c++ - Why is the loop not being vectorized? -

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i know vectorization can take place if objects being accessed contiguous in memory. have created struct has pointer , create vector of struct , ensure pointers inside vector of objects pointing contiguous data blocks set them point elements in vector of double same size. 

#include <iostream> #include <vector>  struct vec {    vec() {}    double* a; };  int main(int argc, char* argv[]) {    std::vector<double> vec_double(10000000, 1.0);    std::vector<vec> vec_vec(10000000);    (unsigned = 0; < 10000000; ++i)       vec_vec[i].a = &(vec_double[i]);     // why loop not vectorized    (unsigned = 0; < 10000000; ++i)       vec_double[i] += *(vec_vec[i].a);     double sum = 0.0;    (unsigned = 0; < 10000000; ++i)       sum += vec_double[i];    std::cout << sum << std::endl;     return 0; }

however, o3 optimization loop @ line number 16 not getting vectorized. can please explain why happening?

just guessing here, when looking @ specific loop, compiler not know vec_vec[i].a points memory location next vec_vec[i+1].a. therefore, cannot calculation without dereferencing each .a member separately.

it know that, when looking @ loop above. if that, @ loop below, calculate final result , print it.


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