Assembly output from JRuby 'fib'

[Charles Oliver Nutter]

An intrinsic was my first thought as well. It would seem quite appropriate to have an operation or set of operations on java.lang.Math for example that we all could use.

Perhaps I should have asked this six months ago :)

- Charlie (mobile)

On May 3, 2011, at 11:44, Christian Thalinger <christian.thalinger at oracle.com> wrote:

> 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
> 
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