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From: "sthlm58 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/67434] New: std::chrono::duration acts like static even if instantiated every time
Date: Wed, 02 Sep 2015 11:15:00 -0000
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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