Can access by local method variables be faster then access by member variables ?

[Ulf Zibis]

Thanks for your first explanations.

The answer from the HotSpot engineers would be interesting. In my
imagination HotSpot should be able to detect and optimize this fact.

-Ulf


Am 16.08.2008 00:44, Charles Oliver Nutter schrieb:
> I'll be interested to hear from the HotSpot engineers, but a few 
> things I know might help:
>
> - local variables will be allocated on the stack and will be much 
> hotter in the cache
> - obviously member variables are going to need to load and dereference 
> for every access, which would be slower than just retrieving a local 
> variable, with or without pushes and pops
> - fields can obviously incur some cross-thread perf penalty if they 
> result in memory caches getting wacked, which you would not have 
> accessing local vars
> - When we're able to move Ruby local variables from a heap-based 
> structure to local variables (stack-based) in JRuby, we see a 
> tremendous improvement in performance. It's probably the single 
> largest perf boost we get from compilation.
>
> - Charlie
>
> Ulf Zibis wrote:
>> Hi experts,
>>
>> to avoid stack push + pop of src, dst I have refactored my code in 
>> moving the local method variables (sp, ...) to member variables 
>> (srcPos, ...).
>>
>> After this I have experienced that my code was little slower.
>>
>> For full source code see:
>> https://java-nio-charset-enhanced.dev.java.net/source/browse/java-nio-charset-enhanced/trunk/src/sun/nio/cs/SingleByteEncoder.java?rev=291&view=markup 
>>
>> https://java-nio-charset-enhanced.dev.java.net/source/browse/java-nio-charset-enhanced/trunk/src/sun/nio/cs/SingleByteEncoder.java?rev=302&view=markup 
>>
>>
>> ======================================== revisions 291 
>> ========================================
>>
>>    private CoderResult encodeArraysLoop(CharBuffer src, ByteBuffer 
>> dst) {
>>        char[] sa = src.array();
>>        int sp = src.arrayOffset() + src.position();
>>        int sl = src.arrayOffset() + src.limit();
>>        assert (sp <= sl);
>>        byte[] da = dst.array();
>>        int dp = dst.arrayOffset() + dst.position();
>>        int dl = dst.arrayOffset() + dst.limit();
>>        assert (dp <= dl);
>>        try {
>>            for (; sp < sl; sp++) {
>>                current = sa[sp];
>>                byte b = encodeSingle();
>>                if (dp >= dl)
>>                    return CoderResult.OVERFLOW;
>>                da[dp++] = b;
>>            }
>>            return CoderResult.UNDERFLOW;
>>        } catch (UnmappableCharacterException e) {
>>            if (Surrogate.isLow(current) || current >= '\uFFFE')
>>                return CODERRESULT_MALFORMED;
>>            if (Surrogate.isHigh(current))
>>                try {
>>                    if (sp+1 >= sl)
>>                        return CoderResult.UNDERFLOW;
>>                    encodeSurrogate(sa[sp+1]);
>>                } catch (UnmappableCharacterException e2) {
>>                    return CODERRESULT_UNMAPPABLE2;
>>                } catch (MalformedInputException e2) {
>>                    return CODERRESULT_MALFORMED2;
>>                }
>>            return CODERRESULT_UNMAPPABLE;
>>        } finally {
>>            src.position(sp - src.arrayOffset());
>>            dst.position(dp - dst.arrayOffset());
>>        }
>>    }
>>
>> ======================================== revisions 302 
>> ========================================
>>
>>    private CoderResult encodeArraysLoop() throws RuntimeException, 
>> UnmappableCharacterException {
>>        for (; srcPos < srcLimit; srcPos++) {
>>            current = srcArray[srcPos];
>>            byte b = encodeSingle();
>>            if (dstPos < dstLimit)
>>                dstArray[dstPos++] = b;
>>            else
>>                return CoderResult.OVERFLOW;
>>        }
>>        return CoderResult.UNDERFLOW;
>>    }
>>
>> =============================================================================================== 
>>
>>
>>
>> Thanks for your explanations,
>>
>> Ulf
>>
>>
>>
>>
>
>