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Re: Eclipse generated hashCode() and Compiler Efficiency

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On 4 Jul., 13:13, lewbloch <lewbl...@gmail.com> wrote:
> On Jul 4, 4:07 am, Robert Klemme <shortcut...@googlemail.com> wrote:
>
>
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>
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> > 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?

Good point!  That was set at default ("preserve unused locals").  When
toggled I get this:

public int hashCode();
  Code:
   Stack=2, Locals=2, Args_size=1
   0:   iconst_1
   1:   istore_1
   2:   bipush  31
   4:   iload_1
   5:   imul
   6:   aload_0
   7:   getfield        #18; //Field i:I
   10:  iadd
   11:  istore_1
   12:  iload_1
   13:  ireturn
  LineNumberTable:
   line 10: 0
   line 11: 2
   line 12: 12

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      14      0    this       Lbytecode/T1;
   2      12      1    result       I

What I find puzzling is that the compiler does only half of an
optimization: the constant is used but the int is still kept around.
That somehow seems inconsequential.  But I guess the scenario is
deemed too rare to be worth a separate treatment in the compiler since
the JVM will likely kill this local anyway.

Kind regards

robert

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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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