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Re: Eclipse generated hashCode() and Compiler Efficiency

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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

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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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