A question about bytecodes + unsigned load performance ./. add performace

[Ulf Zibis]

Also see the regarding bugreport and it's benchmarks in evaluation section:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6642323

-Ulf


Am 10.01.2009 00:44, Ulf Zibis schrieb:
> Am 10.01.2009 00:17, John Rose schrieb:
>>
>>  Note that compilers tend to optimize expressions like myByteArray[i] 
>> & 0xFF into unsigned loads, and packaging this into an intrinsic 
>> method would add predicability of compilation (if anybody cares), and 
>> the case is not frequent enough to warrant shaving a few bytes off 
>> the instruction format.
>>
> ... but myByte + 0x80 is faster than myByte & 0xFF.  For me this is an 
> unintelligible mystery.
> source see here (line 141..144):
>      
> http://hg.openjdk.java.net/jdk7/tl/jdk/file/b89ba9a6d9a6/make/tools/src/build/tools/charsetmapping/GenerateSBCS.java 
>
>
> How can adding be faster than unsigned load of a byte?
>
> -Ulf
>
>