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

[Uros Bizjak <ubizjak@gmail.com>]

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.