c - Convert two's complement to sign-magnitude

Question

I need to convert from two's complement to sign-magnitude in C using only the operators

! ~ & ^ | + << >>

My approach is to find sign: int sign = !(!(a>>31));

basically, if sign == 1 . I want to flip the number and add 1 else just want to display the number.

The thing is I can't use any loops, if statements etc. This is what I'm working on:

int s_M = ((((a+1)>>31)^sign)+1)&sign;

any suggestions?

Answer 1

From http://graphics.stanford.edu/~seander/bithacks.html#IntegerAbs

int const mask = v >> 31;
unsigned int r = (v + mask) ^ mask;

Gives the absolute value (magnitude). If you wish to add the sign bit back simply mask and or the 32nd bit:

unsigned int s_M = r | (v & 0x80000000);

Or if you're looking for a one liner:

unsigned int s_M = ((v + (v >> 31)) ^ (v >> 31)) | (v & 0x80000000);