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Re: Getter performance

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On 10/16/2011 1:12 PM, Jaap Droogers wrote:
> On 15-10-11 23:03, schreef Aéris:
>> Hello everybody.
>>
>> I work on an application where performances are important.
>>
>> To optimize it, I thought a direct access to a variable (foo.bar) would
>> be more efficient than a getter call (foo.getBar()).
>> I thought by avoiding the call to a method and all that goes with it
>> (context switching, stacking, return value…), I can save time, but this
>> code sample prove the contrary :
>> http://pastebin.com/bP1nqxce
>> Direct variable access : 1041 ms
>> Getter call : 556 ms
>>
>> The difference is even more important if I don't modify the variable
>> value (lines 29 and 36 commented) :
>> Direct variable access : 95 ms
>> Getter call : 4 ms
>>
>> How can we explain this not obvious huge difference ( 50 and 95% ) ?
>>
>
> You forgot the JIT compiler. All the nice clean code you have written is
> optimized by the compiler. You don't write C code, remember.

note:
many C and C++ compilers will perform the same optimization in many 
cases. some compilers will require both to be present in the same 
compilation unit, but some others (such as GCC and LLVM/Clang) can make 
use of link-time optimization.

these sorts of optimizations are nothing specific to the JVM.


> Why is Java so fast: it uses the JIT compiler which optimizes the code
> for the machine the program is running on.
> if you write
> int a = myObject.getA();
>
> the compiler compiles it probably as:
> int a = myObject.a;
>
> There is also a change that you try to optimize your code and the JIT
> compiler does not now how to optimize it, so your code runs slower.
>

yep.

at the CPU level the reference may potentially compile down to a single 
"mov" instruction.

if the getter is called but its value isn't used, then there is a chance 
that it may be discarded altogether (so no CPU instructions are produced 
in this case).

if trying to micro-benchmark or micro-optimize things (yes... including 
in C), one will usually spot this case by the running time dropping to a 
very small number (IOW: the code runs impossibly fast).


generally though, it is not really productive to worry about 
micro-optimizing things. better IMO to leave optimizing for when and 
where it actually matters.

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Getter performance Aéris <aeris@imirhil.fr> - 2011-10-15 23:03 +0200
  Re: Getter performance Arne Vajhøj <arne@vajhoej.dk> - 2011-10-15 17:36 -0400
    Re: Getter performance Arne Vajhøj <arne@vajhoej.dk> - 2011-10-15 17:42 -0400
      Re: Getter performance BGB <cr88192@hotmail.com> - 2011-10-15 15:00 -0700
        Re: Getter performance markspace <-@.> - 2011-10-15 15:20 -0700
          Re: Getter performance David Lamb <dalamb@cs.queensu.ca> - 2011-10-20 12:45 -0400
            Re: Getter performance Roedy Green <see_website@mindprod.com.invalid> - 2011-10-21 14:27 -0700
              Re: Getter performance Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-10-21 18:57 -0700
                Re: Getter performance Patricia Shanahan <pats@acm.org> - 2011-10-22 07:27 +0100
                Re: Getter performance Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2011-10-22 09:57 -0300
              Re: Getter performance Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-10-21 22:12 -0400
    Re: Getter performance Aéris <aeris@imirhil.fr> - 2011-10-15 23:59 +0200
      Re: Getter performance Arne Vajhøj <arne@vajhoej.dk> - 2011-10-15 19:44 -0400
        Re: Getter performance Aéris <aeris@imirhil.fr> - 2011-10-16 13:14 +0200
          Re: Getter performance Lars Enderin <lars.enderin@telia.com> - 2011-10-16 16:28 +0200
  Re: Getter performance Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-10-16 09:47 -0400
  Re: Getter performance Jaap Droogers <JaapDroogers@unusable.meel.homelinux.net> - 2011-10-16 22:12 +0200
    Re: Getter performance BGB <cr88192@hotmail.com> - 2011-10-16 13:58 -0700
    Re: Getter performance David Lamb <dalamb@cs.queensu.ca> - 2011-10-20 12:51 -0400
      Re: Getter performance Paul Cager <paul.cager@googlemail.com> - 2011-10-21 08:49 -0700
  Re: Getter performance Roedy Green <see_website@mindprod.com.invalid> - 2011-10-21 08:02 -0700
  Re: Getter performance Wanja Gayk <brixomatic@yahoo.com> - 2011-10-22 21:11 +0200

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