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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/65832] Inefficient vector construction
Date: Tue, 28 Apr 2015 08:51:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65832

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Another example where the vectorizer thinks vectorization is profitable:

#define N 16

unsigned int out[N];
unsigned int in[N] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};

__attribute__ ((noinline)) int
main1 (unsigned int x, unsigned int y)
{
  int i;
  unsigned int a0, a1, a2, a3;

  a0 = in[0];
  a1 = in[1];
  a2 = in[2];
  a3 = in[3];

  out[0] = a0 * x;
  out[1] = a1 * y;
  out[2] = a2 * x;
  out[3] = a3 * y;
}

generates

main1:
.LFB0:
        .cfi_startproc
        movl    %edi, -12(%rsp)
        movd    -12(%rsp), %xmm0
        movl    %esi, -12(%rsp)
        movd    -12(%rsp), %xmm3
        movdqa  in(%rip), %xmm2
        punpckldq       %xmm3, %xmm0
        psrlq   $32, %xmm2
        punpcklqdq      %xmm0, %xmm0
        movdqa  %xmm0, %xmm1
        psrlq   $32, %xmm0
        pmuludq %xmm2, %xmm0
        pshufd  $8, %xmm0, %xmm0
        pmuludq in(%rip), %xmm1
        pshufd  $8, %xmm1, %xmm1
        punpckldq       %xmm0, %xmm1
        movaps  %xmm1, out(%rip)
        ret

slightly less obfuscated when we allow gpr xmm moves with -mtune=intel:

main1:
.LFB0:
        .cfi_startproc
        movd    %edi, %xmm0
        movd    %esi, %xmm3
        movdqa  in(%rip), %xmm2
        punpckldq       %xmm3, %xmm0
        punpcklqdq      %xmm0, %xmm0
        psrlq   $32, %xmm2
        movdqa  %xmm0, %xmm1
        psrlq   $32, %xmm0
        pmuludq in(%rip), %xmm1
        pmuludq %xmm2, %xmm0
        pshufd  $8, %xmm1, %xmm1
        pshufd  $8, %xmm0, %xmm0
        punpckldq       %xmm0, %xmm1
        movdqa  %xmm1, out(%rip)
        ret

so for { x, y, x, y } construction we generate

        movd    %edi, %xmm0
        movd    %esi, %xmm3
        punpckldq       %xmm3, %xmm0
        punpcklqdq      %xmm0, %xmm0

no f*** idea where all the shifting and shuffling comes from...

This is just

  vect_cst_.7_18 = {x_6(D), y_9(D), x_6(D), y_9(D)};
  vect_a0_2.5_17 = MEM[(unsigned int *)&in];
  vect__7.6_19 = vect_a0_2.5_17 * vect_cst_.7_18;
  MEM[(unsigned int *)&out] = vect__7.6_19;