On 05/04/2011 01:41 PM, mark florisson wrote:
On 4 May 2011 13:39, Dag Sverre Seljebotn<d.s.seljeb...@astro.uio.no> wrote:
On 05/04/2011 01:30 PM, mark florisson wrote:
On 4 May 2011 13:15, Dag Sverre Seljebotn<d.s.seljeb...@astro.uio.no>
wrote:
On 05/04/2011 12:59 PM, mark florisson wrote:
On 4 May 2011 12:45, Dag Sverre Seljebotn<d.s.seljeb...@astro.uio.no>
wrote:
On 05/04/2011 12:00 PM, mark florisson wrote:
There are two remaining issue. The first is warnings for potentially
uninitialized variables for prange(). When you do
for i in prange(start, stop, step): ...
it generates code like
nsteps = (stop - start) / step;
#pragma omp parallel for lastprivate(i)
for (temp = 0; temp< nsteps; temp++) {
i = start + temp * step;
...
}
So here it will complain about 'i' being potentially uninitialized, as
it might not be assigned to in the loop. However, simply assigning 0
to 'i' can't work either, as you expect zero iterations not to touch
it. So for now, we have a bunch of warnings, as I don't see a
__attribute__ to suppress it selectively.
Isn't this is orthogonal to OpenMP -- even if it said "range", your
testcase
could get such a warning? If so, the fix is simply to initialize i in
your
testcase code.
No, the problem is that 'i' needs to be lastprivate, and 'i' is
assigned to in the loop body. It's irrelevant whether 'i' is assigned
to before the loop. I think this is the case because the spec says
that lastprivate variables will get the value of the private variable
of the last sequential iteration, but it cannot at compile time know
whether there might be zero iterations, which I believe the spec
doesn't have anything to say about. So basically we could guard
against it by checking if nsteps> 0, but the compiler doesn't detect
this, so it will still issue a warning even if 'i' is initialized (the
warning is at the place of the lastprivate declaration).
Ah. But this is then more important than I initially thought it was. You
are
saying that this is the case:
cdef int i = 0
with nogil:
for i in prange(n):
...
print i # garbage when n == 0?
I think it may be, depending on the implementation. With libgomp it
return 0. With the check it should also return 0.
It would be in the interest of less semantic differences w.r.t. range to
deal better with this case.
Will it silence the warning if we make "i" firstprivate as well as
lastprivate? firstprivate would only affect the case of zero iterations,
since we overwrite with NaN if the loop is entered...
Well, it wouldn't be NaN, it would be start + step * temp :) But, yes,
Doh.
that works. So we need both the check and an initialization in there:
if (nsteps> 0) {
i = 0;
#pragma omp parallel for firstprivate(i) lastprivate(i)
for (temp = 0; ...; ...) ...
}
Why do you need the if-test? Won't simply
#pragma omp parallel for firstprivate(i) lastprivate(i)
for (temp = 0; ...; ...) ...
do the job -- any initial value will be copied into all threads, including
the "last" thread, even if there are no iterations?
It will, but you don't expect your iteration variable to change with
zero iterations.
Look.
i = 42
for i in prange(n):
f(i)
print i # want 42 whenever n == 0
Now, translate this to:
i = 42;
#pragma omp parallel for firstprivate(i) lastprivate(i)
for (temp = 0; ...; ...) {
i = ...
}
#pragma omp parallel end
/* At this point, i == 42 if n == 0 */
Am I missing something?
DS
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