Re: [PATCH][v2] match.pd: drop an operand discarded by a shift

On Fri, Aug 7, 2026 at 9:52 AM Jeffrey Law <[email protected]> wrote:
>
>
>
> On 8/5/2026 5:40 AM, [email protected] wrote:
> > From: Kyrylo Tkachov <[email protected]>
> >
> > Neither an inclusive nor an exclusive or can carry, so an operand whose set
> > bits all lie below the shift count contributes nothing to the result:
> >
> >    int f (int a, int b) { return (a ^ (b & 1)) >> 1; }
> >
> > aarch64 -O2 before:
> >
> >       and     w1, w1, 1
> >       eor     w0, w1, w0
> >       asr     w0, w0, 1
> >
> > after:
> >
> >       asr     w0, w0, 1
> >
> > The set bits are read from tree_nonzero_bits, so the rule also fires when
> > the operand is a boolean, a narrow value or anything else whose range the
> > middle end already knows.  Found by mining the optimized dumps of real code,
> > where the shape comes from flag bits packed into the low bits of a word.
> >
> > Bootstrapped and tested on aarch64-none-linux-gnu.
> > Ok for trunk?
> > Thanks,
> > Kyrill
> >
> > gcc/ChangeLog:
> >
> >       * match.pd ((X | Y) >> C, (X ^ Y) >> C): New simplification.
> >
> > gcc/testsuite/ChangeLog:
> >
> >       * gcc.dg/tree-ssa/shift-drops-bitop-1.c: New test.
> GIven this can fire for !GIMPLE and we totally drop the Y term, don't we
> need to either verify Y has no side effects or guard the transformation
> on GIMPLE?

In the case of generic, genmatch already adds the needed code to deal
with the dropping and/or side effects.

The genmatch code which does this starts with the following comment:
```
          /* Search for captures not used in the result expression and dependent
             on TREE_SIDE_EFFECTS emit omit_one_operand.  */
```

The only case where a match pattern needs to do something manual is if
you make a conditional expression unconditional; usually inside a
COND_EXPR (but might be also from THRUTH_*IF_EXPR too). There is
expr_no_side_effects_p for that check.
Yes there are some match patterns that check TREE_SIDE_EFFECTS
explicitly but that is not needed; I noticed the majority of them were
added by Roger so it might have been out of habit or not realizing
gnematch handles that case already.


Thanks,
Andrea

>
> Jeff