Re: [new-ra] Development status?

[Michael Matz <matz at suse dot de>]

Hi,

On Tue, 20 Jan 2004, Denis Chertykov wrote:

> > > I was interested in comparision of results given by pre-reload based
> > > web_class and regclass based web_class.
> > 
> > They at least shouldn't be worse, I think, are they?
> 
> The results are *equal* !!! (for x86)
> Exclude a few difficult cases.

Hmm, but theoretically regclass can do much better than pre-reload (at the 
expense for sometimes emitting some reloads later).  The situation which 
it handles better for instance is this:

There are register classes A, B and AB (union of A and B).  Now suppose 
there is and insns i1 (with two ops) and i2 (one op) having 
such constraints:
 i1  op0 "a,b"   op1 "b,a"
 i2  op0 "a"

and in the insn stream we have this:
 i1(p0, p1)
 i2(p1)

(p0 and p1 are pseudos, i.e. p0 is operand 0 of i1, and p1 is operand 1 of 
i1 and operand 0 of i2).

Now pre-reload looks at each insn individually and selects one certain 
alternative of it (the cheapest for some definition of cheap).  Without 
other means it would select alternative 0 for insn i1.  Ergo p0 gets class 
A and p1 class B.

Now i2 requires p1 to be class A, ergo we need some fixup code.  Had 
pre-reload selected the second alternative from the beginning it wouldn't 
be needed.  Sometimes there are even cases where selection of an 
alternative should be deferred to even later.

regclass does better in this example.  It would determine that the cost of 
class A for p1 is less than class B.

> After you got the patch and seeing on results I would discuss the
> problem of register classes in new-ra. Is this OK ?

Yes.


Ciao,
Michael.