Is there any counter-indication to doing this ? Or is the behavior well specified?
#pragma omp parallel for
for(auto x : stl_container)
{
...
}
Because it seems that OpenMP specification is only valid for c++98 but I guess there might be more incompatibilities due to C++11 threads, which are not used here. I wanted to be sure, still.
The OpenMP 4.0 specification was finalised and published several days ago here. It still mandates that parallel loops should be in the canonical form (§2.6, p.51):
for (init-expr;test-expr;incr-expr)structured-block
The standard allows for containers that provide random-access iterators to be used in all of the expressions, e.g.:
#pragma omp parallel for
for (it = v.begin(); it < v.end(); it++)
{
...
}
If you still insist on using the C++11 syntactic sugar, and if it takes a (comparatively) lot of time to process each element of stl_container, then you could use the single-producer tasking pattern:
#pragma omp parallel
{
#pragma omp single
{
for (auto x : stl_container)
{
#pragma omp task
{
// Do something with x, e.g.
compute(x);
}
}
}
}
Tasking induces certain overhead so it would make no sense to use this pattern if compute(x); takes very little time to complete.
!= is an Intel-ism - the standard does not allow it to be used, but icpc is smart enough to deduce what's going on in the loop. I had it corrected. That's also one of the reasons why random-access iterators are required - the usual iterators only provide the == and != operators.
std::map has no random access iter, so the < relation doesnt work.OpenMP 5.0 adds the following line on page 99, which makes a lot of range-based for loops OK !
2.12.1.3 A range-based for loop with random access iterator has a canonical loop form.
Source : https://www.openmp.org/wp-content/uploads/OpenMP-API-Specification-5.0.pdf
