http://llvm.org/bugs/show_bug.cgi?id=19542
Bug ID: 19542
Summary: Odd performance drop with mt19937 and
uniform_real_distribution
Product: clang
Version: 3.4
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: C++11
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Classification: Unclassified
When using mt19937 or mt19937_64 in combination with a
uniform_real_distribution, the performance is about an order of magnitude worse
than gcc's. But it only happens with that particular combination. Those
generators are fine with uniform_int_distribution, and another generator like
default_random_engine works fine with uniform_real_distribution. A program to
reproduce this and my results:
#include <iostream>
#include <vector>
#include <chrono>
#include <random>
template< typename T_rng, typename T_dist>
double timer(T_rng& rng, T_dist& dist, int n){
std::vector< typename T_dist::result_type > vec(n, 0);
auto t1 = std::chrono::high_resolution_clock::now();
for (int i = 0; i < n; ++i)
vec[i] = dist(rng);
auto t2 = std::chrono::high_resolution_clock::now();
auto runtime =
std::chrono::duration_cast<std::chrono::microseconds>(t2-t1).count()/1000.0;
return runtime;
}
int main(){
const int n = 10000000;
std::default_random_engine rng_default(1);
std::mt19937 rng_mt (1);
std::mt19937_64 rng_mt_64 (1);
std::uniform_int_distribution<int> dist_int(0,1000);
std::uniform_real_distribution<float> dist_float(0.0, 1.0);
std::cout << "int_default: " << timer(rng_default, dist_int, n) <<
std::endl;
std::cout << "int_mt: " << timer(rng_mt_64, dist_int, n) << std::endl;
std::cout << "int_mt_64: " << timer(rng_mt_64, dist_int, n) << std::endl;
std::cout << "float_default: " << timer(rng_default, dist_float, n) <<
std::endl;
std::cout << "float_mt: " << timer(rng_mt, dist_float, n) << std::endl;
std::cout << "float_mt_64: " << timer(rng_mt_64, dist_float, n) <<
std::endl;
}
result for my linux 64bit:
// g++ 4.8.1
int_default: 184.687
int_mt: 176.092
int_mt_64: 176.076
float_default: 45.478
float_mt: 58.018
float_mt_64: 90.073
// clang 3.4
int_default: 207.594
int_mt: 195.68
int_mt_64: 195.702
float_default: 54.535
float_mt: 747.402 <----- slow
float_mt_64: 781.237 <-- slow
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