[Bug tree-optimization/101842] New: Vectorizer doesn't vectorize when loop bound depends on two independent variables that are unknown

[tnfchris at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101842

            Bug ID: 101842
           Summary: Vectorizer doesn't vectorize when loop bound depends
                    on two independent variables that are unknown
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tnfchris at gcc dot gnu.org
  Target Milestone: ---

The following example

float f(float *p, float d, int len, float lim)
{
  float m[4];
  for (int i = 0; i < len && d >= lim; i += 4)
  {
    m[0] = p[0] * p[0];
    m[1] = p[1] * p[1];
    m[2] = p[2] * p[2];
    m[3] = p[3] * p[3];
    d = d - m[0];
    d = d - m[1];
    d = d - m[2];
    d = d - m[3];
    p += 4;
  }

  return d;
}

isn't vectorized at -Ofast because

```
missed: not vectorized: number of iterations cannot be computed.
```

which seems odd because I would expect that it would be treated as just any
other loop with unbounded iterations.  Commenting out this check results in it
bailing out because of it not knowing how to deal with the reduction.

This loop should be easy to vectorize with vectorizing the multiplications of m
and then reducing the changes of `d - sum (m[0..3])`.