[Bug tree-optimization/24669] New: Loop index variable has offset of 1

[uros at kss-loka dot si]

A simple loop:

void foo (int *a, int *b)
{
  int i;

  for (i = 0; i < 4; i++)
    b[i] = a[i];
}

gets compiled to (gcc -O2 -fomit-frame-pointer):

        pushl   %ebx
        movl    $1, %edx          <<< index starts with 1
        movl    8(%esp), %ebx
        movl    12(%esp), %ecx
        .p2align 4,,15
.L2:
        movl    -4(%ebx,%edx,4), %eax  <<< it is compensated by -4 here
        movl    %eax, -4(%ecx,%edx,4)  <<< and here
        incl    %edx
        cmpl    $5, %edx               <<< compare with N+1
        jne     .L2
        popl    %ebx
        ret

The problem is, that index variable has offset of 1 and it has to be
compensated by appropriate negative offset in the address.

The optimized tree dump shows:

<bb 0>:
  ivtmp.33 = 1;

<L0>:;
  D.1347 = (int *) ivtmp.33;
  MEM[base: b, index: D.1347, step: 4B, offset: -4B] = MEM[base: a, index:
D.1347, step: 4B, offset
: -4B];
  ivtmp.33 = ivtmp.33 + 1;
  if (ivtmp.33 != 5) goto <L0>; else goto <L2>;

<L2>:;
  return;

One would expect the index to start with 0 as this enables use of xor for
initialization. Perhaps non-zero address offset also disables some
optimizations.


-- 
           Summary: Loop index variable has offset of 1
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: uros at kss-loka dot si


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24669