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