OpenMP在函数内部并行化循环

Question

I am trying to parallelize this for loop inside a function using OpenMP, but when I compile the code I still have an error =(

Error 1 error C3010: 'return' : jump out of OpenMP structured block not allowed.

I am using Visual studio 2010 C++ compiler. Can anyone help me? I appreciate any advice.

int match(char* pattern, int patternSize, char* string, int startFrom, unsigned int &comparisons) {
    comparisons = 0;
#pragma omp for 
    for (int i = 0; i < patternSize; i++){
        comparisons++;
        if (pattern[i] != string[i + startFrom])
            return 0;
    }
    return 1;
}

Answer 1

As @Hristo has already mentioned, you are not allowed to branch out of a parallel region in OpenMP. Among other reasons, this is not allowed because the compiler cannot know a priori how many iterations each thread should work on when it splits up a for loop like the one that you have written among the different threads.

Furthermore, even if you could branch out of your loop, you should be able to see that comparisons would be computed incorrectly. As is, you have an inherently serial algorithm that breaks at the first different character. How could you split up this work such that throwing more threads at this algorithm possibly makes it faster?

Finally, note that there is very little work being done in this loop anyway. You would be very unlikely to see any benefit from OpenMP even if you could rewrite this algorithm into a parallel algorithm. My suggestion: drop OpenMP from this loop and look to implement it somewhere else (either at a higher level - maybe you call this method on different strings? - or in a section of your code that does more work).