++nc vs nc = nc + 1

Question

In K&R Ch 1:

The statement ++nc presents a new operator, ++, which means increment by one. You could instead write nc = nc + 1, but ++nc is more concise and often more efficient.

When would pre-increment be more efficient than the alternative? For most things, at least, the assembly for both is the add (edit: or inc) instruction. When do they differ?

Answer 1

That text is long out dated. It might have been true in the 70's that compilers would produce more efficient output for ++n, but not any more. All modern compilers will produce identical code.

Answer 2

For most things, at least, the assembly for both is the add instruction.

That's not quite true: there is often a separate "increment by one" instruction. However, that's irrelevant since any half-decent compiler will produce identical machine code for ++nc and nc = nc + 1.

In other words, there is no performance difference. There may have been when the book was written and compilers were not very good, but there isn't anymore.