Sponsor for 6666666: A better implementation of Character.isSupplementaryCodePoint

[Ulf Zibis]

Am 16.03.2010 21:57, schrieb Martin Buchholz:
> On Tue, Mar 16, 2010 at 12:48, Ulf Zibis<Ulf.Zibis at gmx.de>  wrote:
>    
>>>> - Same shift magic would enhance isISOControl(), isHighSurrogate(),
>>>> isLowSurrogate(), in particular if latter occur consecutive.
>>>>
>>>>          
>>> isISOControl - yes, others - I am not convinced.
>>>
>>>        
>> If virtually shifted by 8, HotSpot could use cheaper 1-byte compare on the
>> high byte.
>> Additionally, those methods are often used consecutively, so all 4 compares
>> would benefit from.
>>
>>      
>>>>   8-bit shift + compare would allow HotSpot to compile to smart 1-byte
>>>> immediate op-codes.
>>>> In encodeBufferLoop() you could use putChar(), putInt() instead put().
>>>> Should perform better.
>>>>
>>>>          
>>> I'm not convinced.  You would need to assemble bytes into an
>>> int, and then break them apart into bytes on the other side?
>>>
>>>        
>> Some time ago, I disassembled such code. I could see, that the int was
>> copied directly to memory by one 32-bit move instruction.
>> In case of using put(byte), I saw 4 8-bit move instructions.
>>      
> Ulf, I'd like to understand this better.
>
> How are you generating the machine code
> (pointer to docs?)?
>    

I must prepare it. Takes some time.

> Bits.java is doing byte-oriented put instructions in any case.
> If the VM can optimize putInt, it should be able to optimize
> the equivalent series of put(byte) as well, no?
>    

Yes, it should, but it doesn't.

> Can you provide a small patch that gives an observable
> performance improvement in a micro-benchmark?
>    

I'll try.

>    
>> I not have dissassembled if a 3-byte value first would be collected in a
>> 3-byte byte[] and then copied by put(byte[]). Maybe HotSpot could optimize
>> here too.
>>
>> Try it out. 2 will see more than 1. Maybe I was in error.
>>
>> BTW: for the same optimization, I would like to have putInt() and putLong()
>> in Charbuffer, ShortBuffer and for the latter in IntBuffer.
>>      
> Perhaps better to get the VM to optimize a series of puts into
> a single instruction?
>    

I have such an RFE in mind.

-Ulf