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From: "chfast at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109667] New: [12/13/14 Regression]
 Unnecessary temporary storage used for 32-byte struct
Date: Fri, 28 Apr 2023 11:46:11 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 12.3.1
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109667

            Bug ID: 109667
           Summary: [12/13/14 Regression] Unnecessary temporary storage
                    used for 32-byte struct
           Product: gcc
           Version: 12.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chfast at gmail dot com
  Target Milestone: ---

Reduced reproducer:

struct i256 {
    long v[4];
};
void assign(struct i256 *v, long z) {
    struct i256 r =3D {};
    for (int i =3D 0; i < 1; ++i)=20
        r.v[i] =3D z;
    *v =3D r;
}

https://godbolt.org/z/avM74o3r6

The compiler allocates temporary storage on stack for `r`:

assign:
        pxor    xmm0, xmm0
        mov     QWORD PTR [rsp-40], rsi
        movups  XMMWORD PTR [rsp-32], xmm0
        movdqa  xmm1, XMMWORD PTR [rsp-40]
        mov     QWORD PTR [rsp-16], 0
        movdqa  xmm2, XMMWORD PTR [rsp-24]
        movups  XMMWORD PTR [rdi], xmm1
        movups  XMMWORD PTR [rdi+16], xmm2
        ret

Regression since 12. The 11 compiles nicely to:

assign:
        mov     QWORD PTR [rdi], rsi
        mov     QWORD PTR [rdi+8], 0
        mov     QWORD PTR [rdi+16], 0
        mov     QWORD PTR [rdi+24], 0
        ret=