Hotspot loves PHP.reboot

[John Rose]

On Sep 6, 2011, at 12:58 PM, John Rose wrote:

> What's needed here is a way to get 33 bits out of a 32-bit add intrinsic.  There's no fully natural way to do this, and about 4 kludgey ways.  Because there are so many poor ways to shape the API, it's hard to pick the best one to invest in.

But, assuming the user wants to force a JO, here's one fairly clean way to do it:

    /**
     * Detect 32-bit overflow if the parameters are summed.
     * @return true if the inputs have the same sign, but their 32-bit sum would have a different sign
     */
    public static boolean addWouldOverflow(int x, int y) {
        //int res = x + y;
        //boolean overflowDetected = (SGN(x) == SGN(y) && SGN(x) != SGN(res));
        //boolean overflowDetected = ((x ^ y) >= 0 & (x ^ res) < 0);
        //boolean overflowDetected = ((x ^ y ^ -1) & (x ^ res)) < 0;
        return (((x ^ y ^ -1) & (x ^ (x+y))) < 0);
    }

That would provide a fairly stable and clear target for the JIT to aim at.  No points for ease-of-use.

-- John
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