"forall" statement

[cole01527]

Hello,
I was just reading a book on parallel programming and there are many C code examples on a statement which looks something like this:

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

I googled around for "forall" and it says everywhere that this is actually HPF (High performance FORTRAN) statement for achieveing loop parallelization on multiprocessor systems but I found no information that this can be used in C. However, in the book all examples are shown in C.
So, are there any C compilers that support "forall" statement? I tried this in GNU gcc 3.x C compiler but it doesn't work. Does GNU gcc 4.x C compiler support this?
Also, since this looks like a very simple loop paralelization method (you just add "forall" and that's it? - even more simple than OpenMP), is this effective parallelizing loops?

thanks for any information.

regards

c.

[mitakeet]

There is no standard for paralell processing in C or C++, you must use third-party libs (which are often macros).

[xErath]

first time for me I see such thing

[dwks]

Is it like foreach in perl?

[cole01527]

In many parallel programming books and tutorials there are examples of this "forall" construsct but I haven't seen any C compilers capable of doing that. What it does is simply create n separate parallel processes executed at the same time - for example if we have:

forall (i=0; i<=10; i++)

..it means that for every single i there is going to be a separate process. This would greately simplify loop parallelization on numerically intensive computations on multiprocessor systems and you wouldn't have to go through all that threads, forks, MPI, OMP, ..you name it.. simply just add "forall" and thatt's it.
I don't know, that's why I'm asking, the book I have - "Parallel programming (Wilkinson, Allen), Prentice/Hall 1999" uses those "forall" constructs all over the book just as if it was a normal C construct without including any libraries !!!???
It does say on one page that sometimes C compilers use "parfor" instead "forall". After I googled around I came across this page:

http://www.cs.nyu.edu/milan/publications/sd97/

par { 
	parfor (int i=0; i<N; i++) {
		statment_1; statment_1;
	 	: : : :
		statement_n; statement_n;
	}
}

..hmm, looks like C languege to me, ..and another example from the above link:

#include "chime.h"
#include <stdlib.h>
#include <iostream.h>
const int N=500;

float A[N][N], B[N][N], C[N][N];
void RandomFill(float mat[N][N], int size){
	for(int i=0; i<size; i++)
	for(int j=0; j<size; j++)
		mat[i][j] = rand();
} /* RandomFill */

void main(int argc, char *argv[]){
	int numOfSubDiv;
	cout << "Input num of sub divisions: ";
	cin >> numOfSubDiv ;
	RandomFill(A,N);
	RandomFill(B,N);
	parfor(int division=0; division < numOfSubDiv; division++){
int from = division * (N/numOfSubDiv);
		int to = from + (N/numOfSubDiv);
		for(int i=from; i<to; i++)
		for(int j=0; j<N; j++){
		C[i][j] = 0;
		for (int k=0; k<N; k++)
			C[i][j] += A[i][k] * B[k][j];
		}
	} /* parfor */
} /* main */

So, "parfor" should be for C ?

Which compilers? Is this "chime.h" needed for this? "forall" also seems to be standard construct in Fortarn 95, but all free/GNU Fortran compilers available on the web failed to build on my machine so I can't try this "magical" construct..

c.

[cole01527]

Is it like foreach in perl?

sorry, I don't know anything about perl languege - I know just Fortran IV, 77, 90, C and a little of C++. What does "foreach" do? From what I understood, "parfor" and "forall" start all loops as separate processes at the same time - sounds simple and VERY useful for clusters etc. that's all..

c.

[Codeplug]

This type of support is typically built into the compiler itself.

Google("parallelizing C compiler")

Happy hunting.

gg

[quzah]

There is no standard for paralell processing in C or C++, you must use third-party libs (which are often macros).

#define forall for

Quzah.

[Raptor007]

#define forall for

Quzah.

True, this will make it work, but it's no longer doing anything in parallel.

Although, I've never heard of a statement that simple setting everything up in parallel. Even MPI, which is pretty easy to use, has a lot more code involved than that. But then again, MPI is the only parallel experience I have so far. Perhaps someone else is familiar with this "forall" approach.

[Codeplug]

Parallelizing C compilers have been around for a long time. There's no standard syntax and the actuall means of parallelization varies.

I've even seen parallelizing pre-processors that turn "forall" statements into actual MPI, PVM, or pthreads code - that a "normal" compiler can then compile.

gg

[quzah]

True, this will make it work, but it's no longer doing anything in parallel.

Some people have no sense of humor.


Quzah.