Simple performance question

Question

Simple performance question. the simplest way possible, assume
I have a int[][][][][] matrix, and a boolean add. The array is several dimensions long.
When add is true, I must add a constant value to each element in the array.
When add is false, I must subtract a constant value to each element in the array.
Assume this is very hot code, i.e. it is called very often. How expensive is the condition checking? I present the two scenarios.
private void process(){
for (int i=0;i<dimension1;i++)
for (int ii=0;ii<dimension1;ii++)
  for (int iii=0;iii<dimension1;iii++)
    for (int iiii=0;iiii<dimension1;iiii++)
         if (add)
         matrix[i][ii][iii][...]  += constant;
         else
         matrix[i][ii][iii][...]  -= constant;
private void process(){
  if (add)
for (int i=0;i<dimension1;i++)
for (int ii=0;ii<dimension1;ii++)
  for (int iii=0;iii<dimension1;iii++)
    for (int iiii=0;iiii<dimension1;iiii++)
         matrix[i][ii][iii][...]  += constant;
else
for (int i=0;i<dimension1;i++)
for (int ii=0;ii<dimension1;ii++)
  for (int iii=0;iii<dimension1;iii++)
    for (int iiii=0;iiii<dimension1;iiii++)
       matrix[i][ii][iii][...]  -= constant;
}Is the second scenario worth a significant performance boost? Without understanding how the compilers generates executable code, it seems that in the first case, n^d conditions are checked, whereas in the second, only 1. It is however, less elegant, but I am willing to do it for a significant improvement.

Answer 1

erjoalgo wrote:
I guess my real question is, will the compiler optimize the condition check out when it realizes the boolean value will not change through these iterations, and if it does not, is it worth doing that micro optimization?Almost certainly not; the main reason being that
matrix[i][ii][iii][...]  +/-= constantis liable to take many times longer than the condition check, and you can't avoid it. That said, Mel's suggestion is probably the best.
but I will follow amickr advice and not worry about it.Good idea. Saves you getting flamed with all the quotes about premature optimization.
Winston