Re: register allocation questions

At 9:36 PM +0200 10/27/04, Leopold Toetsch wrote:
> Dan Sugalski wrote:
> > At 11:09 AM +0200 10/26/04, Leopold Toetsch wrote:
>
> > > 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.
>
> Yep, that's the easy part ;) OTOH when the register allocator is 
> doing register renaming anyway, the most inner loop with a function 
> call should get registers assigned already matching the calling 
> convemtions. With more then one call at that loop level, you have to 
> move around registers anyway.

Oh, sure, but keeping your scratch PMCs out of the way makes life a 
lot easier for the register coloring algorithms. Might not be 
optimal, but if it makes life simpler to start, optimal can come 
later.

> > 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.
>
> Yes, 16 regs are plenty and do suffice for all "normal"[1] code. 
> Assigning to wanted reg numbers for a function is a nice 
> optimization.
>
> [1] all except Dan's 6000 lines subroutines :) Did you start 
> creating real subs for your code already?

I wish. :( Unfortunately not, outside some simple stuff, and I doubt 
I will. The language just doesn't lend itself to that sort of thing. 
We're going to add actual real subroutines to the language after we 
roll out into production, but that doesn't help now, alas.
--
                                Dan

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