From: Richard Biener <richard.guenther@gmail.com>
To: Tamar Christina <Tamar.Christina@arm.com>
Cc: Richard Sandiford <Richard.Sandiford@arm.com>,
Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org>,
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.
Date: Tue, 21 Jun 2022 09:43:16 +0200 [thread overview]
Message-ID: <CAFiYyc327mZ8+fMRTBZ0G38Ojiy3GjKVg-SgMVq0cAFPdBiQCw@mail.gmail.com> (raw)
In-Reply-To: <VI1PR08MB53255417099B2E4CAEA42738FFB09@VI1PR08MB5325.eurprd08.prod.outlook.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" } } */
> > >>
> > >>
> > >>
> > >>
> > >> --
next prev parent reply other threads:[~2022-06-21 7:43 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-16 11:08 Tamar Christina
2022-06-16 11:09 ` [PATCH 2/2]middle-end: Support recognition of three-way max/min Tamar Christina
2022-06-20 8:36 ` Richard Biener
2022-06-20 9:01 ` Tamar Christina
2022-06-21 13:15 ` Richard Biener
2022-06-21 13:42 ` Tamar Christina
2022-06-27 7:52 ` Richard Biener
2022-07-05 15:25 ` Tamar Christina
2022-07-12 9:39 ` Tamar Christina
2022-07-12 13:19 ` Richard Biener
2022-07-27 10:40 ` Tamar Christina
2022-07-27 11:18 ` Richard Biener
2022-08-02 8:32 ` Tamar Christina
2022-08-02 9:11 ` Richard Biener
2022-08-03 8:17 ` Tamar Christina
2022-08-03 8:25 ` Richard Biener
2022-08-03 20:41 ` H.J. Lu
2022-06-20 23:16 ` Andrew Pinski
2022-06-21 6:54 ` Richard Biener
2022-06-21 7:12 ` Tamar Christina
2022-06-20 8:03 ` [PATCH 1/2]middle-end: Simplify subtract where both arguments are being bitwise inverted Richard Biener
2022-06-20 8:18 ` Richard Sandiford
2022-06-20 8:49 ` Tamar Christina
2022-06-21 7:43 ` Richard Biener [this message]
2022-08-03 15:13 ` Tamar Christina
2022-08-04 6:58 ` Richard Biener
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