Fwd: No for each loop comment?

[Paul Benedict]

Bytecode output courtesy of Mikael Ståldal:

With standard loop:

  private static boolean contains(org.apache.logging.log4j.Marker,
org.apache.logging.log4j.Marker...);
    Code:
       0: iconst_0
       1: istore_2
       2: aload_1
       3: arraylength
       4: istore_3
       5: iload_2
       6: iload_3
       7: if_icmpge     29
      10: aload_1
      11: iload_2
      12: aaload
      13: astore        4
      15: aload         4
      17: aload_0
      18: if_acmpne     23
      21: iconst_1
      22: ireturn
      23: iinc          2, 1
      26: goto          5
      29: iconst_0
      30: ireturn


With for-each:

  private static boolean contains(org.apache.logging.log4j.Marker,
org.apache.logging.log4j.Marker...);
    Code:
       0: aload_1
       1: astore_2
       2: aload_2
       3: arraylength
       4: istore_3
       5: iconst_0
       6: istore        4
       8: iload         4
      10: iload_3
      11: if_icmpge     34
      14: aload_2
      15: iload         4
      17: aaload
      18: astore        5
      20: aload         5
      22: aload_0
      23: if_acmpne     28
      26: iconst_1
      27: ireturn
      28: iinc          4, 1
      31: goto          8
      34: iconst_0
      35: ireturn



Cheers,
Paul

On Mon, Sep 29, 2014 at 11:31 AM, Vitaly Davidovich <vitalyd at gmail.com>
wrote:

> I think Paul's email already has jmh output.
>
> I looked at the generated asm on 7u60 x64 linux, and didn't see any
> material difference.
>
> Paul, have you or anyone else looked at the machine code diffs between the
> two? Looking at timing is useful, but it's possible to get caught up in
> noise; the generated assembly should provide more conclusive data on
> whether any real difference is there or not.
>
> On Mon, Sep 29, 2014 at 11:21 AM, Andrew Haley <aph at redhat.com> wrote:
>
>> On 09/29/2014 03:29 PM, Paul Benedict wrote:
>> > Open JDKers, I am forwarding an email to get some clarification. It's
>> been
>> > a common understanding that foreach should perform no differently than
>> the
>> > equivalent for-loop . However, some fellow developers claim there is a
>> > noticable difference in their microbenchmarking. Can you help explain
>> what
>> > is really going on? It's either the case there is a true difference (a
>> > result that would surprise me) or the results are within a margin of
>> error
>> > that make the results insignificant. Please advise.
>>
>> The actual code that such a forEach loop generates is this:
>>
>>     private int forLoop(final int[] array) {
>>         int result = 0;
>>         int[] a = array;
>>         int len = a.length;
>>         for (int i = 0; i < len; i++) {
>>             int element = a[i];
>>             result ^= element;
>>         }
>>         return result;
>>     }
>>
>> If you get different timings for this one, then the measurements are
>> suspect.
>>
>> Java microbenchmarking is notoriously difficult.  Please try to use
>> jmh; you'll get better and easier to interpret results.
>>
>> Andrew.
>>
>>
>> http://openjdk.java.net/projects/code-tools/jmh/
>>
>>
>