jvm handling of long indexed for loops

[Vitaly Davidovich]

Maybe some of your oracle colleagues can comment on plans to optimize
longs. :)

Can you cast down to an int and use that? Do you really have loops that
need more than 2B+ iterations?

As for OSR, I always thought it generated worse code than what normal
compilation would do.

Sent from my phone
On Jun 19, 2014 1:19 PM, "Alexander Weld" <alexander.weld at oracle.com> wrote:

>  Okay, that makes sense. The PrintAssembly flag is quite useful.
>
> Are there plans for optimizing for loops that have a long counter in the
> next releases of hotspot? I guess there is no flag to force the jvm to
> unroll a long-loop.. Is there anything else I can do now, other than
> unrolling the loop by hand? (what I want to avoid)
>
> And for my second question: Do i understand it correctly, that the OSR
> compilation performs better than the "normal" compilation for the
> long-loop?
>
> On 6/18/2014 5:15 PM, Vitaly Davidovich wrote:
>
> Yes, hotspot has a bunch of loop optimizations that only kick in when the
> counter is an int.  My hunch is that, given your code, the difference is
> that the int loop is unrolled and the long one isn't.  Checking the
> generated assembly should deny/confirm that, and generally shed light on
> the compiled differences.
>
> As an aside, System.nanoTime is more appropriate for timing execution;
> prefer that over System.currentTimeMillis.
>
> Sent from my phone
> On Jun 18, 2014 7:52 PM, "Alexander Weld" <alexander.weld at oracle.com>
> wrote:
>
>> Hi,
>>
>> I have some questions on how the JVM handles long values, especially as
>> index in for loops.
>>
>> I have the following test class, which tracks the running time of two
>> versions of a simple for loop, one using an int-value as index and one
>> using a long-value as index:
>>
>> public class Test {
>>
>>   public static long testInt(int maxIter) {
>>     long x = 0;
>>
>>     for (int i = 0; i < maxIter; i++) {
>>       x += i;
>>     }
>>
>>     return x;
>>   }
>>   public static long testLong(long maxIter) {
>>     long x = 0;
>>
>>     for (long l = 0; l < maxIter; l++) {
>>       x += l;
>>     }
>>
>>     return x;
>>   }
>>
>>   public static void main(String[] args) {
>>
>>     final int MAX_ITER_INT = Integer.MAX_VALUE;
>>     final long MAX_ITER_LONG = (long) MAX_ITER_INT;
>>
>>     long t1, t2;
>>
>>     for (int i = 0; i < 10; i++) {
>>       long x = 0;
>>
>>       t1 = System.currentTimeMillis();
>>       x = testInt(MAX_ITER_INT);
>>       t2 = System.currentTimeMillis();
>>
>>       System.out.println("Iteration " + i + ": x = " + x + "  time(int
>> loop) = " + (t2 - t1));
>>     }
>>
>>     for (int i = 0; i < 10; i++) {
>>       long x = 0;
>>
>>       t1 = System.currentTimeMillis();
>>       x = testLong(MAX_ITER_LONG);
>>       t2 = System.currentTimeMillis();
>>
>>       System.out.println("Iteration " + i + ": x = " + x + " time(long
>> loop) = " + (t2 - t1));
>>     }
>>
>>   }
>>
>> }
>>
>> When I run this on our cluster (*) I get the following results:
>>
>> $ java Test
>> Iteration 0: x = 2305843005992468481  time(int loop) = 945
>> Iteration 1: x = 2305843005992468481  time(int loop) = 937
>> Iteration 2: x = 2305843005992468481  time(int loop) = 937
>> Iteration 3: x = 2305843005992468481  time(int loop) = 938
>> Iteration 4: x = 2305843005992468481  time(int loop) = 938
>> Iteration 5: x = 2305843005992468481  time(int loop) = 937
>> Iteration 6: x = 2305843005992468481  time(int loop) = 937
>> Iteration 7: x = 2305843005992468481  time(int loop) = 938
>> Iteration 8: x = 2305843005992468481  time(int loop) = 938
>> Iteration 9: x = 2305843005992468481  time(int loop) = 937
>> Iteration 0: x = 2305843005992468481 time(long loop) = 1510
>> Iteration 1: x = 2305843005992468481 time(long loop) = 1509
>> Iteration 2: x = 2305843005992468481 time(long loop) = 1799
>> Iteration 3: x = 2305843005992468481 time(long loop) = 1799
>> Iteration 4: x = 2305843005992468481 time(long loop) = 1799
>> Iteration 5: x = 2305843005992468481 time(long loop) = 1799
>> Iteration 6: x = 2305843005992468481 time(long loop) = 1799
>> Iteration 7: x = 2305843005992468481 time(long loop) = 1799
>> Iteration 8: x = 2305843005992468481 time(long loop) = 1799
>> Iteration 9: x = 2305843005992468481 time(long loop) = 1799
>>
>> My questions are:
>>
>>   (1) Why is the long-version slower compared to the int-version? Is it
>> the int to long conversion? Or are there some optimizations, that are
>> only applied to int loops?
>>
>>   (2) What is the reason for the long-version getting slower after
>> "Iteration 1"?
>>
>> Thanks,
>> Alex
>>
>> (*) System information:
>>
>> Intel(R) Xeon(R) CPU E5-2650 v2 @ 2.60GHz
>>
>> $ lsb_release -d
>> Description:    Oracle Linux Server release 6.5
>>
>> $ java -version
>> java version "1.7.0_55"
>> Java(TM) SE Runtime Environment (build 1.7.0_55-b13)
>> Java HotSpot(TM) 64-Bit Server VM (build 24.55-b03, mixed mode)
>>
>> (Let me know if you need more information)
>>
>
>
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