http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49575
Summary: OpenMP has a problem with -funroll-loops
Product: gcc
Version: 4.5.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: sailorw...@hotmail.com
-funroll-loops optimisation option does not work on OpenMP in some cases.
Attached an example. Compiled with options
g++ -g -O2 -funroll-loops -fomit-frame-pointer -march=native -fopenmp
On a 4-core single Intel CPU machine with Kubuntu 11.04, the following program
compiled with OpenMP is around 20 times slower than the program compiled
without OpenMP, because -funroll-loops option does not work on the OpenMP
version. Defining k variable as constant or moving its declaration to inside
the for loop can solve the problem, but they should not be necessary. I think
-funroll-loops should work on the following program as it is.
#include <math.h>
#include <iostream>
using namespace std;
int main ()
{
long double i=0;
long double k=0.7;
#pragma omp parallel for firstprivate(k) reduction(+:i)
for(int t=1; t<300000000; t++){
for(int n=1; n<16; n++){
i=i+pow(k,n);
}
}
cout << i<<"\t";
return 0;
}
Initial discussion on this topic was at
http://stackoverflow.com/questions/6506987/why-openmp-version-is-slower
