[Bug target/47949] Missed optimization for -Os using xchg instead of mov.

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47949

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Roger Sayle <sayle@gcc.gnu.org>:

https://gcc.gnu.org/g:fc6ef90173478521982e9df3831a06ea85b4f41e

commit r13-1945-gfc6ef90173478521982e9df3831a06ea85b4f41e
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Wed Aug 3 09:07:36 2022 +0100

    PR target/47949: Use xchg to move from/to AX_REG with -Oz on x86.

    This patch adds a peephole2 to i386.md to implement the suggestion in
    PR target/47949, of using xchg instead of mov for moving values to/from
    the %rax/%eax register, controlled by -Oz, as the xchg instruction is
    one byte shorter than the move it is replacing.

    The new test case is taken from the PR:
    int foo(int x) { return x; }

    where previously we'd generate:
    foo:    mov %edi,%eax  // 2 bytes
            ret

    but with this patch, using -Oz, we generate:
    foo:    xchg %eax,%edi  // 1 byte
            ret

    On the CSiBE benchmark, this saves a total of 10238 bytes (reducing
    the -Oz total from 3661796 bytes to 3651558 bytes, a 0.28% saving).

    Interestingly, some modern architectures (such as Zen 3) implement
    xchg using zero latency register renaming (just like mov), so in theory
    this transformation could be enabled when optimizing for speed, if
    benchmarking shows the improved code density produces consistently
    better performance.  However, this is architecture dependent, and
    there may be interactions using xchg (instead a single_set) in the
    late RTL passes (such as cprop_hardreg), so for now I've restricted
    this to -Oz.

    2022-08-03  Roger Sayle  <roger@nextmovesoftware.com>
                Uroš Bizjak  <ubizjak@gmail.com>

    gcc/ChangeLog
            PR target/47949
            * config/i386/i386.md (peephole2): New peephole2 to convert
            SWI48 moves to/from %rax/%eax where the src is dead to xchg,
            when optimizing for minimal size with -Oz.

    gcc/testsuite/ChangeLog
            PR target/47949
            * gcc.target/i386/pr47949.c: New test case.