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

[Tom Rodriguez]

What pushes and pops are you talking about?  You should consider stack  
and local variable access to be completely free when any reasonable  
JIT is involved.  The bytecodes that operate on the stack and locals  
don't correspond to any code that it's emitted by C1 or C2.   
Operations on member variables are definitely not free, though  
depending on how good your JIT is it may be able to reduce the number  
of actuals loads and stores that are issued.  In general though the  
number issued will never become zero.  C2 is fairly good at this and  
C1 does ok for simple cases but doesn't include any loop optimizations  
so it will be much less efficient for loopy code like you're showing.

Basically the revision 302 code could be transformed into the revision  
291 by a good compiler.  291 will definitely run faster with C1  
because it doesn't have any loop optimizations around memory  
operations.  C2 might be able to reproduce equivalent code, though I'm  
not positive it can delay the stores implied by the srcPos++ dstPos++  
until the whole loop has completed.

tom

On Aug 15, 2008, at 3:02 PM, 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
>
>
>
>