[Bug tree-optimization/49121] [4.7 Regression] FAIL: gcc.dg/tree-ssa/ivopt_infer_2.c scan-tree-dump-times ivopts "Replacing" 0

[vries at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49121

--- Comment #3 from vries at gcc dot gnu.org 2011-05-23 16:07:04 UTC ---
test case:
...
#ifndef TYPE
#define TYPE char*
#endif

extern int a[];

/* Can not infer loop iteration from array -- exit test can not be replaced. 
*/
void foo (int i_width, TYPE dst, TYPE src1, TYPE src2)
{
      TYPE dstn= dst + i_width;
      TYPE dst0 = dst;
      unsigned long long i = 0;
       for( ; dst <= dstn; )
       {
           dst0[i] = ( src1[i] + src2[i] + 1 +a[i]) >> 1;
           dst++;
       i += 16;
       }
}
...

test case representation at ivopts:
...
foo (int i_width, char * dst, char * src1, char * src2)
{
  long long unsigned int i;
  char * dstn;
  char D.2707;
  int D.2706;
  int D.2705;
  int D.2704;
  int D.2703;
  int D.2702;
  int D.2701;
  char D.2700;
  char * D.2699;
  int D.2698;
  char D.2697;
  char * D.2696;
  char * D.2695;
  long unsigned int D.2694;

<bb 2>:
  D.2694_4 = (long unsigned int) i_width_3(D);
  dstn_6 = dst_5(D) + D.2694_4;
  if (dst_5(D) <= dstn_6)
    goto <bb 3>;
  else
    goto <bb 7>;

<bb 3>:

<bb 4>:
  # dst_32 = PHI <dst_24(5), dst_5(D)(3)>
  # i_33 = PHI <i_25(5), 0(3)>
  D.2695_9 = dst_5(D) + i_33;
  D.2696_11 = src1_10(D) + i_33;
  D.2697_12 = *D.2696_11;
  D.2698_13 = (int) D.2697_12;
  D.2699_15 = src2_14(D) + i_33;
  D.2700_16 = *D.2699_15;
  D.2701_17 = (int) D.2700_16;
  D.2702_18 = D.2698_13 + D.2701_17;
  D.2703_19 = D.2702_18 + 1;
  D.2704_20 = a[i_33];
  D.2705_21 = D.2703_19 + D.2704_20;
  D.2706_22 = D.2705_21 >> 1;
  D.2707_23 = (char) D.2706_22;
  *D.2695_9 = D.2707_23;
  dst_24 = dst_32 + 1;
  i_25 = i_33 + 16;
  if (dstn_6 >= dst_24)
    goto <bb 5>;
  else
    goto <bb 6>;

<bb 5>:
  goto <bb 4>;

<bb 6>:

<bb 7>:
  return;

}
...

the patch causes estimated_loop_iterations (loop, true, &max_niter) in
may_eliminate_iv to return a different value:
without patch: 0x10000000000000000
with    patch: 0x01000000000000000

The analysis in the patch concludes that the pointer arithmetic statement
  D.2695_9 = dst_5(D) + i_33
can only be executed 0x01000000000000000 times, based on the fact that i is
incremented with 16 each iteration. Note that the statement corresponds to the
pointer arithmetic in 'dst0[i]'.

Since the proposed new char pointer iterator can represent 0x10000000000000000
distinct values, may_eliminate_iv returns true, and the exit test is replaced:
...
Replacing exit test: if (dstn_6 >= dst_24)
...

This causes the check:
...
/* { dg-final { scan-tree-dump-times "Replacing" 0 "ivopts"} } */
...
to fail.

The test case needs to be adapted.