On 05/04/2011 01:59 PM, mark florisson wrote:
On 4 May 2011 13:54, Dag Sverre Seljebotn<d.s.seljeb...@astro.uio.no> wrote:
On 05/04/2011 01:48 PM, mark florisson wrote:
On 4 May 2011 13:47, mark florisson<markflorisso...@gmail.com> wrote:
On 4 May 2011 13:45, Dag Sverre Seljebotn<d.s.seljeb...@astro.uio.no>
wrote:
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?
Yes, 'i' may be uninitialized with nsteps> 0 (this should be valid
code). So if nsteps> 0, we need to initialize 'i' to something to get
correct behaviour with firstprivate.
This I don't see. I think I need to be spoon-fed on this one.
So assume this code
cdef int i
for i in prange(10): ...
Now if we transform this without the guard we get
int i;
#pragma omp parallel for firstprivate(i) lastprivate(i)
for (...) { ...}
This is invalid C code, but valid Cython code. So we need to
initialize 'i', but then we get our "leave it unaffected for 0
iterations" paradox. So we need a guard.
You mean C code won't compile if i is firstprivate and not initialized?
(Sorry, I'm not aware of such things.)
My first instinct is to initialize i to 0xbadabada. After all, its value
is not specified -- we're not violating any Cython specs by initializing
it to garbage ourselves.
OTOH, I see that your approach with an if-test is more
Valgrind-friendly, so I'm OK with that.
Would it work to do
if (nsteps > 0) {
#pragma omp parallel
i = 0;
#pragma omp for lastprivate(i)
for (temp = 0; ...) ...
...
}
instead, to get rid of the warning without using a firstprivate? Not
sure if there's an efficiency difference here, I suppose a good C
compiler could compile them to the same thing.
Dag Sverre
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