> To improve the performance, take the redundant multiplication out of the loop like below:

Um, that multiplication is already outside the loop. You said before that one shouldn't trust the optimiser, but it did the right thing here...

EDIT: (After reading the entire thing) The memory aliasing point completely correct and very important (even though you probably want the `b[i] = val` outside the inner loop...); in C, I'd argue that this point needs even more attention if we're micro-optimising already.

The strlen call is only repeated (leading to quadratic behaviour) because the string may be modified in the inner loop. Might be worth mentioning that the calls will be hoisted (i.e. merged to one) if the string is constant and nothing can alias to it.

The "replacing multiplication by additions", the replacing multiplication with a shift and the hoisting I mentioned above are all redundant optimisations for a programmer: compilers will trivially do that already. The first two are instances of "strength reduction" and feature in most compilers' -O1 set.

Conclusion: I do like the first part about the actual trivia, but I think the micro-optimisation part misses the point somewhat.