or not a pseudo which gets
> > assigned
> > a hard reg is put into live_throughout or dead_or_set_p in the reload chain
> > structures, which in turn changes what pseudos get reassigned hard regs
> > during
> > reload. This is a somewhat odd effect and should be investigated further.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43413
