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* [Bug libstdc++/67434] New: std::chrono::duration acts like static even if instantiated every time
@ 2015-09-02 11:15 sthlm58 at gmail dot com
2015-09-02 11:28 ` [Bug libstdc++/67434] " redi at gcc dot gnu.org
2015-09-03 11:55 ` sthlm58 at gmail dot com
0 siblings, 2 replies; 3+ messages in thread
From: sthlm58 at gmail dot com @ 2015-09-02 11:15 UTC (permalink / raw)
To: gcc-bugs
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
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug libstdc++/67434] std::chrono::duration acts like static even if instantiated every time
2015-09-02 11:15 [Bug libstdc++/67434] New: std::chrono::duration acts like static even if instantiated every time sthlm58 at gmail dot com
@ 2015-09-02 11:28 ` redi at gcc dot gnu.org
2015-09-03 11:55 ` sthlm58 at gmail dot com
1 sibling, 0 replies; 3+ messages in thread
From: redi at gcc dot gnu.org @ 2015-09-02 11:28 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug libstdc++/67434] std::chrono::duration acts like static even if instantiated every time
2015-09-02 11:15 [Bug libstdc++/67434] New: std::chrono::duration acts like static even if instantiated every time sthlm58 at gmail dot com
2015-09-02 11:28 ` [Bug libstdc++/67434] " redi at gcc dot gnu.org
@ 2015-09-03 11:55 ` sthlm58 at gmail dot com
1 sibling, 0 replies; 3+ messages in thread
From: sthlm58 at gmail dot com @ 2015-09-03 11:55 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2015-09-03 11:55 UTC | newest]
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2015-09-02 11:15 [Bug libstdc++/67434] New: std::chrono::duration acts like static even if instantiated every time sthlm58 at gmail dot com
2015-09-02 11:28 ` [Bug libstdc++/67434] " redi at gcc dot gnu.org
2015-09-03 11:55 ` sthlm58 at gmail dot com
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