[x86 PATCH] Improve ix86_expand_int_movcc to allow condition (mask) sharing.

[Uros Bizjak]

On Mon, Jan 2, 2023 at 11:50 AM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch modifies the way that ix86_expand_int_movcc generates RTL,
> to allow the condition mask to be shared/reused between multiple
> conditional move sequences.  Such redundancy is common when RTL
> if-conversion transforms non-trivial basic blocks.
>
> As a motivating example, consider the new test case:
>
> int a, b, c, d;
> int foo(int x)
> {
>     if (x == 0) {
>         a = 3;
>         b = 1;
>         c = 4;
>         d = 1;
>     } else {
>         a = 5;
>         b = 9;
>         c = 2;
>         d = 7;
>     }
>     return x;
> }
>
> This is currently compiled, with -O2, to:
>
> foo:    cmpl    $1, %edi
>         movl    %edi, %eax
>         sbbl    %edi, %edi
>         andl    $-2, %edi
>         addl    $5, %edi
>         cmpl    $1, %eax
>         sbbl    %esi, %esi
>         movl    %edi, a(%rip)
>         andl    $-8, %esi
>         addl    $9, %esi
>         cmpl    $1, %eax
>         sbbl    %ecx, %ecx
>         movl    %esi, b(%rip)
>         andl    $2, %ecx
>         addl    $2, %ecx
>         cmpl    $1, %eax
>         sbbl    %edx, %edx
>         movl    %ecx, c(%rip)
>         andl    $-6, %edx
>         addl    $7, %edx
>         movl    %edx, d(%rip)
>         ret
>
> Notice that the if-then-else blocks have been if-converted into four
> conditional move sequences/assignments, each consisting of cmpl, sbbl,
> andl and addl.  However, as the conditions are the same, the cmpl and
> sbbl instructions used to generate the mask could be shared by CSE.
>
> This patch enables that, so we now generate:
>
> foo:    cmpl    $1, %edi
>         movl    %edi, %eax
>         sbbl    %edx, %edx
>         movl    %edx, %edi
>         movl    %edx, %esi
>         movl    %edx, %ecx
>         andl    $-6, %edx
>         andl    $-2, %edi
>         andl    $-8, %esi
>         andl    $2, %ecx
>         addl    $7, %edx
>         addl    $5, %edi
>         addl    $9, %esi
>         addl    $2, %ecx
>         movl    %edx, d(%rip)
>         movl    %edi, a(%rip)
>         movl    %esi, b(%rip)
>         movl    %ecx, c(%rip)
>         ret
>
> Notice, the code now contains only a single cmpl and a single sbbl,
> with their result being shared (via movl).
>
> 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?
>
>
> 2023-01-02  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         * config/i386/i386-expand.cc (ix86_expand_int_movcc): Rewrite
>         RTL expansion to allow condition (mask) to be shared/reused,
>         by avoiding overwriting pseudos and adding REG_EQUAL notes.
>
> gcc/testsuite/ChangeLog
>         * gcc.target/i386/cmov10.c: New test case.

OK.

Thanks,
Uros.

>
>
> Thanks in advance,
> Roger
> --
>