Re: OpenMP directive turns a statement into a real code block [with proper example]

[Peter Cech <pcech at vision dot ee dot ethz dot ch>]

On Sat, Jun 20, 2009 at 06:42:22AM -0700, Tim Prince wrote:
> Peter Cech wrote:
> 
> > I was adding some OpenMP paralellization into an existing code and run
> > into a puzzling problem. Here is a snippet to illustrate:
> > 
> >   // Inside a parallel for loop
> > 
> >   // The code statement below modifies a shared data structure
> >   #pragma omp critical
> >   Class instance = create_and_register_new_instance()
> > 
> >   // Problem here - compiler claims that "instance" was not declared in
> >   // this scope
> >   instance.do_something()
> > 
> > So it seems that g++ 4.3 turns the code into:
> > 
> >   
> >   // The code statement below modifies a shared data structure
> >   #pragma omp critical
> >   {
> >     Class instance = create_and_register_new_instance()
> >   }
> > 
> >   // "instance" not declared here
> >   instance.do_something()
> > 
> 
> This looks right.  If you wish the critical region to include more than a
> single statement, you need an explicit block with {}.

Ah, sorry, I was not very clear. I should not have been lazy to prepare
proper example code. here is something you can feed to a compiler
directly:

  #include <map>

  // Just a dummy class with a method
  class Class
  {
    public:
      Class() {}

      void do_something() {
	// some computations here
	return;
      }
  };


  // something that modifies shared/global data structure
  Class & create_and_register_new_instance()
  {
    static std::map<unsigned int, Class> registry;
    return registry[registry.size()];
  }

  void loop1()
  {
    for (int i = 0; i < 10; ++i) {
      Class instance = create_and_register_new_instance();

      instance.do_something();
    }
  }

  void loop2()
  {
  # pragma omp parallel for
    for (int i = 0; i < 10; ++i) {
  #   pragma omp critical
      Class instance = create_and_register_new_instance();

      instance.do_something();
    }
  }


If I do not enable OpenMP, it compiles fine (just warning about ignoring the pragmas):

  $ g++ -W -Wall openmp.cpp -c
  openmp.cpp:34: warning: ignoring #pragma omp parallel
  openmp.cpp:36: warning: ignoring #pragma omp critical

But if I enable OpenMP it does not compile anymore:

  $ g++ -W -Wall openmp.cpp -c -fopenmp
  openmp.cpp: In function 'void loop2()':
  openmp.cpp:39: error: 'instance' was not declared in this scope

I would expect a clean compilation in both cases.

Thanks,
Peter