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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