On Wed, 3 Aug 2022, Tamar Christina wrote:
> > -----Original Message-----
> > From: Richard Biener <richard.guent...@gmail.com>
> > Sent: Tuesday, June 21, 2022 8:43 AM
> > To: Tamar Christina <tamar.christ...@arm.com>
> > Cc: Richard Sandiford <richard.sandif...@arm.com>; Richard Biener via Gcc-
> > patches <gcc-patches@gcc.gnu.org>; Richard Guenther
> > <rguent...@suse.de>; nd <n...@arm.com>
> > Subject: Re: [PATCH 1/2]middle-end: Simplify subtract where both
> > arguments are being bitwise inverted.
> >
> > On Mon, Jun 20, 2022 at 10:49 AM Tamar Christina
> > <tamar.christ...@arm.com> wrote:
> > >
> > > > -----Original Message-----
> > > > From: Richard Sandiford <richard.sandif...@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.christ...@arm.com>; Richard Biener
> > > > <richard.guent...@gmail.com>; Richard Guenther
> > <rguent...@suse.de>;
> > > > nd <n...@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.
>
> The only special thing we do for signed numbers if to do the subtract as
> unsigned. As I mentioned
> before GCC already does this transformation as part of the fold machinery,
> but that only only happens
> when a very simple tree is matched and only when single use. i.e.
> https://godbolt.org/z/EWsdhfrKj
>
> I'm only attempting to make it apply more generally as the result is always
> beneficial.
>
> I've respun the patch to the same as we already do.
>
> Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
>
> Ok for master?
OK.
> 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
> 330c1db0c8e12b0fb010b1958729444672403866..00b3e07b2a5216b19ed58500923680d83c67d8cf
> 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -1308,6 +1308,11 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> (simplify
> (bit_not (plus:c (bit_not @0) @1))
> (minus @0 @1))
> +/* (~X - ~Y) -> Y - X. */
> +(simplify
> + (minus (bit_not @0) (bit_not @1))
> + (with { tree utype = unsigned_type_for (type); }
> + (convert (minus (convert:utype @1) (convert:utype @0)))))
>
> /* ~(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..d621bacd27bd3d19a010e4c9f831aa77d28bd02d
> --- /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" } } */
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)
