the code is not exactly memcmp, because the sign of the result doesn't match the sign of memcmp. I verified this with my favorite open source C program prover, CBMC http://www.cprover.org/cbmc/

You can see that CBMC proves that (1) a zero result from mycmp guarantees a zero result from memcmp, and (2) a nonzero result from mycmp guarantees a nonzero result fro mycmp. But (3) a negative result from mycmp doesn't guarantee a negative result from memcmp and (4) a positive result doesn't guarantee a positive result.

So, modulo problems with "strict aliasing" in modern C standards, you can use this code to compare blocks of memory for equality, but you cannot use it to order blocks according to a less-than or greater-than predicate.

https://gist.github.com/jepler/760cfcd4b326d6d11241256a6a4c7...