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#include <array> | |
#include <chrono> | |
#include <iostream> | |
// Width of each dimension | |
constexpr size_t N = 200; | |
// Number of iterations for the test | |
constexpr size_t NUM_ITERATIONS = 100; | |
// Data type we'll use for testing | |
using DataType = short; | |
// 1D array index from a 3D coordinate | |
int index(int x, int y, int z) { | |
return N*(N*y + z) + x; | |
} | |
void test1() { | |
using namespace std::chrono; | |
const DataType data[N*N*N] = {0}; | |
int sum = 0; | |
const auto start = high_resolution_clock::now(); | |
for(int iteration = 0; iteration < NUM_ITERATIONS; ++iteration) { | |
for(int x = 0; x < N; ++x) { | |
for(int z = 0; z < N; ++z) { | |
for(int y = 0; y < N; ++y) { | |
if(x > 0 && x + 1 < N) { | |
sum += data[index(x - 1, y, z)] / 2; | |
sum += data[index(x, y, z)]; | |
sum += data[index(x + 1, y, z)] / 2; | |
} else { | |
sum += data[index(x, y, z)]; | |
} | |
} | |
} | |
} | |
} | |
const auto elapsed = high_resolution_clock::now() - start; | |
const auto ms = duration_cast<milliseconds>(elapsed).count(); | |
std::cout << sum << " in " << ((1.0 * ms) / NUM_ITERATIONS) << "ms per loop\n"; | |
} | |
void test2() { | |
using namespace std::chrono; | |
const DataType data[N*N*N] = {0}; | |
int sum = 0; | |
const auto start = high_resolution_clock::now(); | |
for(int iteration = 0; iteration < NUM_ITERATIONS; ++iteration) { | |
for(int x = 0; x < N; ++x) { | |
for(int z = 0; z < N; ++z) { | |
for(int y = 0; y < N; ++y) { | |
if(x > 0 && x + 1 < N) { | |
sum += data[index(x - 1, y, z)] / 2; | |
sum += data[index(x, y, z)]; | |
sum += data[index(x + 1, y, z)] / 2; | |
} else { | |
sum += data[index(x, y, z)]; | |
} | |
const int prefetch_index = index(x, y + 20, z); | |
__builtin_prefetch(data + prefetch_index, 0, 0); | |
} | |
} | |
} | |
} | |
const auto elapsed = high_resolution_clock::now() - start; | |
const auto ms = duration_cast<milliseconds>(elapsed).count(); | |
std::cout << sum << " in " << ((1.0 * ms) / NUM_ITERATIONS) << "ms per loop\n"; | |
} | |
int main(int argc, char **argv) { | |
test1(); | |
test2(); | |
return 0; | |
} |
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$ clang++ -O2 -std=c++1y main.cpp | |
$ ./a.out | |
0 in 145.29ms per loop | |
0 in 84.02ms per loop |
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