RE: Optimising redundant compares

["Jon Beniston" <jbeniston at compxs dot com>]

> -----Original Message-----
> From: Jim Wilson [mailto:wilson@specifixinc.com] 
> Sent: 26 September 2003 07:56
> To: jbeniston@compxs.com
> Cc: gcc@gcc.gnu.org
> Subject: Re: Optimising redundant compares
> 
> 
> Jon Beniston wrote:
> > Yep, my port is very similar. The only apparent significant 
> difference 
> > 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?

Cheers,
Jon