[PATCH] Fix PR46556 (poor address generation)

[William J. Schmidt]

On Wed, 2011-10-05 at 18:21 +0200, Paolo Bonzini wrote:
> On 10/05/2011 06:13 PM, William J. Schmidt wrote:
> > One other general question about the pattern-match transformation:  Is
> > this an appropriate transformation for all targets, or should it be
> > somehow gated on available addressing modes on the target processor?
> >
> > Bootstrapped and regression tested on powerpc64-linux-gnu.  Verified no
> > performance degradations on that target for SPEC CPU2000 and CPU2006.
> >
> > I'm looking for eventual approval for trunk after any comments are
> > resolved.  Thanks!
> 
> How do the costs look like for the two transforms you mention in the 
> head comment of locally_poor_mem_replacement?
> 
> Paolo
> 

I don't know off the top of my head -- I'll have to gather that
information.  The issue is that the profitability is really
context-sensitive, so just the isolated costs of insns aren't enough.
The forward propagation of the add into (mem (reg REG)) looks like a
slam dunk in the absence of other information, but if there are other
nearby references using nonzero offsets from REG, this just extends the
lifetimes of X and Y without eliminating the need for REG.