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From: "crazylht at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/97891] [x86] Consider using registers on large
 initializations
Date: Thu, 24 Dec 2020 02:34:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97891

--- Comment #6 from Hongtao.liu <crazylht at gmail dot com> ---
cat test.c

typedef struct {
  long a;
  long b;
}TI;

extern TI r;
void
foo ()
{
  r.a =3D 0;
  r.b =3D 0;
}

gcc -Ofast -march=3Dcascadelake -S got

foo:
.LFB0:
        .cfi_startproc
        movq    $0, r(%rip)
        movq    $0, r+8(%rip)
        ret
        .cfi_endproc
.LFE0:


SLP failed to vectorize due to cost

test1.c:10:7: note:   =3D=3D=3D vect_slp_analyze_instance_alignment =3D=3D=
=3D
test1.c:10:7: note:   vect_compute_data_ref_alignment:
test1.c:10:7: note:   can't force alignment of ref: r.a
test1.c:10:7: note:   =3D=3D=3D vect_slp_analyze_instance_dependence =3D=3D=
=3D
test1.c:10:7: note:   =3D=3D=3D vect_slp_analyze_operations =3D=3D=3D
test1.c:10:7: note:   =3D=3D> examining statement: r.a =3D 0;
test1.c:10:7: note:   vect_is_simple_use: operand 0, type of def: constant
test1.c:10:7: note:   Vectorizing an unaligned access.
test1.c:10:7: note:   vect_model_store_cost: unaligned supported by hardwar=
e.
test1.c:10:7: note:   vect_model_store_cost: inside_cost =3D 16, prologue_c=
ost =3D
0 .
test1.c:10:7: note:   =3D=3D=3D vect_bb_partition_graph =3D=3D=3D
test1.c:10:7: note: ***** Analysis succeeded with vector mode V16QI
test1.c:10:7: note: SLPing BB part
0x48bc3d0 0 1 times unaligned_store (misalign -1) costs 16 in body
0x48bc3d0 <unknown> 1 times vector_load costs 12 in prologue
0x48bbf50 0 1 times scalar_store costs 12 in body
0x48bbf50 0 1 times scalar_store costs 12 in body
test1.c:10:7: note: Cost model analysis:=20
  Vector inside of basic block cost: 16
  Vector prologue cost: 12
  Vector epilogue cost: 0
  Scalar cost of basic block: 24

Shouldn't cost of zero vector CTOR be different from normal ones?
Since x86 could have pxor, similar for aarch64 (eor??).=