Re: [PATCH] Improve CSE (PR rtl-optimization/70467)

[Bernd Schmidt <bschmidt at redhat dot com>]

On 04/01/2016 04:51 PM, Jakub Jelinek wrote:
> On Fri, Apr 01, 2016 at 03:35:19PM +0200, Bernd Schmidt wrote:
> > On 04/01/2016 03:14 PM, Jakub Jelinek wrote:
> > > As the testcase below shows, we can end up with lots of useless
> > > instructions from multi-word arithmetics.
> > > simplify-rtx.c can optimize x {&,|,^}= {0,-1}, but while
> > > x &= 0 or x {|,^}= -1 are optimized into constants and CSE can handle those
> > > fine, we keep x &= -1 and x {|,^}= 0 in the IL until expansion if x
> > > is a MEM.  There are two issues, one is that cse_insn has for a few years
> > > code that wants to prevent partially overlapping MEM->MEM moves,
> > > but actually doesn't realize that fully overlapping MEM->MEM noop moves
> > > are fine.  And the second one is that on most backends, there are no
> > > MEM->MEM move instructions, so we need to delete the useless insns instead,
> > > because it can't match.
> > >
> > > Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux.
> > > Is this something we want for 6.x or defer for stage1?
> >
> > It seems like a stage1 thing to me unless it's a regression. But you're in a
> > better position to make that call.
>
> I guess it can wait for stage1.
>
> > > +	  /* Similarly, lots of targets don't allow no-op
> > > +	     (set (mem x) (mem x)) moves.  */
> > > +	  else if (n_sets == 1
> > > +		   && MEM_P (trial)
> > > +		   && MEM_P (dest)
> > > +		   && rtx_equal_p (trial, dest)
> > > +		   && !side_effects_p (dest)
> > > +		   && (cfun->can_delete_dead_exceptions
> > > +		       || insn_nothrow_p (insn)))
> >
> > Looks like this block of code is practically duplicated - I'd prefer a
> > helper function set_of_equal_mems_removable_p or something. Ok with that
> > change.
>
> Perhaps instead just set a bool in the second hunk and just test that at the
> third hunk's condition?

Also works for me. Or maybe set trial and dest to pc_rtx and merge the 
new case with the preexisting one. As long as we don't get a big block 
of duplicated conditions.


Bernd