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From: "michael.kenzel at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/115351] New: [14 regression] pointless movs when passing by
 value on x86-64
Date: Wed, 05 Jun 2024 03:14:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115351

            Bug ID: 115351
           Summary: [14 regression] pointless movs when passing by value
                    on x86-64
           Product: gcc
           Version: 14.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: michael.kenzel at gmail dot com
  Target Milestone: ---

When passing structs of certain shape by value on x86-64 (seems to be speci=
fic
to this target), gcc 14 emits (fails to optimize away?) a number of redunda=
nt
moves:

        struct complex
        {
                double real;
                double imag;
        };

        complex blub(complex z)
        {
                return {
                        z.real * z.real - z.imag * z.imag,
                        2 * z.real * z.imag
                };
        }

on gcc 13 with -O3 results in

        blub(complex):
                movapd  xmm3, xmm1
                movapd  xmm2, xmm0
                mulsd   xmm3, xmm1
                addsd   xmm2, xmm2
                mulsd   xmm0, xmm0
                mulsd   xmm1, xmm2
                subsd   xmm0, xmm3
                ret

gcc 14 and later with -O3 emit

        blub(complex):
                        movq    rax, xmm1
                        movq    rdx, xmm0
                        xchg    rdx, rax
                        movq    xmm2, rax
                        movq    xmm1, rdx
                        movapd  xmm0, xmm2
                        movapd  xmm3, xmm1
                        mulsd   xmm3, xmm1
                        mulsd   xmm0, xmm2
                        addsd   xmm2, xmm2
                        mulsd   xmm1, xmm2
                        subsd   xmm0, xmm3
                        ret

godbolt: https://godbolt.org/z/hzEfs3ob4=