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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/97756] Inefficient handling of 128-bit arguments
Date: Wed, 10 Mar 2021 12:55:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97756

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Perhaps related to this PR: On x86_64, the following basic wrapper around
int128 addition

  __uint128_t f(__uint128_t x, __uint128_t y) { return x + y; }

gets compiled (/w -O3, -O2 or -Os) to the seemingly suboptimal

        movq    %rdi, %r9
        movq    %rdx, %rax
        movq    %rsi, %r8
        movq    %rcx, %rdx
        addq    %r9, %rax
        adcq    %r8, %rdx
        ret

Clang does:

        movq    %rdi, %rax
        addq    %rdx, %rax
        adcq    %rcx, %rsi
        movq    %rsi, %rdx
        retq=