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.


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.