Hotspot loves PHP.reboot

[Rémi Forax]

On 09/06/2011 10:19 PM, John Rose wrote:
> 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.

An exception is perhaps more easier to use,
because if it overflow you may have to deoptimize, for that you need the 
stack and local values,
it's easier to jump to a exception handler that will push all these 
values and call the interpreter.

>
> -- John

Rémi

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