Why do we need arrays when we have vectors in C++
Arrays allow us to use the __restrict__ keyword. For instance, the following snippet, when compiled (with g++ -O3) and run on my system.
#include <cstdlib> #include <iostream> #include <vector> #include <sys/time.h> #include <unistd.h> using namespace std; void with_vectors(vector<int> a, vector<int> b) { for (int i = 0; i < a.size(); i++) { b[i] *= a[i]; } } void with_arrays(int *__restrict__ a, int *__restrict__ b, int length) { for (int i = 0; i < length; i++) { b[i] *= a[i]; } } long current_time_usecs() { struct timeval time; gettimeofday(&time, NULL); return time.tv_sec * 1000000 + time.tv_usec; } int main() { const int kLength = 5 * 1024 * 1024; int *array_0 = new int[kLength]; int *array_1 = new int[kLength]; for (int i = 0; i < kLength; i++) { array_0[i] = rand(); array_1[i] = rand(); } vector<int> vec_0(array_0, array_0 + kLength); vector<int> vec_1(array_1, array_1 + kLength); long begin_time = current_time_usecs(); with_vectors(vec_0, vec_1); cout << "vectors took " << current_time_usecs() - begin_time << " usecs" << endl; begin_time = current_time_usecs(); with_arrays(array_0, array_1, kLength); cout << "arrays took " << current_time_usecs() - begin_time << " usecs" << endl; return 0; }
outputs
1 2 |
vectors took 23522 usecs arrays took 5447 usecs |
The code using arrays is significantly
faster.
If you look at the assembly output (again, on g++ -O3), you‘ll
see that the vector version is a simple scalar loop but the version using
arrays is vectorized (ironic use of terms, I know :) ); and hence can make use
of the processor‘s SIMD extensions.
Puritans may note that this isn‘t a
fundamental theoretical issue, but a pragmatic one. You may have
super-sophisticated compilers which can "see" that with_vectors may be
vectorized as well.
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