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/
>>
>>
>
