Assembly output from JRuby 'fib'

[Christian Thalinger]

On May 3, 2011, at 5:06 PM, Rémi Forax wrote:
> On 05/03/2011 03:59 PM, Christian Thalinger wrote:
>> 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
> 
> The pattern occurs enough often to be recognized as a pattern.
> 
> Here is the version used by JSR 310 reference implementation:
> http://threeten.svn.sourceforge.net/viewvc/threeten/trunk/threeten/src/main/java/javax/time/MathUtils.java?revision=1417&view=markup
> (look for safeAdd)
> 
> It uses a && instead of a & in the middle,
> I don't know which one is the better.

That's interesting, thank you.  Intrinsifying a Java method would definitely be easier than doing something in the backend but I will look at it again.

-- Christian