You can wrap the if (token_cache[token] == SUPP_UNKNOWN) check, as well as the if (token > OPAL_LAST) check above it, with unlikely(), since this function is in the hot path. Out-of-bounds tokens are very rare, and after the cache is populated, the SUPP_UNKNOWN condition is expected to be false in almost all cases. Wrapping these checks with unlikely() will help the compiler optimize branch