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Re: [PATCH] New if-combining pass, addresses PRs 15353 and 31657
On Thu, Apr 26, 2007 at 05:47:47PM +0200, Richard Guenther wrote:
> On Thu, 26 Apr 2007, Andrew Pinski wrote:
>
> > On 4/26/07, Richard Guenther <rguenther@suse.de> wrote:
> > > On Wed, 25 Apr 2007, Andrew Pinski wrote:
> > >
> > > > On 4/25/07, Richard Guenther <rguenther@suse.de> wrote:
> > > >
> > > > > + /* returning 1 causes phiopt to trigger in */
> > > > If this is true, then this pass should run before phiopt so you can
> > > > catch more stuff.
> > >
> > > Well, I thought it is not profitable to do the transformation if phiopt
> > > can remove the same amount of control flow. Basically replacing
> > >
> > > x = (a >> b)
> > > if (x & 1)
> > > y = (a >> c)
> > > if (y & 1)
> > > return 1;
> > > else
> > > return 0;
> > >
> > > with
> > >
> > > tmp = (1 << b) | (1 << c)
> > > if (a & tmp == tmp)
> > > return 1;
> > > else
> > > return 0;
> > >
> > > instead of
> > >
> > > x = (a >> b)
> > > if (x & 1)
> > > y = (a >> c)
> > > return y & 1;
> >
> > Well it depends on the target :). Some (like spu-elf) non hinted
> > branches (and mispredicted hinted branches) are really really bad so
> > the first case of getting rid of an extra branch is a good thing.
>
Will it help
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25553
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31460
H.J.