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RE: Optimising redundant compares
- From: "Jon Beniston" <jbeniston at compxs dot com>
- To: "'Jim Wilson'" <wilson at specifixinc dot com>
- Cc: <gcc at gcc dot gnu dot org>
- Date: Fri, 26 Sep 2003 11:39:06 +0100
- Subject: RE: Optimising redundant compares
- Organization: CompXs
- Reply-to: <jbeniston at compxs dot com>
> -----Original Message-----
> From: Jim Wilson [mailto:firstname.lastname@example.org]
> Sent: 26 September 2003 07:56
> To: email@example.com
> Cc: firstname.lastname@example.org
> Subject: Re: Optimising redundant compares
> Jon Beniston wrote:
> > Yep, my port is very similar. The only apparent significant
> > is that I was using a fixed hard register, where as I
> believe the PPC
> > allows CC regs to be pseduos.
> The PPC has 8 condition code registers. Using pseudos lets
> us allocate
> values to the 8 available cc registers. If you only have one, then
> using a hard register should be fine.
After playing around for a while, it would appear that if I use a hard
register, then GCC will not attempt to optimise the compare
instructions. However, if I use a pseudo it will, but it will also try
lots more optimisations that would mean I have to write rules for
spilling and restoring the CC reg which I don't want to do at this point
in time. Does this sound correct?