Re: Optimizing a function?

From:

Rune Allnor <allnor@tele.ntnu.no>

Newsgroups:

comp.lang.c++

Date:

Sun, 13 Dec 2009 11:03:31 -0800 (PST)

Message-ID:

<d63b7004-6260-4900-a619-6392efa1b36c@m3g2000yqf.googlegroups.com>

On 12 Des, 16:05, "carl" <carl@.com> wrote:

I have a function that gets called for 512*512*350 times. In this functio=

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_spl=

it;

     key = x_dir + y_dir + z_dir;
     return key;
   }

The number of elements in the std::vector<MyObj> is fixed. When the ve=

ctors

contains 216 elements the program takes around 50 seconds to complete (on=

3 GHZ machine with 6GB RAM and code compiled for release, using Visual
Studio 2008).

From what I can see, the code contains a lot of vector
accesses. If correct, make sure your code is compiled
with pre-processor directive SECURE_SCL=0, no buffer
security check (flag /GS-) and with the enhanced
instruction set enabled (flag /arch:SSE2).

Rune