Is there any way to do 128-bit shifts on gcc <4

Question

gcc 4.4 seems to be the first version when they added int128_t. I need to use bit shifting and I have run out of room for some bit fields.

Edit: It might be because I'm on a 32-bit computer, there's no way to have it for a 32-bit computer (Intel Atom), is there? I wouldn't care if it generated tricky slow machine code if I would work as expected with bit shifting.

Answer 1

You could use two 64-bit ints, but then you need to keep track of the bits moving between.

Answer 2

Bit shifting is very easy in any arbitrary number of bits. Just remember to shift the overflowed bits to the next limb. That's all

typedef struct {
   int64_t high;
   uint64_t low;
} int128_t;


int128_t shift_left(int128_t v, unsigned shiftcount)
{
   int128_t result;
   result.high = (v.high << shiftcount) | (v.low >> (64 - shiftcount));
   result.low  =  v.low  << shiftcount;
   return result;
}

Similar for shift right

int128_t shift_right(int128_t v, unsigned shiftcount)
{
   int128_t result;
   result.low  = (v.low  >> shiftcount) | (v.high << (64 - shiftcount));
   result.high =  v.high >> shiftcount;
   return result;
}

Answer 3

You could also use a library. This would have the advantage that it is portable (regarding platform and compiler) and you could easily switch to even bigger datatype. One I could recommend is gmp (even if its intention is not to handle bitwidth x, but variable as big as you want).

Answer 4

I'm pretty sure that __int128_t is available on earlier versions of gcc. Just checked on 4.2.1 and FreeBSD and sizeof(__int128_t) gives 16.