Question

I was trying to compile the following code:

#pragma omp parallel shared (j)
{
   #pragma omp for schedule(dynamic)
   for(i = 0; i != j; i++)
   {
      // do something
   }
}

but I got the following error: error: invalid controlling predicate.

The OpenMP standard states that for parallel for constructor it "only" allows one of the following operators: <, <=, > >=.

I do not understand the rationale for not allowing i != j. I could understand, in the case of the static schedule, since the compiler needs to pre-compute the number of iterations assigned to each thread. But I can’t understand why this limitation in such case for example. Any clues?

---

EDIT: even if I make for(i = 0; i != 100; i++), although I could just have put "<" or "<=" .

Answer 1

.

I sent an email to OpenMP developers about this subject, the answer I got:

For signed int, the wrap around behavior is undefined. If we allow !=, programmers may get unexpected tripcount. The problem is whether the compiler can generate code to compute a trip count for the loop.

For a simple loop, like:

for( i = 0; i < n; ++i )

the compiler can determine that there are ‘n’ iterations, if n>=0, and zero iterations if n < 0.

For a loop like:

for( i = 0; i != n; ++i ) 

again, a compiler should be able to determine that there are ‘n’ iterations, if n >= 0; if n < 0, we don’t know how many iterations it has.

For a loop like:

for( i = 0; i < n; i += 2 )

the compiler can generate code to compute the trip count (loop iteration count) as floor((n+1)/2) if n >= 0, and 0 if n < 0.

For a loop like:

for( i = 0; i != n; i += 2 )

the compiler can’t determine whether ‘i’ will ever hit ‘n’. What if ‘n’ is an odd number?

For a loop like:

for( i = 0; i < n; i += k )

the compiler can generate code to compute the trip count as floor((n+k-1)/k) if n >= 0, and 0 if n < 0, because the compiler knows that the loop must count up; in this case, if k < 0, it’s not a legal OpenMP program.

For a loop like:

for( i = 0; i != n; i += k )

the compiler doesn’t even know if i is counting up or down. It doesn’t know if ‘i’ will ever hit ‘n’. It may be an infinite loop.

Credits: OpenMP ARB