Help in CRC custom instructions please.

The definition of __builtin_custom_ini() is in the gcc sources, there isn't anything 'special' about it you could write it yourself. It expends to a single nios instruction.

I've a very similar one:

__attribute__((always_inline))
static __inline__ unsigned int
custom_inic(const unsigned int op, unsigned int a, const unsigned int b)
{
    uint32_t result;
    __asm__ ( "custom\t%1, %0, %2, c%3"
        : "=r" (result) : "n" (op), "r" (a), "n" (b));
    return result;
}

for custom instructions where I use the 'regb' field as a sub-opcode

(In particular for supporting all the byte and bit swaps in a single instruction).

The custom instruction number is allocated by sopc builder/qsys when you add the instruction, unfortunately you don't seem to be able to choose your own number [1].

The instruction number is then written to one of the generated .h files.

[1] This is a problem if you are trying to define multiple nios cpu with different (overlapping) sets of custom instructions. Or want to be able to compile code without the sopc/qsys generated header file.