Strange performance behavior

[Bill Baxter]

Marius Muja wrote:
> I have noticed the following strange (at least for me) performance 
> behavior with one of my programs. It is a program that does some 
> scientific computations and while trying to optimize it I noticed that 
> the code from case_B below executes faster (almost twice as fast) as the 
> code in case_A. This is a bit counterintuitive for me, since in case _B 
> there is also the cost of the function call (or should be the same if 
> the function is inlined).
> Can anybody shed some light on why it's behaving this way?
> 
> case_A:
> -------------------------------
> foreach (i,index; indices) {
>    foreach (k, inout value; centers[belongs_to[i]])
>     value += vecs[index][k];
> }
> ----------------------------------
> 
> case_B:
> -------------------------------
> void addTo(T,U)(T[] a, U[] b) {
>    foreach(index, inout value; a) {
>       value += b[index];
>    }
> }
> ....
> foreach (i,index; indices) {
>    addTo(centers[belongs_to[i]],vecs[index]);
> }
> _______________________________

My guess would be the compiler's failure to optimize[*] away the [index] 
indexing in A.  So you do 2 lookups per iteration rather than just one. 
  If so then this should be just as fast as case_B:

foreach (i,index; indices) {
    auto vecs_i = vecs[index];
    foreach (k, inout value; centers[belongs_to[i]])
       value += vecs_i[k];
}


--bb