Re: [RFA] [PR rtl-optimization/90275] Handle nop reg->reg copies in cse

[Segher Boessenkool <segher@kernel.crashing.org>]

On Thu, Mar 12, 2020 at 12:03:08PM -0600, Jeff Law wrote:
> On Sat, 2020-02-08 at 10:41 -0600, Segher Boessenkool wrote:
> > I don't think each stanza of code should use it's own "noop-ness", no.
> Richard's patch is actually better than mine in that regard as it handles mem and
> reg nop moves in an indentical way.  I don't think refactoring the cse.c code is
> advisable now though -- but I do want to fix the multiply-reported ICE on ARM and
> Richard's cse changes are the cleanest way to do that that I can see.

It looks pretty simple, yeah...  All of CSE is hopelessly fragile, but
this patch does not make things worse.

> > I don't know if this patch makes matters worse or not.  It doesn't seem
> > suitable for stage 4 though.  And Richard said the cse.c part breaks
> > rs6000, if that is true, yes I do object ;-)
> The rs6000 port breakage is trivial to fix.  In fact, I did so and ran it through
> my tester, which includes ppc64le and ppc64 a slew of *-elf targets x86 native
> and more.

I don't see anything rs6000 below?  Is it just this generic code?

> @@ -5324,9 +5324,11 @@ cse_insn (rtx_insn *insn)
>  	    }
>  
>  	  /* Similarly, lots of targets don't allow no-op
> -	     (set (mem x) (mem x)) moves.  */
> +	     (set (mem x) (mem x)) moves.  Even (set (reg x) (reg x))
> +	     might be impossible for certain registers (like CC registers).  */
>  	  else if (n_sets == 1
> -		   && MEM_P (trial)
> +		   && ! CALL_P (insn)
> +		   && (MEM_P (trial) || REG_P (trial))
>  		   && MEM_P (dest)
>  		   && rtx_equal_p (trial, dest)
>  		   && !side_effects_p (dest)

This adds the !CALL_P (no space btw) condition, why is that?

(Is that CCmode reg-reg move comment about rs6000?  Huh, we *do* have
patterns for that, or *should* have at least!)


Segher