Question

I’d like to get the sign of a float value as an int value of -1 or 1.

Avoiding conditionals is always a good idea in reducing computational cost. For instance, one way I can think of would be to use a fast bit-shift to get the sign:

float a = ...;
int sign = a >> 31; //0 for pos, 1 for neg
sign = ~sign; //1 for pos, 0 for neg
sign = sign << 1; //2 for pos, 0 for neg
sign -= 1; //-1 for pos, 1 for neg -- perfect.

Or more concisely:

int sign = (~(a >> 31) << 1) - 1;

1. Does this seem like a good approach?
2. Will this work for all platforms, given endianness concerns (as MSB holds sign)?

Answer 1

Any reasons why you don’t simply use:

int sign = (int) Math.signum(a); //1 cast for floating-points, 2 for Integer types

Additionally most Number implementations have a signum method taking a primitive of that type and returning an int, so you can avoid casting for extra performance.

int sign1 = Integer.signum(12); //no casting
int sign2 = Long.signum(-24l); //no casting

It will return +1 / 0 / -1 and it has been optimized to deliver a good performance.

For reference, you can have a look at the implementation in openJDK. The relevant bits are:

public static float signum(float f) {
    return (f == 0.0f || isNaN(f)) ? f : copySign(1.0f, f);
}

public static boolean isNaN(float f) {
    return (f != f);
}

public static float copySign(float magnitude, float sign) {
    return rawCopySign(magnitude, (isNaN(sign) ? 1.0f : sign));
}

public static float rawCopySign(float magnitude, float sign) {
    return Float.intBitsToFloat((Float.floatToRawIntBits(sign)
            & (FloatConsts.SIGN_BIT_MASK))
            | (Float.floatToRawIntBits(magnitude)
            & (FloatConsts.EXP_BIT_MASK
            | FloatConsts.SIGNIF_BIT_MASK)));
}

static class FloatConsts {
    public static final int SIGN_BIT_MASK = -2147483648;
    public static final int EXP_BIT_MASK = 2139095040;
    public static final int SIGNIF_BIT_MASK = 8388607;
}