Re: [PATCH 1/2]middle-end: Simplify subtract where both arguments are being bitwise inverted.

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Jun 20, 2022 at 10:49 AM Tamar Christina
<Tamar.Christina@arm.com> wrote:
>
> > -----Original Message-----
> > From: Richard Sandiford <richard.sandiford@arm.com>
> > Sent: Monday, June 20, 2022 9:19 AM
> > To: Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org>
> > Cc: Tamar Christina <Tamar.Christina@arm.com>; Richard Biener
> > <richard.guenther@gmail.com>; Richard Guenther <rguenther@suse.de>;
> > nd <nd@arm.com>
> > Subject: Re: [PATCH 1/2]middle-end: Simplify subtract where both
> > arguments are being bitwise inverted.
> >
> > Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > > On Thu, Jun 16, 2022 at 1:10 PM Tamar Christina via Gcc-patches
> > > <gcc-patches@gcc.gnu.org> wrote:
> > >>
> > >> Hi All,
> > >>
> > >> This adds a match.pd rule that drops the bitwwise nots when both
> > >> arguments to a subtract is inverted. i.e. for:
> > >>
> > >> float g(float a, float b)
> > >> {
> > >>   return ~(int)a - ~(int)b;
> > >> }
> > >>
> > >> we instead generate
> > >>
> > >> float g(float a, float b)
> > >> {
> > >>   return (int)a - (int)b;
> > >> }
> > >>
> > >> We already do a limited version of this from the fold_binary fold
> > >> functions but this makes a more general version in match.pd that applies
> > more often.
> > >>
> > >> Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> > >>
> > >> Ok for master?
> > >>
> > >> Thanks,
> > >> Tamar
> > >>
> > >> gcc/ChangeLog:
> > >>
> > >>         * match.pd: New bit_not rule.
> > >>
> > >> gcc/testsuite/ChangeLog:
> > >>
> > >>         * gcc.dg/subnot.c: New test.
> > >>
> > >> --- inline copy of patch --
> > >> diff --git a/gcc/match.pd b/gcc/match.pd index
> > >>
> > a59b6778f661cf9121dd3503f43472871e4da445..51b0a1b562409af535e53828a1
> > 0
> > >> c30b8a3e1ae2e 100644
> > >> --- a/gcc/match.pd
> > >> +++ b/gcc/match.pd
> > >> @@ -1258,6 +1258,10 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> > >> (simplify
> > >>   (bit_not (plus:c (bit_not @0) @1))
> > >>   (minus @0 @1))
> > >> +/* (~X - ~Y) -> X - Y.  */
> > >> +(simplify
> > >> + (minus (bit_not @0) (bit_not @1))
> > >> + (minus @0 @1))
> > >
> > > It doesn't seem correct.
> > >
> > > (gdb) p/x ~-1 - ~0x80000000
> > > $3 = 0x80000001
> > > (gdb) p/x -1 - 0x80000000
> > > $4 = 0x7fffffff
> > >
> > > where I was looking for a case exposing undefined integer overflow.
> >
> > Yeah, shouldn't it be folding to (minus @1 @0) instead?
> >
> >   ~X = (-X - 1)
> >   -Y = (-Y - 1)
> >
> > so:
> >
> >   ~X - ~Y = (-X - 1) - (-Y - 1)
> >           = -X - 1 + Y + 1
> >           = Y - X
> >
>
> You're right, sorry, I should have paid more attention when I wrote the patch.

You still need to watch out for undefined overflow cases in the result
that were well-defined in the original expression I think.

Richard.

> Tamar
> > Richard
> >
> >
> > > Richard.
> > >
> > >>
> > >>  /* ~(X - Y) -> ~X + Y.  */
> > >>  (simplify
> > >> diff --git a/gcc/testsuite/gcc.dg/subnot.c
> > >> b/gcc/testsuite/gcc.dg/subnot.c new file mode 100644 index
> > >>
> > 0000000000000000000000000000000000000000..d621bacd27bd3d19a010e4c9f
> > 83
> > >> 1aa77d28bd02d
> > >> --- /dev/null
> > >> +++ b/gcc/testsuite/gcc.dg/subnot.c
> > >> @@ -0,0 +1,9 @@
> > >> +/* { dg-do compile } */
> > >> +/* { dg-options "-O -fdump-tree-optimized" } */
> > >> +
> > >> +float g(float a, float b)
> > >> +{
> > >> +  return ~(int)a - ~(int)b;
> > >> +}
> > >> +
> > >> +/* { dg-final { scan-tree-dump-not "~" "optimized" } } */
> > >>
> > >>
> > >>
> > >>
> > >> --