Checking whether a number is positive or negative using bitwise operators

Question

I can check whether a number is odd/even using bitwise operators. Can I check whether a number is positive/zero/negative without using any conditional statements/operators l

Answer 1

Here is an update related to C++11 for this old question. It is also worth considering std::signbit.

On Compiler Explorer using gcc 7.3 64bit with -O3 optimization, this code

bool s1(double d)
{
    return d < 0.0;
}

generates

s1(double):
  pxor xmm1, xmm1
  ucomisd xmm1, xmm0
  seta al
  ret

And this code

bool s2(double d)
{
    return std::signbit(d);
}

generates

s2(double):
  movmskpd eax, xmm0
  and eax, 1
  ret

You would need to profile to ensure that there is any speed difference, but the signbit version does use 1 less opcode.