Question

Strings are fixed length 8, contain alphanumeric characters and right-padded with spaces.

I.e,

"STRING1 "
"STR2    "
"S       "

etc..

I was thinking memcmp might be the fastest here?

Answer 1

If you ensure that the strings are aligned on an 8-byte boundary via compiler-specific attributes, you can do:

uint64_t a = *((uint64_t *) "STRING1 ");
uint64_t b = *((uint64_t *) "STR2    ");

Then a == b should yield to a single 64-bit instruction.

Or, if they are just constant immutable strings (stored in a read-only area of the process), you can go on with comparing the const char * pointers themselves. It’s still a reliable test since a string literal that appears twice in the current translation unit should refer to the same memory:

/* fails because the two strings are stored at different locations */
"STRING1 " == "STR2    "
/* should succeed, even the silliest compiler should merge both literals */
"STRING1 " == "STRING1 "