Re: [xmlvm-users] Arrays manipulation on iPhone

[Sascha H. <sa...@xm...>]

Hello Jacob,
thank you for your great analysis and question.

You are absolutely right. As you can see, we are trying to mimic a JVM as
close as possible. While we think that the performance is fine for most use
cases, some use cases (like yours) can actually perform much worse when
being cross-compiled with XMLVM. Therefore, one of our current tasks is to
look at converting our stack-bases model to a register-based one, so that
languages like Objective-C can benefit.

However, we are still in the planning stage of this and I cannot give you
any deadlines. Would this be something you are interested in contributing to
or do you have any other ideas?

Thank you
// Sascha

On Sun, Sep 20, 2009 at 11:29 PM, Jacob Nordfalk
<jac...@gm...>wrote:

> Hi,
>
> I am considering porting an image manipulation program in Java to iPhone
> using XMLVM.
>
> However, code such as
>
>   public byte[] xxx(byte[] rgb)  {
>
>     for (int i = 0; i<rgb.length; i+=3) {
>       rgb[i] += 1;
>       rgb[i+1]/=2;
>       rgb[i+2]/=2;
>     }
>
>     return rgb;
>   }
>
> seems to run 20 times slower than JIT-enabled Java when compiling xmlvm
> generated objective-C.
>
> When I looked at the code I understood why - the generated objective-C is
> something like 26 lines
> per line of Java (here below is for the line: rgb[i+1]/=2).
>
>
> - (NSMutableArray*) optimize___byte_ARRAYTYPE :(NSMutableArray*)n1
> {
>     XMLVMElem _stack[4];
>     XMLVMElem _locals[5];
>     int _sp = 0;
>     XMLVMElem _op1, _op2, _op3;
> ...
>
> *//       rgb[i+1]/=2;*
>     _stack[_sp++].o = _locals[1].o;
>     _op1.i = _locals[4].i;
>     _stack[_sp++].i = _op1.i;
>     _stack[_sp++].i = 1;
>     _op2.i = _stack[--_sp].i;
>     _op1.i = _stack[--_sp].i;
>     _stack[_sp++].i = _op1.i + _op2.i;
>     _op1 = _stack[_sp - 2];
>     _op2 = _stack[_sp - 1];
>     _stack[_sp++] = _op1;
>     _stack[_sp++] = _op2;
>     _op1.i = _stack[--_sp].i;
>     _op2.o = _stack[--_sp].o;
>     _stack[_sp++].i = [[_op2.o objectAtIndex: _op1.i] intValue];
>     _stack[_sp++].i = 2;
>     _op2.i = _stack[--_sp].i;
>   _op1.i = _stack[--_sp].i;
>   _stack[_sp++].i = _op1.i / _op2.i;
>     _op1.i = _stack[--_sp].i;
>     _stack[_sp++].i = (int) (_op1.i & 0xFF);
>     _op1.i = _stack[--_sp].i;
>     _op2.i = _stack[--_sp].i;
>     _op3.o = _stack[--_sp].o;
>     [_op3.o replaceObjectAtIndex: _op2.i withObject: [NSNumber
> numberWithInt: _op1.i]];
>
>
> Now, I understand that the speed factor 20 is becaurse XMLVM is emulating
> the stack-based JVM in Objective-C,
> but I as I have really LOT of array manipulation code I am asking you:
>
>
> Do you have any advice or ideas on how to make Java code using simple array
> manipulations run relatively fast on the iPhone ?
>
>
>
> Thanks you,
> Jacob Nordfalk
>
> --
> Jacob Nordfalk
> एस्पेरान्तो के हो?  http://www.esperanto.org.np/.
> Memoraĵoj de KEF -. http://kef.saluton.dk/memorajoj/
>
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