Re: [PATCH 1/2] match: Optimize A < 0 ? ARG1 OP 2^n-1 : ARG1

[Raphael Zinsly]

On Fri, May 15, 2026 at 10:09 AM Richard Biener <rguent...@suse.de> wrote:

> On Wed, 13 May 2026, Raphael Moreira Zinsly wrote:
> ...
> > +/* Simplify (a < 0) ? ARG1 OP CONST : ARG1 to ((a < 0) ? 0 : CONST) OP
> ARG1
> > +   when CONST is a 2^n-1 constant.  */
> > +(for op (plus bit_ior bit_xor)
> > + (simplify
> > +  (cond (lt @0 integer_zerop@1) (op:c @2 INTEGER_CST@3) @2)
>
> No :c necessary on 'op'
>
>
Ok.


> > +   (if (wi::exact_log2 (wi::to_wide (@3) + 1) != -1)
>
> Why is it cheap to splat the sign bit?  If it's cheap to splat
> the sign bit it's cheap to select between zero and a constant:
> <splat-sign-bit-to-full-width> & COSTANT.
>
> That you leave in the (cond ...) hints strongly in that it's
> an oddly special "canonicalization" and that it is really
> a target dependent RTL thing?
>
>
It is a canonicalization, but I believe it's worth it.
The message of this commit is misleading, which is my mistake, I was so
immersed in trying to generate a splat sign bit that I kept the original
commit description without much thought.
I'm in fact generating a conditional move that can be transformed into a
splat sign bit in the targets that benefit from it.
I will describe it better in V2.

The current code produces for 'c < 0 ? a : a + 7':

  if (c_2(D) >= 0)
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 3>
  _4 = a_3(D) + 7;

  <bb 4>
  # _1 = PHI <a_3(D)(2), _4(3)>
  return _1;

After this patch we get:

  _1 = c_2(D) < 0;
  _4 = _1 ? 0 : 7;
  _8 = a_3(D) + _4;
  return _8;

Is it reasonable?

> +    (op:c (cond (lt @0 @1) @3 { build_zero_cst (type); }) @2))))
>
> :c doesn't do anything in the result (I wonder why we don't diagnose it
> ...)  Please re-use a captured (lt @0 @1).
>
> Similar below.
>

Ok


Thanks,
Raphael