Re: [x86 PATCH] PR rtl-optimization/96692: ((A|B)^C)^A using andn with -mbmi.

[Uros Bizjak <ubizjak@gmail.com>]

On Mon, Jul 4, 2022 at 7:27 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> Hi Uros,
> Thanks for the review.  This patch implements all of your suggestions, both
> removing ix86_pre_reload_split from the combine splitter(s), and dividing
> the original splitter up into four simpler variants, that use match_dup to
> handle the variants/permutations caused by operator commutativity.
>
> This revised patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32} with no
> new failures.  Ok for mainline?
>
> 2022-07-04  Roger Sayle  <roger@nextmovesoftware.com>
>             Uroš Bizjak  <ubizjak@gmail.com>
>
> gcc/ChangeLog
>         PR rtl-optimization/96692
>         * config/i386/i386.md (define_split): Split ((A | B) ^ C) ^ D
>         as (X & ~Y) ^ Z on target BMI when either C or D is A or B.
>
> gcc/testsuite/ChangeLog
>         PR rtl-optimization/96692
>         * gcc.target/i386/bmi-andn-4.c: New test case.

OK.

Thanks,
Uros.

>
> Thanks again,
> Roger
> --
>
> > -----Original Message-----
> > From: Uros Bizjak <ubizjak@gmail.com>
> > Sent: 26 June 2022 18:08
> > To: Roger Sayle <roger@nextmovesoftware.com>
> > Cc: gcc-patches@gcc.gnu.org
> > Subject: Re: [x86 PATCH] PR rtl-optimization/96692: ((A|B)^C)^A using andn with
> > -mbmi.
> >
> > On Sun, Jun 26, 2022 at 2:04 PM Roger Sayle <roger@nextmovesoftware.com>
> > wrote:
> > >
> > >
> > > This patch addresses PR rtl-optimization/96692 on x86_64, by providing
> > > a define_split for combine to convert the three operation ((A|B)^C)^D
> > > into a two operation sequence using andn when either A or B is the
> > > same register as C or D.  This is essentially a reassociation problem
> > > that's only a win if the target supports an and-not instruction (as with -mbmi).
> > >
> > > Hence for the new test case:
> > >
> > > int f(int a, int b, int c)
> > > {
> > >     return (a ^ b) ^ (a | c);
> > > }
> > >
> > > GCC on x86_64-pc-linux-gnu wth -O2 -mbmi would previously generate:
> > >
> > >         xorl    %edi, %esi
> > >         orl     %edx, %edi
> > >         movl    %esi, %eax
> > >         xorl    %edi, %eax
> > >         ret
> > >
> > > but with this patch now generates:
> > >
> > >         andn    %edx, %edi, %eax
> > >         xorl    %esi, %eax
> > >         ret
> > >
> > > I'll investigate whether this optimization can also be implemented
> > > more generically in simplify_rtx when the backend provides accurate
> > > rtx_costs for "(and (not ..." (as there's no optab for andn).
> > >
> > > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> > > and make -k check, both with and without --target_board=unix{-m32},
> > > with no new failures.  Ok for mainline?
> > >
> > >
> > > 2022-06-26  Roger Sayle  <roger@nextmovesoftware.com>
> > >
> > > gcc/ChangeLog
> > >         PR rtl-optimization/96692
> > >         * config/i386/i386.md (define_split): Split ((A | B) ^ C) ^ D
> > >         as (X & ~Y) ^ Z on target BMI when either C or D is A or B.
> > >
> > > gcc/testsuite/ChangeLog
> > >         PR rtl-optimization/96692
> > >         * gcc.target/i386/bmi-andn-4.c: New test case.
> >
> > +  "TARGET_BMI
> > +   && ix86_pre_reload_split ()
> > +   && (rtx_equal_p (operands[1], operands[3])
> > +       || rtx_equal_p (operands[1], operands[4])
> > +       || (REG_P (operands[2])
> > +   && (rtx_equal_p (operands[2], operands[3])
> > +       || rtx_equal_p (operands[2], operands[4]))))"
> >
> > You don't need a ix86_pre_reload_split for combine splitter*
> >
> > OTOH, please split the pattern to two for each commutative operand and use
> > (match_dup x) instead. Something similar to [1].
> >
> > *combine splitter is described in the documentation as the splitter pattern that
> > does *not* match any existing insn pattern.
> >
> > [1] https://gcc.gnu.org/pipermail/gcc-patches/2022-June/596804.html
> >
> > Uros.