Re: [x86 PATCH] PR target/31985: Improve memory operand use with doubleword add.

[Uros Bizjak <ubizjak@gmail.com>]

On Wed, Jun 7, 2023 at 1:05 AM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch addresses the last remaining issue with PR target/31985, that
> GCC could make better use of memory addressing modes when implementing
> double word addition.  This is achieved by adding a define_insn_and_split
> that combines an *add<dwi>3_doubleword with a *concat<mode><dwi>3, so
> that the components of the concat can be used directly, without first
> being loaded into a double word register.
>
> For test_c in the bugzilla PR:
>
> Before:
>         pushl   %ebx
>         subl    $16, %esp
>         movl    28(%esp), %eax
>         movl    36(%esp), %ecx
>         movl    32(%esp), %ebx
>         movl    24(%esp), %edx
>         addl    %ecx, %eax
>         adcl    %ebx, %edx
>         movl    %eax, 8(%esp)
>         movl    %edx, 12(%esp)
>         addl    $16, %esp
>         popl    %ebx
>         ret
>
> After:
> test_c:
>         subl    $20, %esp
>         movl    36(%esp), %eax
>         movl    32(%esp), %edx
>         addl    28(%esp), %eax
>         adcl    24(%esp), %edx
>         movl    %eax, 8(%esp)
>         movl    %edx, 12(%esp)
>         addl    $20, %esp
>         ret
>
>
> If this approach is considered acceptable, similar splitters can be
> used for other doubleword operations.

Yes, the approach looks good to me. But please take care not to
interfere with STV transformations.

> 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?
>
> 2023-06-07  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         PR target/31985
>         * config/i386/i386.md (*add<dwi>3_doubleword_concat): New
>         define_insn_and_split combine *add<dwi>3_doubleword with a
>         *concat<mode><dwi>3 for more efficient lowering after reload.
>
> gcc/testsuite/ChangeLog
>         PR target/31985
>         * gcc.target/i386/pr31985.c: New test case.

OK.

Thanks,
Uros.

>
>
> Roger
> --
>