Re: Register Allocation

[Peter Bergner <bergner@vnet.ibm.com>]

On Wed, 2005-11-23 at 15:05 +0100, Michael Matz wrote:
> > Spill Cost Engine [page(s) 26-29]:
> >     * The register allocator should not be estimating the execution
> >       frequency of a basic block as 10^nesting level.  That information
> >       should be coming from the cfg which comes from profile data or
> >       from a good static profile.  The problem with 10^loop nesting
> >       level is that we can overestimate the spill costs for some
> >       pseudos.  For example:
> >     	while (...) {
> >     	  <use of "a">
> >     	  if (...)
> >     	    <use of "b">
> >     	  else
> >     	    <use of "b"
> >     	}
> >       In the code above, "b"'s spill cost will be twice that of "a",
> >       when they really should have the same spill cost.
> 
> Nearly.  "b" _is_ more costly to spill, code size wise.  All else being 
> equal it's better to spill "a" in this case.  But the cost is of course 
> not twice as large, as you say.  I.e. I agree with you that the metric 
> should be based exclusively on the BB frequencies attached to the CFG, not 
> any nesting level.  Also like in new-ra ;)

The spill cost for a pseudo in a classic Chaitin/Briggs allocator does
not take number of spill instructions inserted into account, so "b"'s
spill cost would be twice that of "a" if we were to use 10^nesting
level.  That said, I think we're all in agreement that using basic
block frequencies from the cfg is the correct thing to do and that
taking static spill instruction counts into account is a good idea
which Andrew's proposal does by using it as a tie breaker.

I assume it goes without saying that when using -Os, spill cost will
be used as the tie breaker when two pseudos have the same static spill
instruction counts.

Peter