Re: [committed] [v2] More logical op simplifications in simplify-rtx.cc

[Andrew Pinski <pinskia@gmail.com>]

On Sat, May 25, 2024 at 11:42 AM Jeff Law <jlaw@ventanamicro.com> wrote:
>
> This is a revamp of what started as a target specific patch.
>
> Basically xalan (corrected, I originally thought it was perlbench) has a
> bitset implementation with a bit of an oddity.  Specifically setBit will
> clear the bit before it is set:
>
> >             if (bitToSet < 32)
> >             {
> >                 fBits1 &= ~mask;
> >                 fBits1 |= mask;
> >             }
> >              else
> >             {
> >                 fBits2 &= ~mask;
> >                 fBits2 |= mask;
> >             }
>
> We can clean this up pretty easily in RTL with a small bit of code in
> simplify-rtx.  While xalan doesn't have other cases, we can synthesize
> tests pretty easily and handle them as well.
>
>
> It turns out we don't actually have to recognize this stuff at the bit
> level, just standard logical identities are sufficient.  For example
>
> (X | Y) & ~Y -> X & ~Y
>
>
>
> Andrew P. might poke at this at the gimple level.  The type changes
> kindof get in the way in gimple but he's much better at match.pd than I
> am, so if he wants to chase it from the gimple side, I'll fully support
> that.

So we already have this pattern (without the type change) in gimple:
 /* (~x | y) & x -> x & y */
 /* (~x & y) | x -> x | y */
 (simplify
  (bitop:c (rbitop:c @2 @1) @0)
  (with { bool wascmp; }
   (if (bitwise_inverted_equal_p (@0, @2, wascmp)
        && (!wascmp || element_precision (type) == 1))
    (bitop @0 @1))))

The problem is bitwise_inverted_equal_p does not see that:
  c.0_4 = (signed char) _1;
  _5 = ~c.0_4;
  _16 = (charD.11) _5;

and
  c_11 = (charD.11) _1;

are bitwise inversions of each other.

I filed https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115449 to keep
track of this.

Thanks,
Andrew Pinski




>
> Bootstrapped and regression tested on x86.  Also run through my tester
> on its embedded targets.
>
> Pushing to the trunk.
>
> jeff
>