[Bug 1598618] [NEW] 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)

[Alex]

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)

Status in glibc package in Ubuntu:
  New

Bug description:
  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)

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