Re: scavanging a reg

[law at redhat dot com]

> On Tue, 2001-11-13 at 11:25, law@redhat.com wrote:
> >   > i need a GPR at epilogue/prologue time so i can load and save to the
> >   > VRSAVE register (which can only be moved to and fro GPRs).
> >   > 
> >   > what's the best route for this?  can i iterate through "regs_ever_live"
> >   > and get one from there?  i mean, reload is already done so we won't nee
> d
> >   > any more registers.
> > True, but you also can't make a call-saved register live that wasn't live
> > before.  So you have to find a call-clobbered register.  
> 
> doh.. i thought about that right after i sent the mail...
> 
> how about this?
> 
>     /* Scavange any non fixed register we can use as scratch at
>        pro/epilogue time.  Pick a register out of the call clobbered set
>        because no one expects it to hold anything meaningful.  */
>     
>     int
>     get_scratch_gpr (void)
>     {
>       int i;
>     
>       for (i = GP_ARG_MAX + 1; i <= 32; ++i)
>         if (! fixed_regs[i] && call_used_regs[i])
>           return i;
>     
>       /* If	we can't find anything,	return	r12.  */
>       return 12;
>     }
>     
> although i'm thinking it's prob best to follow David's suggestion and
> just use r12 regardless.
> 
> is the above ok?
Either-or should be OK.  

jeff