C/C++ Bit Twiddling
I would try
x -= (pow2 ^ (~sgn+1)) + sgnor, as suggested by lijie in the comments
x -= (pow2 ^ -sgn) + sgnIf sgn is 0, ~sgn+1 is also 0, so pow2 ^ (~sgn+1) == pow2. If sgn is 1, (~sgn+1) is 0xFFFFFFFF, and (pow2 ^ (~sgn+1)) + sgn == -pow2.
mask = sgn - 1; // generate mask: sgn == 0 => mask = -1, sgn == 1 => mask = 0x = x + (mask & (-pow2)) + (~mask & (pow2)); // use mask to select +/- pow2 for addition
Off the top of my head:
int subMask = sgn - 1;x -= pow2 & subMask;int addMask = -sgn;x += pow2 & addMask;No guarantees on whether it works or whether this is smart, this is just a random idea that popped into my head.
EDIT: let's make this a bit less readable (aka more compact):
x += (pow2 & -sgn) - (pow2 & (sgn-1));