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From: "hjl.tools at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/103393] [12 Regression] Generating 256bit register
 usage with -mprefer-avx128 -mprefer-vector-width=128
Date: Wed, 02 Mar 2022 14:52:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103393

--- Comment #24 from H.J. Lu <hjl.tools at gmail dot com> ---
Another testcase:

[hjl@gnu-tgl-2 pr103393]$ cat x.c
struct TestData {
  float arr[8];
};
void cpy(struct TestData *s1, struct TestData *s2 ) {
  for(int i=3D0; i<16; ++i) {
    s1->arr[i] =3D s2->arr[i];
  }
}
[hjl@gnu-tgl-2 pr103393]$ make x.s
/export/build/gnu/tools-build/gcc-debug/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/tools-build/gcc-debug/build-x86_64-linux/gcc/ -O2
-march=3Dskylake-avx512 -S x.c
[hjl@gnu-tgl-2 pr103393]$ cat x.s
        .file   "x.c"
        .text
        .p2align 4
        .globl  cpy
        .type   cpy, @function
cpy:
.LFB0:
        .cfi_startproc
        vmovdqu64       (%rsi), %zmm0
        vmovdqu64       %zmm0, (%rdi)
        vzeroupper
        ret
        .cfi_endproc
.LFE0:
        .size   cpy, .-cpy
        .ident  "GCC: (GNU) 12.0.1 20220301 (experimental)"
        .section        .note.GNU-stack,"",@progbits
[hjl@gnu-tgl-2 pr103393]$=20

ZMM is used when we try to avoid it.=