If you look at where they are used, then it looks like these macros do not need to compare m[1] to c1 because whenever they are used it is already known that m[

If you look at where they are used, then it looks like these macros do not need to compare m[1] to c1 because whenever they are used it is already known that m[1] == 'O' (hence "str3O" in the name -- that's 3 and the letter O, not number zero). I guess c1 is included in the first case because it's faster to compare the whole 32-bit word than mask out the known byte (and since c0, c1, c2 and c3 are compile time constants, the whole right-hand-side of the comparison will be optimized by constant folding anyway). (It's ridiculous micro-optimization in my opinion, but at least it is correct, and it is limited to a single file.)