Assembly output from JRuby 'fib'

[Christian Thalinger]

On May 2, 2011, at 9:55 PM, Charles Oliver Nutter wrote:
> On Thu, Apr 28, 2011 at 11:18 AM, Rémi Forax <forax at univ-mlv.fr> wrote:
>> Do you specialize the overflow check depending on the callsite ?
>> for fib(n - 1), you just have to check if n is different from
>> Integer.MIN_INT,
>> for fib(n - 2), if n is <= to Integer.MIN_INT - 1
>> and for + use the double xor tricks.
> 
> Here's a related question. Is there a way we can structure the double
> xor trick to get Hotspot to intrinsify it as a jc instruction or
> similar?
> 
> As I posted earlier, we use the "double xor" trick, which I would
> expect JVM could recognize as a carry check and do the right thing. Or
> perhaps there's an intrinsic somewhere in JVM/JDK we could call?

I look a little into this and as far as I know there is nothing you could call.  I also tried to hack something into the backend and I think that could work.  I'm just not sure how many users would trigger that pattern.

-- Christian

> 
> I did a couple comparisons, and it seems like for math-heavy logic the
> current overflow check adds about 10% overhead...much more than I'd
> like :(
> 
> - Charlie
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