[RS6000] rs6000_rtx_costs cost IOR

[Segher Boessenkool]

Hi!

On Thu, Sep 17, 2020 at 01:12:19PM +0930, Alan Modra wrote:
> On Wed, Sep 16, 2020 at 07:02:06PM -0500, Segher Boessenkool wrote:
> > > +		  /* Test both regs even though the one in the mask is
> > > +		     constrained to be equal to the output.  Increasing
> > > +		     cost may well result in rejecting an invalid insn
> > > +		     earlier.  */
> > 
> > Is that ever actually useful?
> 
> Possibly not in this particular case, but I did see cases where
> invalid insns were rejected early by costing non-reg sub-expressions.

But does that ever change generated code?

This makes the compiler a lot harder to read and understand.  To the
point that such micro-optimisations makes worthwhile optimisations hard
or impossible to do.


Segher