[Cython] prange CEP updated

[Dag Sverre Seljebotn]

On 05/04/2011 01:41 PM, mark florisson wrote:
> On 4 May 2011 13:39, Dag Sverre Seljebotn<d.s.seljebotn at 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.seljebotn at 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.seljebotn at 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