https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67434
Bug ID: 67434
Summary: std::chrono::duration acts like static even if
instantiated every time
Product: gcc
Version: 5.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: sthlm58 at gmail dot com
Target Milestone: ---
For some reason std::chrono:duration maintains state (when instantiated as
local variable) if compiled together with std::random_device in the same scope.
Found on GCC:
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-unknown-linux-gnu/5.2.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ./configure --prefix=/usr/local --disable-multilib
--without-mpc --without-mpfr --without-gmp --without-cloog --without-isl
--enable-languages=c,c++
Thread model: posix
gcc version 5.2.0 (GCC)
Reproduce with:
#include <iostream>
#include <algorithm>
#include <random>
#include <chrono>
std::chrono::duration<double> benchmark( )
{
std::random_device rd;
std::chrono::duration<double> total;
for (int i = 0; i < 100; i++)
{
auto t1 = std::chrono::high_resolution_clock::now();
auto t2 = std::chrono::high_resolution_clock::now();
total += std::chrono::duration_cast<std::chrono::duration<double>>(t2 -
t1);
}
return total;
}
int main()
{
for (int i = 0; i < 10; i++)
{
std::cout << "time: " << benchmark().count() << std::endl;
}
return 0;
}
Build & run:
g++ -std=c++14 -O2 main.cpp -Wall -Wextra && ./a.out
Output (e.g.):
http://coliru.stacked-crooked.com/a/0d4dcd432a4b0f51
time: 0.000112018
time: 0.000222767
time: 0.000333607
time: 0.000444344
time: 0.000554722
time: 0.00066537
time: 0.000775786
time: 0.000886287
time: 0.000996599
time: 0.0011071
Expected output (when commenting line 8) (e.g.):
http://coliru.stacked-crooked.com/a/e1f9fa0323d4d3f4
time: 0.00010953
time: 0.000110805
time: 0.000111902
time: 0.000112299
time: 0.000109301
time: 0.000109781
time: 0.000109413
time: 0.000109506
time: 0.000109258
time: 0.000110727
