FWIW, I've verified the same byte codes on Oracle Java 7 and 8 on Windows 7
(all 64 bit).
Gary
On Mon, Sep 29, 2014 at 1:58 PM, Paul Benedict <pbened...@apache.org> wrote:
> 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 <vita...@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 <a...@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/
>>>
>>>
>>
>
--
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