https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81877
--- Comment #11 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #10)
> Now - for refs that have an invariant address in such loop the interleaving
> effectively means that they are independent even in the same iteration.
Not if there's no "other iteration", i.e. runtime iteration count is 1.
[...]
> for example. I suppose it's not their intent. So maybe there's an
> additional
> restriction on the interleaving? Preserve iteration order of individual
> stmts? That would prevent autopar in face of just ivdep for example.
>
> Note that any "handle must-defs 'correctly'" writing is inherently fishy.
I think you're saying that pragma-ivdep and do-concurrent are too hand-wavy
about how the compiler may or must privatize variables, whether it must detect
and handle reductions/inductions etc. But note that LIM is keying on 'simdlen',
and simdlen is also set by OpenMP-SIMD which is more rigorous in that regard,
i.e. privatization is explicit in GIMPLE. And there I believe LIM does not have
the license to disregard may-alias relations *unless* it verifies that loop
iterates at least twice and repeated writes are UB. On this example:
void g(int p, int *out)
{
int x, y = 0, *r = p ? &x : &y;
unsigned n = 0;
asm("" : "+r" (n));
#pragma omp simd
for (int i = 0; i <= n; i++)
{
//#pragma omp ordered simd
x = 42;
out[i] = *r;
}
}
I believe LSM is wrong for n=0, and for any n if the pragma-ordered is
uncommented.
