Optimised string comparison.

strncmp_fast.c

#include <stddef.h>
#include <stdint.h>

int strncmp_fast(const char *str1, const char *str2, size_t size)
{
	typedef uint_fast32_t word_t;

	int blocks = size >> 2;
	if (size > sizeof(word_t)) {
		word_t *word1 = (word_t *)str1;
		word_t *word2 = (word_t *)str2;
		for (int block = 0; block < blocks; ++block) {
			if (!(word1[block] ^ word2[block]))
				continue;
			int i = block << 2;
			const char *a = str1 + i, *b = str2 + i;
			if ((a[0] ^ b[0]) || !a[0]) return a[0] - b[0];
			if ((a[1] ^ b[1]) || !a[1]) return a[1] - b[1];
			if ((a[2] ^ b[2]) || !a[2]) return a[2] - b[2];
			if ((a[3] ^ b[3]) || !a[3]) return a[3] - b[3];
		}
	}

	int offset = blocks << 2;
	if (offset < size) {
		const char *a = str1 + offset, *b = str2 + offset;
		if ((a[0] ^ b[0]))
			return a[0] - b[0];
		if (offset < size - 1) {
			if ((a[1] ^ b[1]))
				return a[1] - b[1];
			if (offset < size - 2 && (a[2] ^ b[2]))
				return a[2] - b[2];
		}
	}

	return 0;
}