[PATCH] Improve integer bit test on atomic builtin return

[H.J. Lu]

On Tue, Oct 5, 2021 at 3:07 AM Richard Biener <rguenther@suse.de> wrote:
>
> On Mon, 4 Oct 2021, H.J. Lu wrote:
>
> > commit adedd5c173388ae505470df152b9cb3947339566
> > Author: Jakub Jelinek <jakub@redhat.com>
> > Date:   Tue May 3 13:37:25 2016 +0200
> >
> >     re PR target/49244 (__sync or __atomic builtins will not emit 'lock bts/btr/btc')
> >
> > optimized bit test on atomic builtin return with lock bts/btr/btc.  But
> > it works only for unsigned integers since atomic builtins operate on the
> > 'uintptr_t' type.  It fails on bool:
> >
> >   _1 = atomic builtin;
> >   _4 = (_Bool) _1;
> >
> > and signed integers:
> >
> >   _1 = atomic builtin;
> >   _2 = (int) _1;
> >   _5 = _2 & (1 << N);
> >
> > Improve bit test on atomic builtin return by converting:
> >
> >   _1 = atomic builtin;
> >   _4 = (_Bool) _1;
> >
> > to
> >
> >   _1 = atomic builtin;
> >   _5 = _1 & (1 << 0);
> >   _4 = (_Bool) _5;
> >
> > and converting:
> >
> >   _1 = atomic builtin;
> >   _2 = (int) _1;
> >   _5 = _2 & (1 << N);
> >
> > to
> >   _1 = atomic builtin;
> >   _6 = _1 & (1 << N);
> >   _5 = (int) _6;
>
> Why not do this last bit with match.pd patterns (and independent on
> whether _1 is defined by an atomic builtin)?  For the first suggested

The full picture is

 _1 = _atomic_fetch_or_* (ptr_6, mask, _3);
  _2 = (int) _1;
  _5 = _2 & mask;

to

  _1 = _atomic_fetch_or_* (ptr_6, mask, _3);
  _6 = _1 & mask;
  _5 = (int) _6;

It is useful only if 2 masks are the same.

> transform that's likely going to be undone by folding, no?
>

The bool case is

  _1 = __atomic_fetch_or_* (ptr_6, 1, _3);
  _4 = (_Bool) _1;

to

  _1 = __atomic_fetch_or_* (ptr_6, 1, _3);
  _5 = _1 & 1;
  _4 = (_Bool) _5;

Without __atomic_fetch_or_*, the conversion isn't needed.
After the conversion, optimize_atomic_bit_test_and will
immediately optimize the code sequence to

  _6 = .ATOMIC_BIT_TEST_AND_SET (&v, 0, 0, 0);
  _4 = (_Bool) _6;

and there is nothing to fold after it.

-- 
H.J.