Re: register allocation questions

At 11:09 AM +0200 10/26/04, Leopold Toetsch wrote:
> Bill Coffman <bill.coffman@xxxxxxxxx> wrote:
>  > 1)  In the existing parrot code, when a register is assigned, it uses
> >  the following code:
>
> >                          int c = (color + MAX_COLOR/2) % MAX_COLOR;
>
> >  Thus, it seems to prefer to use register #16 and up, first, before it
> >  uses 0-15.  This is mistifying to me, since it's not so trivial to
> >  code it this way,
>
> Well, some time ago, register allocation started at zero. The split of
> register frames in upper and lower halfs *plus* the premature
> optimization to save only the upper half of registers made it necessary
> to allocate from 16 up.
>
> But things are a bit more compilcated. For function calls, we are
> passing arguments from register x5 ..x15 and I0..I4 plus some more have
> a special meaning. See docs/ppds/pdd03_calling_conventions for all the
> details. The same convention is used for function returns.
>
> So, if you want that really super efficient, you would allocate
> registers around function calls directly to that wanted register number,
> which should be in the SymReg's want_regno.

While true, in the general case leaving 0-15 as non-preferred 
registers will probably make things easier. Those registers, 
especially the PMC ones, are going to see a lot of thrash as function 
calls are made, and it'll probably be easier to have them as scratch 
registers.

It's distinctly possible, of course, that there'll be very little 
pressure to actually *use* them for most code, as we've got plenty of 
registers in general. That's the hope, at least.

--
                                Dan

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Dan Sugalski                          even samurai
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