[Bug tree-optimization/99364] New: Regression: loops not optimized to count to 0/-1

[stefan at franke dot ms via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99364

            Bug ID: 99364
           Summary: Regression: loops not optimized to count to 0/-1
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stefan at franke dot ms
  Target Milestone: ---

consider this simple loop:
```
void ArrayFill2(int *array, int len) {
  int i = 0;
  for(; i < len; ++i)
    array[i] = 50;
}
```

gcc 2.93 was able to convert this into a loop counting to zero.
gcc 6/7/8/9/10/11 can't do that.

if you change the code above:
```
void ArrayFill2(int *array) {
  int i = 0;
  int len = 42;
  for(; i < len; ++i)
    array[i] = 50;
}
```
then it works and you get this after ivcanon:
```
  int i;
  unsigned int i.0_4;
  unsigned int _5;
  int * _7;
  unsigned int ivtmp_15;
  unsigned int ivtmp_16;

  <bb 2>:

  <bb 3>:
  # i_13 = PHI <i_9(4), 0(2)>
  # ivtmp_16 = PHI <ivtmp_15(4), 42(2)>
  i.0_4 = (unsigned int) i_13;
  _5 = i.0_4 * 4;
  _7 = array_6(D) + _5;
  *_7 = 50;
  i_9 = i_13 + 1;
  ivtmp_15 = ivtmp_16 - 1;
  if (ivtmp_15 != 0)
    goto <bb 4>;
  else
    goto <bb 5>;

  <bb 4>:
  goto <bb 3>;

  <bb 5>:
  return;

```

so the expected result for the version above, with len as parameter would be
something like this where the constant loop count is replaced by len-1:

```
  int i;
  unsigned int i.0_4;
  unsigned int _5;
  int * _7;
  int ivtmp_14
  unsigned int ivtmp_15;
  unsigned int ivtmp_16;

  <bb 2>:
  ivtmp_14 = len_4(D) - 1;

  <bb 3>:
  # i_13 = PHI <i_9(4), 0(2)>
  # ivtmp_16 = PHI <ivtmp_15(4), ivtmp_14(2)>
  i.0_4 = (unsigned int) i_13;
  _5 = i.0_4 * 4;
  _7 = array_6(D) + _5;
  *_7 = 50;
  i_9 = i_13 + 1;
  ivtmp_15 = ivtmp_16 - 1;
  if (ivtmp_15 != 0)
    goto <bb 4>;
  else
    goto <bb 5>;

  <bb 4>:
  goto <bb 3>;

  <bb 5>:
  return;
```

To handle this tree-ssa-loop-ivcanon must be able to handle no only const_int
loop limits but also a const loop limit provided in a variable.