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From: "cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/43644] __uint128_t missed optimizations.
Date: Sun, 07 May 2023 06:57:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D43644

--- Comment #2 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:d8a6945c6ea22efa4d5e42fe1922d2b27953c8cd

commit r14-554-gd8a6945c6ea22efa4d5e42fe1922d2b27953c8cd
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Sun May 7 07:52:15 2023 +0100

    Don't call emit_clobber in lower-subreg.cc's resolve_simple_move.

    Following up on posts/reviews by Segher and Uros, there's some question
    over why the middle-end's lower subreg pass emits a clobber (of a
    multi-word register) into the instruction stream before emitting the
    sequence of moves of the word-sized parts.  This clobber interferes
    with (LRA) register allocation, preventing the multi-word pseudo to
    remain in the same hard registers.  This patch eliminates this
    (presumably superfluous) clobber and thereby improves register allocati=
on.

    A concrete example of the observed improvement is PR target/43644.
    For the test case:
    __int128 foo(__int128 x, __int128 y) { return x+y; }

    on x86_64-pc-linux-gnu, gcc -O2 currently generates:

    foo:    movq    %rsi, %rax
            movq    %rdi, %r8
            movq    %rax, %rdi
            movq    %rdx, %rax
            movq    %rcx, %rdx
            addq    %r8, %rax
            adcq    %rdi, %rdx
            ret

    with this patch, we now generate the much improved:

    foo:    movq    %rdx, %rax
            movq    %rcx, %rdx
            addq    %rdi, %rax
            adcq    %rsi, %rdx
            ret

    2023-05-07  Roger Sayle  <roger@nextmovesoftware.com>

    gcc/ChangeLog
            PR target/43644
            * lower-subreg.cc (resolve_simple_move): Don't emit a clobber
            immediately before moving a multi-word register by parts.

    gcc/testsuite/ChangeLog
            PR target/43644
            * gcc.target/i386/pr43644.c: New test case.=