[Bug libstdc++/67434] New: std::chrono::duration acts like static even if instantiated every time

[Bug libstdc++/67434] New: std::chrono::duration acts like static even if instantiated every time

[sthlm58 at gmail dot com]

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

[Bug libstdc++/67434] std::chrono::duration acts like static even if instantiated every time

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67434

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Michal Kucharski from comment #0)
> std::chrono::duration<double> benchmark( ) 
> {
>   std::random_device rd;
> 
>   std::chrono::duration<double> total;

You have not initialized this variable.

>   
>   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);

This has undefined behaviour because you are performing addition on an
uninitialized value. You could have found this with valgrind.

[Bug libstdc++/67434] std::chrono::duration acts like static even if instantiated every time

[sthlm58 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67434

--- Comment #2 from sthlm58 at gmail dot com ---
(In reply to Jonathan Wakely from comment #1)
> (In reply to Michal Kucharski from comment #0)
> > std::chrono::duration<double> benchmark( ) 
> > {
> >   std::random_device rd;
> > 
> >   std::chrono::duration<double> total;
> 
> You have not initialized this variable.
> 
> >   
> >   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);
> 
> This has undefined behaviour because you are performing addition on an
> uninitialized value. You could have found this with valgrind.

In other words, here 'total' was default-initialized (hence unspecified) thus
causing undefined behavior. It it were value-initialized e.g. with '{}' the
problem would not happen.