[Bug tree-optimization/113026] Bogus -Wstringop-overflow warning on simple memcpy type loop

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113026

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org
     Ever confirmed|0                           |1
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2023-12-15

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
We are diagnosing the scalar store in the epilogue of the vectorized "loop"
where we somehow optimized the vector loop to terminate immediately (based
on dst[] size I guess) and also peeled the epilogue but failed to eliminate
that completely (but then -Wstringop-overflow correctly figures the access
would be out-of-bounds).

A bit reduced (interestingly when avoiding versioning for aliasing
we manage to avoid the diagnostic because of the way we re-use the
epilog when n < 16

char dst[16];
void
foo (char *src, long n)
{
  for (long i = 0; i < n; i++)
    dst[i] = src[i];
}

in the end this is a missed optimization.  We do know an upper bound
for the loop so we should have disabled epilog peeling based on that.

The peeling code assumes the vector loop can be skipped (niter < vf)
only when we either vectorize the epilogue or do not version the loop
while the code computing wheter we need peeling takes into account the
versioning cost model threshold only.