[PATCH][combine][RFC] Don't transform sign and zero extends inside mults

[Segher Boessenkool]

On Mon, Nov 09, 2015 at 08:52:13AM +0100, Uros Bizjak wrote:
> On Sun, Nov 8, 2015 at 9:58 PM, Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> > On Fri, Nov 06, 2015 at 04:00:08PM -0600, Segher Boessenkool wrote:
> >> This patch stops combine from generating widening muls of anything else
> >> but registers (immediates, memory, ...).  This probably is a reasonable
> >> tradeoff for all targets, even those (if any) that have such insns.
> >>
> >> > >I'll let you put it through it's paces on your setup :)
> >>
> >> > I'll let Segher give the final yes/no on this, but it generally looks
> >> > good to me.
> >>
> >> It looks okay to me too.  Testing now, combine patches have the tendency
> >> to do unforeseen things on other targets ;-)
> >
> > Testing shows it makes a difference only very rarely.  For many targets
> > it makes no difference, for a few it is a small win.  For 32-bit x86 it
> > creates slightly bigger code.
> >
> > I think it looks good, but let's wait to hear Uros' opinion.
> 
> >From the original patch submission, it looks that this patch would
> also benefit x86_32.

Yes, that is what I thought too.

> Regarding the above code size increase -  do you perhaps have a
> testcase, to see what causes the difference?

I could extract some.  It happens quite rarely on usual code.

> It isn't necessary due to
> the patch, but perhaps some loads are moved to the insn and aren't
> CSE'd anymore.


Segher