[Bug rtl-optimization/30970] New: Register zeroing by xor N,N should be moved out of loop

[ubizjak at gmail dot com]

The testcase:

--cut here--
#define N 256
int b[N];

void test()
{  
  int i;

  for (i = 0; i < N; i++)
    b[i] = 0;
}
--cut here--

compiles with '-O2 -msse2 -ftree-vectorize' into:

test:
        movl    $16, %eax
        pxor    %xmm0, %xmm0
        movdqa  %xmm0, b
        .p2align 4,,7
.L2:
        pxor    %xmm0, %xmm0
        movdqa  %xmm0, b(%eax)
        addl    $16, %eax
        cmpl    $1024, %eax
        jne     .L2
        rep ; ret

Please note second pxor that is _not_ needed. Also, it should be moved out of
loop as it is loop invariant.

For slightly different testcase, where 'b[i] = 1' (or anything != 0) we get
optimized code:

test:
        movl    $16, %eax
        movdqa  .LC0, %xmm0
        movdqa  %xmm0, b
        .p2align 4,,7
.L2:
        movdqa  %xmm0, b(%eax)
        addl    $16, %eax
        cmpl    $1024, %eax
        jne     .L2
        rep ; ret

It looks like (g)cse doesn't know what 'xor N,N' means.


-- 
           Summary: Register zeroing by xor N,N should be moved out of loop
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ubizjak at gmail dot com
  GCC host triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30970