Simple CRC-16-MCRF4XX C implementation.

crc16_mcrf4xx.c

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

uint16_t crc16_mcrf4xx(uint16_t crc, uint8_t *data, size_t len)
{
    if (!data || len < 0)
        return crc;

    while (len--) {
        crc ^= *data++;
        for (int i=0; i<8; i++) {
            if (crc & 1)  crc = (crc >> 1) ^ 0x8408;
            else          crc = (crc >> 1);
        }
    }
    return crc;
}

You're right, it should be if (!data || len <= 0) for performance, but the functionality comes out the same because len-- returns 0 so it skips down to return crc; anyways. The CRC of an empty array is valid and should equal the CRC start value.

With just -O1 there is not much difference. Tested with 12 bytes:

$ g++ -O0 crc.cpp -o crc
$ ./crc                 
10M runs crc16_mcrf4xx: 945.964 ms
10M runs crc16_mcrf4xx_fast: 509.515 ms
$ g++ -O1 crc.cpp -o crc
$ ./crc                 
10M runs crc16_mcrf4xx: 12.783 ms
10M runs crc16_mcrf4xx_fast: 12.746 ms

1024 bytes:

$ g++ -O0 crc.cpp -o crc
$ ./crc                 
10k runs crc16_mcrf4xx: 352.066 ms
10k runs crc16_mcrf4xx_fast: 73.724 ms
$ g++ -O1 crc.cpp -o crc
$ ./crc                 
10k runs crc16_mcrf4xx: 0.016 ms
10k runs crc16_mcrf4xx_fast: 0.015 ms

The initial implementation and the one by @mudgek1 produce different results for some numbers (e.g. 0x76). Which one is correct?