Add 32-bit words with saturation

Question

Do you know any way to add with saturation 32-bit signed words using MMX/SSE assembler instructions? I can find 8/16 bits versions but no 32-bit ones.

Answer 1

You can emulate saturated signed adds by performing the following steps:

int saturated_add(int a, int b)
{
    int sum = a + (unsigned)b;                // avoid signed-overflow UB
    if (a >= 0 && b >= 0)
        return sum > 0 ? sum : INT32_MAX;     // catch positive wraparound
    else if (a < 0 && b < 0)
        return sum > 0 ? INT32_MIN : sum;     // catch negative wraparound
    else
        return sum;                           // sum of pos + neg always fits
}

Unsigned, it's even simpler, see this stackoverflow posting

In SSE2, the above maps to a sequence of parallel compares and AND/ANDN operations. No single operation is available in hardware, unfortunately.

Answer 2

Saturated unsigned subtraction is easy, because for `a -= b', we can do

    asm (
        "pmaxud %1, %0\n\t" // a = max (a,b)
        "psubd %1, %0" // a -= b
        : "+x" (a)
        : "xm" (b)
    );

with SSE.

I was looking for unsigned addition, but possibly, the only way is to transform to a saturated unsigned subtraction, perform it, and transform back. Same for signed variants.

EDIT: with unsigned addition, you get min (a, ~b) + b this way, which of course works. With signed addition and subtraction, you have two saturation boundaries, which makes things complicated.