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From: Cecil Westerhof <Cecil@decebal.nl>
Newsgroups: comp.lang.java.programmer
Subject: Re: One code is aggressively optimised, while another not. Why?
Date: Thu, 13 Jul 2017 18:52:25 +0200
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Eric Sosman <esosman@comcast-dot-net.invalid> writes:

>     Remark: What's the point of all those Threads?  The code
> never starts even one of them, never uses any of them for
> anything at all.

I use the Threads as parameter to the test function.


>     Explanation/Rationale: Each test runs a loop to compute a sum,
> but the sum is never subsequently used for anything at all: It is
> not printed, it is not tested, it participates in no further
> computation of any kind.  The JVM may well notice this and decide
> not to bother computing the sum since it isn't needed.  A loop
> that starts out as
>
> 	long sum = 0;
> 	for (int i = start; i < end; ++i) {
> 	    sum += i;
> 	}
>
> ... may then be transformed to
>
> 	for (int i = start; i < end; ++i) {
> 	}
>
> ... which (since the loop has no effect on anything except `i',
> which is itself not used outside the loop) may be further
> optimized to
>
> 	{
> 	}
>
> ... which casts just a wee bit of suspicion on the timings ;).

OK, I change the code to return the sum.


>     I took the liberty of rewriting your test to eliminate the
> Threads, to print the calculated sums (restraining the JVM's
> eagerness to optimize), and to run a warm-up phase before the
> "in earnest" loops.  I'd be happy to mail you the code if you'd
> like; maybe you can find as many "improvements" to mine as I did
> to yours.

I would appreciate it when you could email the code: I can only learn
from it.

-- 
Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof