[Bug target/11877] gcc should use xor trick with -Os

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877

--- Comment #10 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:9cedbaab8e048b90ceb9ceef0d851385fae67cde

commit r12-1668-g9cedbaab8e048b90ceb9ceef0d851385fae67cde
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Mon Jun 21 08:54:50 2021 +0100

    PR target/11877: Use xor to write zero to memory with -Os

    The following patch attempts to resolve PR target/11877 (without
    triggering PR/23102).  On x86_64, writing an SImode or DImode zero
    to memory uses an instruction encoding that is larger than first
    clearing a register (using xor) then writing that to memory.  Hence,
    after reload, the peephole2 pass can determine if there's a suitable
    free register, and if so, use that to shrink the code size with -Os.

    To improve code size, and avoid inserting a large number of xor
    instructions (PR target/23102), this patch makes use of peephole2's
    efficient pattern matching to use a single temporary for a run of
    consecutive writes.  In theory, one could do better still with a
    new target-specific pass, gated on -Os, to shrink these instructions
    (like stv), but that's probably overkill for the little remaining
    space savings.

    Evaluating this patch on the CSiBE benchmark (v2.1.1) results in a
    0.26% code size improvement (3715273 bytes down to 3705477) on x86_64
    with -Os [saving 1 byte every 400].  549 of 894 tests improve, two
    tests grow larger.  Analysis of these 2 pathological cases reveals
    that although peephole2's match_scratch prefers to use a call-clobbered
    register (to avoid requiring a new stack frame), very rarely this
    interacts with GCC's shrink wrapping optimization, which may previously
    have avoided saving/restoring a call clobbered register, such as %eax,
    in the calling function.

    2021-06-21  Roger Sayle  <roger@nextmovesoftware.com>

    gcc/ChangeLog
            PR target/11877
            * config/i386/i386.md: New define_peephole2s to shrink writing
            1, 2 or 4 consecutive zeros to memory when optimizing for size.

    gcc/testsuite/ChangeLog
            PR target/11877
            * gcc.target/i386/pr11877.c: New test case.

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