[Bug rtl-optimization/53533] [4.7/4.8 regression] vectorization causes loop unrolling test slowdown as measured by Adobe's C++Benchmark

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53533

--- Comment #8 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-06-12 
10:27:15 UTC ---
Small testcase:

int a[256];
int b[256];

void foo (void)
{
  int i;
  for (i = 0; i < 256; ++i)
    {
      b[i] = a[i] * 23;
    }
}

you can see that we shuffle even the vector with constants around!  Not taking
into account the REG_EQUAL note which is gone at split1 time, removed by
either loop2_invariant or loop2_unswitch.

(insn 26 24 27 3 (set (reg:V4SI 82 [ vect_var_.10 ])
        (mult:V4SI (reg:V4SI 83 [ MEM[symbol: a, index: ivtmp.20_9, offset: 0B]
])
            (reg:V4SI 85))) t.c:9 1496 {*sse2_mulv4si3}
     (expr_list:REG_EQUAL (mult:V4SI (reg:V4SI 83 [ MEM[symbol: a, index:
ivtmp.20_9, offset: 0B] ])
            (const_vector:V4SI [
                    (const_int 23 [0x17])
                    (const_int 23 [0x17])
                    (const_int 23 [0x17])
                    (const_int 23 [0x17])
                ]))
        (expr_list:REG_DEAD (reg:V4SI 84)
            (expr_list:REG_DEAD (reg:V4SI 83 [ MEM[symbol: a, index:
ivtmp.20_9, offset: 0B] ])
                (nil)))))