[Bug tree-optimization/115363] New: Missing loop vectorization due to loop bound load not being pulled out

[Bug tree-optimization/115363] New: Missing loop vectorization due to loop bound load not being pulled out

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115363

            Bug ID: 115363
           Summary: Missing loop vectorization due to loop bound load not
                    being pulled out
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
struct out {
  unsigned *array;
};

struct m{
  void f(out *output);
  void f1(out *output);
  int size;
};

void  m::f(out *output)
{
      for (int k = 0; k < size; k++) {
        output->array[k] += 1;
      }
}

void  m::f1(out *output)
{
  int tmp = size;
  for (int k = 0; k < size; k++) {
    output->array[k] += 1;
  }
}
```

We should be able to vectorize `m::f` but currently does not since this->size
might alias array[k].
But we could version the loop to pull out the this->size out of the loop and we
could vectorize the loop then.

[Bug tree-optimization/115363] Missing loop vectorization due to loop bound load not being pulled out

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115363

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-06-06

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Invariant motion doesn't do versioning for aliasing.  But in fact once the
loop iterates array[k] can no longer alias this->size but this is difficult
to exploit (peeling the loop once would help).

I'm not sure we should start to version all those loops where the exit
condition depends on a not hoistable but invariant expression?

But maybe we can diagnose this so people can rewrite their code.

[Bug tree-optimization/115363] Missing loop vectorization due to loop bound load not being pulled out

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115363

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> Invariant motion doesn't do versioning for aliasing.  But in fact once the
> loop iterates array[k] can no longer alias this->size but this is difficult
> to exploit (peeling the loop once would help).

So I should mention that this was noticed from some code internally which is
optimized by LLVM's polly (graphite) infrastructure and yes it is an important
loop in this code.