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.
> Dag Sverre
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