Bitwise operations on 128-bit values on a non-sse2 arch - c

Question

I am writing a routine in C, targeted for an embedded platform.In the routine I need to perform bitwise XOR and SHIFT RIGHT operations on 128-bit values. The target arch doesn't have SSE2, hence no native 128-bit operations supported. I came across this answer which simulates the SHIFT operations in software.
My question is, are there better ways of doing this, I mean with better data structure to represent 128-bit values and optimal way to simulate the SHIFT and XOR operations than using recursion(as done in the answer in the link). I wish to minimise usage of the limited stack memory.

Answer 1

You can use a structure to store 128 bit data as follows
typedef struct
{
uint32_t a;
uint32_t b;
uint32_t c;
uint32_t d;
} Type_128bit;
Then you can write a left shift function as follows
int leftshift(Type_128bit in, Type_128bit out, int value)
{
int val;
if (value >= 128)
{
return (-1); // error condition
}
else if (value < 32)
{
out->a = (in->a << value) | (in->b >> value);
out->b = (in->b << value) | (in->c >> value);
out->c = (in->c << value) | (in->d >> value);
out->d = in->d << value;
}
else if (value < 64)
{
val = value - 32;
out->a = (in->b << val) | (in->c >> val);
out->b = (in->c << val) | (in->d >> val);
out->c = (in->d << val);
out->d = 0x00;
}
else if (value < 96)
{
val = value - 64;
out->a = (in->c << val) | (in->d >> val);
out->b = (in->d << val);
out->c = 0x00;
out->d = 0x00;
}
else // value < 128
{
val = value - 96;
out->a = (in->d << val);
out->b = 0x00;
out->c = 0x00;
out->d = 0x00;
}
return (0); //success
}
This will avoid the recursion of the mentioned solution and give better runtime. But code size will increase and you need to carefully test the code.

Answer 2

uint32_t *shiftL(uint32_t *val, const size_t size, const size_t nbits) // <= 32
{
uint32_t mask = (1 << nbits) - 1;
mask <<= 32 - nbits;
for(size_t cword = size; cword - 1 ; cword --)
{
uint32_t temp = (val[cword - 2] & mask) >> nbits
val[cword - 1] <<= nbits;
val |= temp;
}
val[0] <<= nbits;
return val;
}