Why is valarray so slow?

Question

I am trying to use valarray since it is much like MATLAB while operating vector and matrices. I first did some performance check and found that valarray cannot achieve the p

Answer 1

I suspect that the reason c = a*b is so much slower than performing the operations an element at a time is that the

template valarray operator*
    (const valarray&, const valarray&);

operator must allocate memory to put the result into, then returns that by value.

Even if a "swaptimization" is used to perform the copy, that function still has the overhead of

• allocating the new block for the resulting valarray
• initializing the new valarray (it's possible that this might be optimized away)
• putting the results into the new valarray
• paging in the memory for the new valarray as it is initialized or set with result values
• deallocating the old valarray that gets replaced by the result