I have a C++ program that runs slower under 4.0 CVS than 3.4. So, I am trying to make some test-cases that might help deduce the reason. However, when I reduced this testcase sufficiently, it began behaving badly under BOTH 3.4 and 4.0.... but I guess I should start with the most reduced case first.
Basically, the code just does a lot of multiplies and adds. However, if I take the main loop outside of an if-block, it goes 5x faster. Also, if I implement an array as 'double*' instead of 'vector<double>' it also goes 5x faster. Using valarray<double> instead of vector<double> does not give any improvement.
MATH INSIDE IF-BLOCK % time ./2h 1 double addition result = 83283300.006041
real 0m0.995s user 0m1.000s sys 0m0.000s
MATH OUTSIDE IF-BLOCK % time ./2i 1 result = 83283299.999998
real 0m0.218s user 0m0.220s sys 0m0.000s
Should I submit a PR? Any help would be appreciated...
-BenRI
------------ begin testcase ------------- #include <vector>
const int OUTER = 100000; const int INNER = 1000;
using namespace std;
int main(int argn, char *argv[])
{
int s = atoi(argv[1]); double result;
if (s == 1) { //remove this condition to get a 5x speedup
// initialize d
vector<double> d(INNER); //change to double* to get 5x speedup
for (int i = 0; i < INNER; i++)
d[i] = double(1+i) / INNER; // calc result
result=0;
for (int i = 0; i < OUTER; ++i)
for (int j = 1; j < INNER; ++j)
result += d[j]*d[j-1] + d[j-1];
}
else
exit(-1); printf("result = %f\n",result);
return 0;
}
----------- end testcase --------------