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  <roger@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.


Thanks again,
Roger
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