Question

From a long time ago I have a memory which has stuck with me that says comparisons against zero are faster than any other value (ahem Z80).

In some C code I’m writing I want to skip values which have all their bits set. Currently the type of these values is char but may change. I have two different alternatives to perform the test:

if (!~b)
    /* skip */

and

if (b == 0xff)
    /* skip */

Apart from the latter making the assumption that b is an 8bit char whereas the former does not, would the former ever be faster due to the old compare to zero optimization trick, or are the CPUs of today way beyond this kind of thing?

Answer 1

If it is faster, the compiler will substitute it for you.

In general, you can’t write C better than the compiler can optimize it. And it is architecture specific anyway.

In short, don’t worry about it unless that sub-micro-nano-second is ultra important