openmp g++ error: collapsed loops not perfectly nested

问题:

I try to compile

#include <omp.h>  using namespace std;  vector< vector<int> > multiplyMatrixes(const vector< vector<int> > &a, const vector<     vector<int> > &b, int aHeight, int aWidth, int bHeight, int bWidth) {     vector < vector<int> > c(aHeight, vector<int>(bWidth, 0));     #pragma omp parallel for collapse(2)     for(int row = 0; row < aHeight; row++) {             for(int col = 0; col < bWidth; col++) {                    int value = 0;                    for(int i = 0; i < aWidth; i++) {                           value += a[row][i] * b[i][col];                    }                    c[row][col] = value;                    cout<<"Tread #"<<omp_get_thread_num()<<"\n";             }             std::cout<<'\n';     }     return c; }  int main() {}

with 'g++ -fopenmp hello.cpp -o hello' command, gcc version is 4.7, but i get following 'hello.cpp:19:17: error: collapsed loops not perfectly nested' What does it mean?

回答1:

Googling for the error finds "The loops must be perfectly nested; that is, there is no intervening code nor any OpenMP pragma between the loops which are collapsed"

I think that means the code before and after the for(i) loop is not allowed.

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