On Tue, Aug 2, 2022 at 1:31 PM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> This patch adds an extra optimization to *cmp<dwi>_doubleword to improve
> the code generated for comparisons against -1. Hypothetically, if a
> comparison against -1 reached this splitter we'd currently generate code
> that looks like:
>
> notq %rdx ; 3 bytes
> notq %rax ; 3 bytes
> orq %rdx, %rax ; 3 bytes
> setne %al
>
> With this patch we would instead generate the superior:
>
> andq %rdx, %rax ; 3 bytes
> cmpq $-1, %rax ; 4 bytes
> setne %al
>
> which is both faster and smaller, and also what's currently generated
> thanks to the middle-end splitting double word comparisons against
> zero and minus one during RTL expansion. Should that change, this would
> become a missed-optimization regression, but this patch also (potentially)
> helps suitable comparisons created by CSE and combine.
>
> This patch has been tested on x86_64-pc-linux-gnu, on its own and in
> combination with a middle-end patch tweaking RTL expansion, both with
> and without --target-board=unix{-m32}, with no new failures.
> Ok for mainline?
>
>
> 2022-08-02 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/ChangeLog
> * config/i386/i386.md (*cmp<dwi>_doubleword): Add a special case
> to split comparisons against -1 using AND and CMP -1 instructions.
OK.
Thanks,
Uros.
