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Re: Eclipse generated hashCode() and Compiler Efficiency

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On 04-07-11 13:13, schreef lewbloch:
> On Jul 4, 4:07 am, Robert Klemme<shortcut...@googlemail.com>  wrote:
>> Hi,
>>
>> Eclipse generates hashCode() methods like this for a class with a
>> single int field "i":
>>
>>    @Override
>>    public int hashCode() {
>>      final int prime = 31;
>>      int result = 1;
>>      result = prime * result + i;
>>      return result;
>>    }
>>
>> I always found it weird to define the constant "prime" at method
>> level.  I wanted to know what happens and disassembled with
>>
>> $ javap -c -verbose -private -classpath classes bytecode.T1
>>
>> This is what I got for the method:
>>
>> public int hashCode();
>>    Code:
>>     Stack=2, Locals=3, Args_size=1
>>     0:   bipush  31
>>     2:   istore_1
>>     3:   iconst_1
>>     4:   istore_2
>>     5:   bipush  31
>>     7:   iload_2
>>     8:   imul
>>     9:   aload_0
>>     10:  getfield        #18; //Field i:I
>>     13:  iadd
>>     14:  istore_2
>>     15:  iload_2
>>     16:  ireturn
>>    LineNumberTable:
>>     line 9: 0
>>     line 10: 3
>>     line 11: 5
>>     line 12: 15
>>
>>    LocalVariableTable:
>>     Start  Length  Slot  Name   Signature
>>     0      17      0    this       Lbytecode/T1;
>>     3      14      1    prime       I
>>     5      12      2    result       I
>>
>> (This was from Eclipse compiler but Sun's javac yields the same albeit
>> with a different ordering of constants.)
>>
>> Apart from other inefficiencies the weird thing is that in lines 0 and
>> 2 the constant 31 is stored in local variable 1 ("prime") although it
>> is never read again.  Is this an optimization they didn't bother to do
>> (or left for the JVM) or is there any reason why the local variable
>> must be there?  So far I could not find any hints in JLS or JVM spec.
>>
>> You can find the full examples here - including a variant where
>> "prime" is a constant on class level:https://gist.github.com/1063153
>>
>> Instruction reference: http://java.sun.com/docs/books/jvms/second_edition/html/Overview.doc....
>>
>> Kind regards
>>
>
> Eclipse's compiler has an option whether to retain or eliminate unused
> local variables.  How do you have that switch set?
>
> --
> Lew

Don't forget the JIT compiler, she makes your code beautiful to run (and 
awkward to read, but you never do).

Jaap

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Eclipse generated hashCode() and Compiler Efficiency Robert Klemme <shortcutter@googlemail.com> - 2011-07-04 04:07 -0700
  Re: Eclipse generated hashCode() and Compiler Efficiency lewbloch <lewbloch@gmail.com> - 2011-07-04 04:13 -0700
    Re: Eclipse generated hashCode() and Compiler Efficiency jaap <niemand@meel.homelinux.net> - 2011-07-04 17:43 +0200
    Re: Eclipse generated hashCode() and Compiler Efficiency Robert Klemme <shortcutter@googlemail.com> - 2011-07-06 00:20 -0700
  Re: Eclipse generated hashCode() and Compiler Efficiency Roedy Green <see_website@mindprod.com.invalid> - 2011-07-04 08:54 -0700

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