OpenMP parallel for with floating-point range

Question

I have the following program:

int main(){
   double sum=0;
   #pragma omp parallel for reduction(+:sum)
   for(double x=0;x<10;x+=0.1)
   sum+=x*x;
}

When I compile it, I get the error invalid type for iteration variable ‘x’.

I take this to mean that I can only apply a parallel for construct to integer-based loops. But the internals of my loop really do depend on it being floating-point.

Is there a way to convince OpenMP to do this? Is there a recommended alternative method?

Answer 1

From comments:

No, OpenMP won't do this for you for the same reasons as in this answer given to a question about OpenMP loops with integer arithmetic; it's extremely difficult for the compiler to reason about floating point numbers - in particular, the compiler needs to know before entering the loop the loop's tripcount, and floating point arithmetic in the loop makes that very difficult in general, even if there are some simple cases that would be ok (say, looping by 0.5 to 10.0).

For the same reason, even purely serial optimization/vectorization of loops of this form would suffer. The best way to do it is to make an equivalent integer loop and calculate your float based on the integer index;

for (int i=0; i<100; i++) { 
    double x=0.1*i; 
    sum += x*x; 
}

Answer 2

More generally, you need to go and read What Every Computer Scientist Should Know About Floating-Point Arithmetic. The fact that you're adding 0.1 and expecting precise results suggest that you haven't read it yet :-)

Answer 3

OpenMP standard defines "for" loop only for integer, pointer and iterator types, not for floating point types:

http://www.openmp.org/wp-content/uploads/openmp-4.5.pdf#page=62

2.7.1 Loop Construct

  #pragma omp for [clause[ [,] clause] ... ] new-line
  for-loops

Specifically, all associated for-loops must have canonical loop form (see Section 2.6 on page 53).

2.6 Canonical Loop Form

A loop has canonical loop form if it conforms to the following:

for (init-expr; test-expr; incr-expr) structured-block

init-expr One of the following:

 var = lb
 integer-type var = lb
 random-access-iterator-type var = lb
 pointer-type var = lb

...

incr-expr One of the following:

++var
var++
-- var
var --
var += incr
var -= incr
var = var + incr
var = incr + var
var = var - incr

var One of the following:

• A variable of a signed or unsigned integer type.
• For C++, a variable of a random access iterator type.
• For C, a variable of a pointer type.

The canonical form allows the iteration count of all associated loops to be computed before executing the outermost loop

So, for non-canonical types compiler can't compute iteration counts.