Add a new combine pass

Segher Boessenkool
Fri Dec 6 22:51:00 GMT 2019

On Wed, Dec 04, 2019 at 07:43:30PM +0900, Oleg Endo wrote:
> On Tue, 2019-12-03 at 12:05 -0600, Segher Boessenkool wrote:
> > > Hmm ... the R0 problem ... SH doesn't override class_likely_spilled
> > > explicitly, but it's got a R0_REGS class with only one said reg in it. 
> > > So the default impl of class_likely_spilled should do its thing.
> > 
> > Yes, good point.  So what happened here?
> "Something, somewhere, went terribly wrong"...
> insn 18 wants to do
>     mov.l @(r4,r6),r0
> But it can't because the reg+reg address mode has a R0 constraint
> itself.  So it needs to be changed to
>     mov   r4,r0
>     mov.l @(r0,r6),r0
> And it can't handle that.  Or only sometimes?  Don't remember.
> >   Is it just RA messing things
> > up, unrelated to the new pass?
> Yep, I think so.  The additional pass seems to create "tougher" code so
> reload passes out earlier than usual.  We've had the same issue when
> trying address mode selection optimization.  In fact that was one huge
> showstopper.

So maybe you should have a define_insn_and_split that allows any two
regs and replaces one by r0 if neither is (and a move to r0 before the
load)?  Split after reload of course.

It may be admitting defeat, but it may even result in better code as
well ;-)


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