In C++, which is faster? (2 * i + 1) or (i << 1 | 1)?

Question

I realize that the answer is probably hardware specific, but I'm curious if there was a more general intuition that I'm missing?

I asked this question & given the answer, now I'm wondering if I should alter my approach in general to use "(i << 1|1)" instead of "(2*i + 1)"??

Answer 1

Since the ISO standard doesn't actually mandate performance requirements, this will depend on the implementation, the compiler flags chosen, the target CPU and quite possibly the phase of the moon.

These sort of optimisations (saving a couple of cycles) almost always pale into insignificance in terms of return on investment, against macro-level optimisations like algorithm selection.

Aim for readability of code first and foremost. If your intent is to shift bits and OR, use the bit-shift version. If your intent is to multiply, use the * version. Only worry about performance once you've established there's an issue.

Any decent compiler will optimise it far better than you can anyway :-)