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.
Cheers,
Paul
---------- Forwarded message ----------
From: Remko Popma <remko.po...@gmail.com>
Date: Fri, Sep 26, 2014 at 8:43 AM
Subject: Re: No for each loop comment?
To: Log4J Developers List <log4j-...@logging.apache.org>
On Windows it looks like normal for loops are slightly faster than for-each
loops, especially for small arrays of primitives. This could be noise,
since we are talking about 5 nanoseconds where the baseline (an empty
method invocation) is 12 nanos.
On Solaris 10 and Red Hat Enterprise Linux the baseline is so large (255
nanos and 1910 nanos respectively) that any difference we are seeing is
just noise.
All benchmarks were run with one fork, one thread, 10 warmup iterations and
10 test iterations.
*Windows 7 (64bit) with Java 1.8.0_05, 2-core Intel i5-3317u CPU @1.70Ghz
with hyperthreading switched on (4 virtual cores)*
Benchmark Mode Samples
Score Score error Units
o.a.l.l.p.j.LoopsBenchmark.baseline sample 154947
12.432 0.550 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10000ForEachLoop sample 126597
2759.592 3.431 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10000ForLoop sample 126494
2761.729 3.127 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray1000ForEachLoop sample 154124
292.880 1.065 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray1000ForLoop sample 156155
288.751 1.101 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray100ForEachLoop sample 191980
41.826 0.870 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray100ForLoop sample 193770
36.894 0.782 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10ForEachLoop sample 190847
22.393 0.618 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10ForLoop sample 192552
17.146 0.560 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10000ForEachLoop sample 173839
31959.057 14.341 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10000ForLoop sample 171137
32461.985 14.353 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray1000ForEachLoop sample 97495
3591.200 4.852 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray1000ForLoop sample 101560
3445.998 4.010 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray100ForEachLoop sample 102796
438.207 1.923 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray100ForLoop sample 102333
439.576 2.139 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10ForEachLoop sample 113924
58.957 1.247 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10ForLoop sample 120416
60.712 1.284 ns/op
// For loops for Object arrays are similar but return the total XOR of the
element hashcodes.
private int forEachLoop(final int[] array) {
int result = 0;
for (final int element : array) {
result ^= element;
}
return result;
}
private int forLoop(final int[] array) {
int result = 0;
for (int i = 0; i < array.length; i++) {
result ^= array[i];
}
return result;
}
*Solaris 10 (64bit) with JDK1.7.0_06-b24 (Oracle Hotspot), 2 quad-core Xeon
X5570 dual CPUs @2.93Ghz with hyperthreading switched on (16 virtual cores)*
Benchmark Mode Samples
Score Score error Units
o.a.l.l.p.j.LoopsBenchmark.baseline sample 110212
255.300 0.201 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10000ForEachLoop sample 187808
3938.055 1.207 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10000ForLoop sample 187897
3937.929 0.748 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray1000ForEachLoop sample 123989
606.631 0.626 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray1000ForLoop sample 123973
609.565 0.416 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray100ForEachLoop sample 126933
294.204 0.280 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray100ForLoop sample 127070
296.411 0.223 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10ForEachLoop sample 113400
261.519 0.181 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10ForLoop sample 111637
260.435 0.115 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10000ForEachLoop sample 115872
48154.673 18.846 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10000ForLoop sample 116777
47793.868 17.615 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray1000ForEachLoop sample 138432
5256.767 2.451 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray1000ForLoop sample 136644
5325.377 2.388 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray100ForEachLoop sample 166653
773.541 0.330 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray100ForLoop sample 166570
774.513 0.574 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10ForEachLoop sample 178754
317.232 0.134 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10ForLoop sample 180165
316.189 0.238 ns/op
*64 bit RHEL 6.5 (Linux 2.6.32-431.el6.x86_64) with JDK1.7.0_05-b06 (Oracle
Hotspot), 4 quad-core Xeon X5570 CPUs @2.93GHz with hyperthreading switched
on (16 virtual cores)*
Benchmark Mode Samples
Score Score error Units
o.a.l.l.p.j.LoopsBenchmark.baseline sample 114783
1910.576 29.256 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10000ForEachLoop sample 194584
5132.885 25.137 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10000ForLoop sample 196672
4811.572 52.072 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray1000ForEachLoop sample 133119
1967.213 28.970 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray1000ForLoop sample 133804
2004.501 31.554 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray100ForEachLoop sample 142439
1575.329 6.457 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray100ForLoop sample 142215
1957.714 27.815 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10ForEachLoop sample 130826
1980.301 30.818 ns/op
o.a.l.l.p.j.LoopsBenchmark.intArray10ForLoop sample 132654
1589.120 8.449 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10000ForEachLoop sample 126947
43301.320 50.589 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10000ForLoop sample 126117
43574.129 55.272 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray1000ForEachLoop sample 143697
5831.250 19.667 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray1000ForLoop sample 163244
4823.096 13.180 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray100ForEachLoop sample 162502
1930.819 24.136 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray100ForLoop sample 171619
1625.806 10.385 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10ForEachLoop sample 172226
1888.683 22.554 ns/op
o.a.l.l.p.j.LoopsBenchmark.objArray10ForLoop sample 188838
1581.979 6.322 ns/op
On Fri, Sep 26, 2014 at 2:56 AM, Matt Sicker <boa...@gmail.com> wrote:
> The foreach over an array looks like it's supposed to compile to the same
> thing:
>
> https://jcp.org/aboutJava/communityprocess/jsr/tiger/enhanced-for.html
>
> Same goes for .length which is supposed to be a final field which would
> allow for inlining by the JIT I'd imagine (hence why we use final
> everywhere):
>
> http://docs.oracle.com/javase/specs/jls/se7/html/jls-10.html#jls-10.7
>
>
>>>> On 24 September 2014 22:12, Gary Gregory <garydgreg...@gmail.com>
>>>> wrote:
>>>>
>>>>> Why does this "//noinspection ForLoopReplaceableByForEach" comment
>>>>> mean?
>>>>>
>>>>> Why not for an enhanced for each loop?
>>>>>
>>>>> private static boolean contains(final Marker parent, final Marker...
>>>>> localParents) {
>>>>> //noinspection ForLoopReplaceableByForEach
>>>>> for (int i = 0, localParentsLength = localParents.length;
>>>>> i < localParentsLength; i++) {
>>>>> final Marker marker = localParents[i];
>>>>> if (marker == parent) {
>>>>> return true;
>>>>> }
>>>>> }
>>>>> return false;
>>>>> }
>>>>>
>>>>> Thanks,
>>>>> Gary
>>>>>
>>>>>
>>>>>
