review request for 6622432

[Xiaobin Lu]

On 02/17/09 03:15, Christian Thalinger wrote:
> On Mon, 2009-02-16 at 14:33 -0800, Xiaobin Lu wrote:
>   
>> Webrev: http://webrev.invokedynamic.info/xiaobin.lu/6622432/
>>
>> 6622432: RFE: Performance improvements to java.math.BigDecimal
>>     
>
> <snip>
>
>   
>> As you know, the division operation is expensive and the 
>> algorithm to compare with the ten's power array can be made more 
>> efficiently by correlating the bit length of the number with the index 
>> to the array so that the time to compute the precision is O(1) rather 
>> than being O(n) (where n is the length of the array). And that is 
>> exactly what we are doing in the webrev. First, we get the bit length of 
>> the number and then multiply it with log2 (10 base), use the result to 
>> index to a dynamically expanded ten's power array so that division 
>> operation can be avoided completely.
>>     
>
> Actually I'm not reviewing the webrev but I have some questions.
>
> 1. Is bitCount() called often and performance critical?  There is a RFE
> (6378821) to intrinsify it and I am thinking about to implement it.
>   
No, I don't think it is a hot method in both derby and SPECjbb2005 based 
on the profiling information from Analyzer. I might be wrong, but you 
can verify it easily.
> 2. Why is BigInteger using it's own implementation of bit-count
> (bitCnt()) instead of using Integer.bitCount()?
>   
That is a good catch. I will fix it.

-Xiaobin
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