Problem with libstdc++ in GCC 4.4.0 port to interix

[Robert Oeffner]

Hi,

Probably a long shot but I wonder if anyone would have a useful tip on a 
problem porting gcc4.4.0 to interix (a BSD-like OS running on top of the 
Windows kernel).

As libgomp in GCC so far isn't targeting interix I have made some changes to 
libgomp in my copy of the GCC 4.4.0
distribution.  A new source file was created, 
gcc-4.4.0/libgomp/config/posix/interix/proc.c, which is templated on the
existing gcc-4.4.0/libgomp/config/posix/proc.c and 
gcc-4.4.0/libgomp/config/posix/mingw32/proc.c in the distribution (see 
http://www.oeffner.net/stuff/gcc-4.4.0_interix_changes.zip or 
http://www.suacommunity.com/forum/tm.aspx?m=16600 ). With this file and 
modifications to GCC configuration files in the distribution I can bootstrap 
GCC 4.4.0 to build gcc and g++ compilers on interix.

The port produces fast code for single threaded running programs. However, 
there's a major problem with OpenMP. It's something to do with libstdc++ 
that tends to go in overdrive when you request OpenMP to create more than 
one thread for the compiled program. When calling string.erase() from 
libstdc++ it somehow hogs the CPU with high kernel times and runs orders of 
magnitudes slower. The code below demonstrates the problem. It runs fast 
when using just one thread but abysmally slow when two or more threads are 
present, even though the loop doing the work is actually single threaded. 
Windows Taskmanager shows that execution times is roughly 50% kernel and 50% 
user time whenever you run more than one thread. Invoked with a single 
threaded all execution times is just spend in user mode.

If there is anyone here on this list who has a good suggestion that would be 
much appreciated.

Many thanks,

Rob


#include <iostream>
#include <omp.h>

using namespace std;

const long lmax = 50000;

int main()
{
   int nthreads = 1;
   cout<<"Enter number of OpenMP threads to create: ";
   cin >> nthreads;
   omp_set_num_threads(nthreads);

#pragma omp parallel
   {
#pragma omp single
       cout << "Doing string stuff with "<<omp_get_num_threads()<<" 
thread(s)"<<endl;
   }

   time_t start, now;
   time( &start );

   string pairlbl("");

   for (long m = 0; m< lmax; m++)
   {
       if ((m % (lmax/20))==0)
       cout << "m = "<<m<<endl;

       for (int j=1;j<=2000;j++)
       {
           pairlbl.clear();
       }
   }

   time( &now);
   cout<<"\ntime= "<<difftime( now, start )<<" sec\n";

   return 0;
}