Public bug reported:
I started to use Ubuntu 16.04 with gcc 5.3.1-14ubuntu2.1, libc 2.23-0ubuntu3.
And I wrote a simple program:
#include <cmath>
#include <iostream>
#include <chrono>
#define CYCLE_NUM 1E+7
using namespace std;
using namespace std::chrono;
int main()
{
double sum = 0.0;
auto t1 = high_resolution_clock::now();
for(int i = 1; i < CYCLE_NUM; i++)
{
sum += sin((double)(i)) / (double)(i);
}
auto t2 = high_resolution_clock::now();
microseconds::rep t = duration_cast<microseconds>(t2-t1).count();
cout << "sum = " << sum << endl;
cout << "time = " << (double)(t) / 1.E+6 << endl;
return 0;
}
which compiled with command:
g++ -std=c++11 -O3 main.cpp -o sum
This program work 2.5 times slower than the same program in Ubuntu
14.04.3 with gcc 4.8.4 libc-2.19 compiled with the same command.
I've tried to compile the same code in VS C++ 2015 with the same level
of optimization and I've got almost the same time as I've got in Ubuntu
14.04 (gcc 4.8.4 libc-2.19).
My proc is i7-5820, haswell.
It seems that math functions (no only sin, also cos, exp) now process
significantly slower compared to older versions of c++ libraries or
compiler.
I've created topic on stackoverflow.com :
http://stackoverflow.com/questions/38172066/the-program-works-3-times-slower-compiled-with-g-5-3-1-than-the-same-program-c
ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: libc-bin 2.23-0ubuntu3
ProcVersionSignature: Ubuntu 4.4.0-28.47-generic 4.4.13
Uname: Linux 4.4.0-28-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Jul 3 23:24:33 2016
Dependencies:
gcc-6-base 6.0.1-0ubuntu1
libc6 2.23-0ubuntu3
libgcc1 1:6.0.1-0ubuntu1
InstallationDate: Installed on 2016-07-02 (1 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: glibc
UpgradeStatus: No upgrade log present (probably fresh install)
** Affects: glibc (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug xenial
** Attachment added: "Assembler & preprocessed listings of programm pointed in
message"
https://bugs.launchpad.net/bugs/1598618/+attachment/4694855/+files/asm_preprocessed_listing.tar.gz
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https://bugs.launchpad.net/bugs/1598618
Title:
Math calls to libc-2.23 compiled with g++ 5.3.1 (Ubuntu 16.04) process
2.5 time slower compared to libc-2.19 compiled with g++ 4.8.4 (Ubuntu
14.04)
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