[Bug rtl-optimization/34849] New: Missed autoincrement oppurtunities thanks to a different basic block structure.

["ramana dot radhakrishnan at celunite dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Whilst investigating a missed optimization oppurtunity in comparison to gcc 3.4
I came across this case. 

void foo (int n, int in[], int res[])
{
  int i;
  for (i=0; i<n;i++)
    if (in[i])
      res[i]= 0x1234;
    else
      res[i]= 0x9876;
}

The basic block structure generated is as follows 
foo (n, in, res)
{
  unsigned int ivtmp.19;
  int i;

<bb 2>:
  if (n > 0)
    goto <bb 3>;
  else
    goto <bb 8>;

<bb 3>:
  i = 0;
  ivtmp.19 = 0;

<bb 4>:
  if (MEM[base: in, index: ivtmp.19] != 0)
    goto <bb 5>;
  else
    goto <bb 6>;

<bb 5>:
  MEM[base: res, index: ivtmp.19] = 4660;
  goto <bb 7>;

<bb 6>:
  MEM[base: res, index: ivtmp.19] = 39030;

<bb 7>:
  i = i + 1;
  ivtmp.19 = ivtmp.19 + 4;
  if (n > i)
    goto <bb 4>;
  else
    goto <bb 8>;

<bb 8>:
  return;

}

If you notice ivtmp.19 can be used for post-increment based addressing modes. 


Note that GCC 3.4 did not have another basic block for the else case, the basic
block for the else case got merged with the tail block of the loop and hence
auto-inc could get generated in the else case and not in the if side of things.
Can be reproduced with today's head of 4.3.0


-- 
           Summary: Missed autoincrement oppurtunities thanks to a different
                    basic block structure.
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ramana dot radhakrishnan at celunite dot com
 GCC build triplet: i686-linux-gnu
  GCC host triplet: i686-linux-gnu
GCC target triplet: arm-none-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34849