Re: Optimizing a function?

"Alf P. Steinbach" <>
Sat, 12 Dec 2009 16:12:54 +0100
* carl:

I have a function that gets called for 512*512*350 times. In this
function there is a loop that gets run each time the function is called:

myFun(VectorType point) {
 std::vector<MyObj> & myobjs =this->FindMyObjs(point);
 int num = myobjs.size();

 for (int i=0; i<num; i++) {
   // Do stuff with each neighbor

Where FindMyObjs(point) is a function that looks up the point in:

  std::map<int, std::vector<MyObj>> m_myMap;

which has been created before the program is run:

std::vector<MyObj> FindMyObjs(VectorType point) {
 int key = computeKey(point);
 return m_myMap[key];

The computeKey(point) looks like this:

  unsigned int computeKey(VectorType location ) {
    unsigned int key = 0;
    int x_dir = floor((location[0]/m_cellSize[0]));
    int y_dir = floor((location[1]/m_cellSize[1]))*m_split;
    int z_dir = floor((location[2]/m_cellSize[2]))*m_split*m_split;
    key = x_dir + y_dir + z_dir;
    return key;

The number of elements in the std::vector<MyObj> is fixed. When the
vectors contains 216 elements the program takes around 50 seconds to
complete (on a 3 GHZ machine with 6GB RAM and code compiled for release,
using Visual Studio 2008).

I have tried to remove the code from the while body but is has almost no
effect on the computation time.

Am I missing something very basic c++ optimization skills there or is
the program not meant to run faster on this machine?

I have made sure that only references to the already existing data
structures are used. Could it be the computeKey (3 floor operations)
that is expensive when called so many times?

It's all very roundabout.

Try an array.

Cheers & hth.,

- Alf

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