How Compilers Transform OpenMP Code

To better describe OpenMP debugging, it is helpful to understand how OpenMP code is transformed by the compilers. Consider the following Fortran example:

1    program example
2        integer i, n
3        parameter (n = 1000000)
4        real sum, a(n)
5    
6        do i = 1, n
7        a(i) = i*i
8        end do
9    
10        sum = 0
11    
12    !$OMP PARALLEL DO DEFAULT(PRIVATE), SHARED(a, sum)
13    
14        do i = 1, n
15        sum = sum + a(i)
16        end do
17    
18    !$OMP END PARALLEL DO
19    
20        print*, sum
21        end program example

The code in line 12 through line 18 is a parallel region. The f95 compiler converts this section of code to an outlined subroutine that will be called from the OpenMP runtime library. This outlined subroutine has an internally generated name, in this case _$d1A12.MAIN_. The f95 compiler then replaces the code for the parallel region with a call to the OpenMP runtime library and passes the outlined subroutine as one of its arguments. The OpenMP runtime library handles all the thread-related issues and dispatches slave threads that execute the outlined subroutine in parallel. The C compiler works in the same way.

When debugging an OpenMP program, the outlined subroutine is treated by dbx as any other function, with the exception that you cannot explicitly set a breakpoint in that function by using its internally generated name.