[Bug tree-optimization/51179] poor vectorization on interlagos.

[ubizjak at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51179

Uros Bizjak <ubizjak at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011-11-22
                 CC|                            |irar at il dot ibm.com
          Component|target                      |tree-optimization
     Ever Confirmed|0                           |1

--- Comment #2 from Uros Bizjak <ubizjak at gmail dot com> 2011-11-22 11:33:24 UTC ---
We can start here with something that hopefully resembles your original fortran
code:

--cut here--
double C[10][4], B[10][10], A[10][4];

void test (void)
{
  int i = 0, j = 0, l = 0;

//for (; j < 10; j += 2)
//  for (; l < 10; l++)
      for (; i < 4; i++)
        {
          C[j+0][i] = C[j+0][i] + A[l][i] * B[j+0][l];
          C[j+1][i] = C[j+1][i] + A[l][i] * B[j+1][l];
        }
}
--cut here--

gcc -O3 -ffast-math -mfma4 -mavx:

test:
        vmovapd A(%rip), %ymm0
        vbroadcastsd    B(%rip), %ymm1
        vfmaddpd        C(%rip), %ymm1, %ymm0, %ymm1
        vmovapd %ymm1, C(%rip)
        vbroadcastsd    B+80(%rip), %ymm1
        vfmaddpd        C+32(%rip), %ymm1, %ymm0, %ymm0
        vmovapd %ymm0, C+32(%rip)
        vzeroupper
        ret

Nice.

Now uncomment the second loop ("l" index) and this kernel will break:

        < ... lots of code deleted ... >
.L3:
        vmovupd (%r8,%rax), %xmm1
        addl    $1, %esi
        vinsertf128     $0x1, 16(%r8,%rax), %ymm1, %ymm1
        vfmaddpd        %ymm0, %ymm5, %ymm1, %ymm0
        vmovapd %ymm0, (%rbx,%rax)
        vmovupd (%rcx,%rax), %xmm0
        vinsertf128     $0x1, 16(%rcx,%rax), %ymm0, %ymm0
        vfmaddpd        %ymm0, %ymm4, %ymm1, %ymm0
        vmovupd %xmm0, (%rcx,%rax)
        vextractf128    $0x1, %ymm0, 16(%rcx,%rax)
        addq    $32, %rax
        cmpl    %r10d, %esi
        jb      .L3
        < ... lots of code deleted ... >

This already happens in the tree optimizers (vectorizer), RTL is just following
this trail.

Confirmed as a vectorizer problem.