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

[Tamar Christina <Tamar.Christina@arm.com>]

> -----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.
 
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" } } */
> >>
> >>
> >>
> >>
> >> --