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Re: One code is aggressively optimised, while another not. Why?

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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

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Thread

One code is aggressively optimised, while another not. Why? Cecil Westerhof <Cecil@decebal.nl> - 2017-07-12 10:07 +0200
  Re: One code is aggressively optimised, while another not. Why? Eric Sosman <esosman@comcast-dot-net.invalid> - 2017-07-12 09:38 -0400
    Re: One code is aggressively optimised, while another not. Why? Cecil Westerhof <Cecil@decebal.nl> - 2017-07-13 09:51 +0200
      Re: One code is aggressively optimised, while another not. Why? Eric Sosman <esosman@comcast-dot-net.invalid> - 2017-07-13 11:05 -0400
        Re: One code is aggressively optimised, while another not. Why? Cecil Westerhof <Cecil@decebal.nl> - 2017-07-13 18:52 +0200
          Re: One code is aggressively optimised, while another not. Why? Cecil Westerhof <Cecil@decebal.nl> - 2017-07-13 19:51 +0200
  Re: One code is aggressively optimised, while another not. Why? Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid> - 2017-07-12 09:19 -0500
    Re: One code is aggressively optimised, while another not. Why? Martin Gregorie <martin@address-in-sig.invalid> - 2017-07-12 17:04 +0000
  Re: One code is aggressively optimised, while another not. Why? Arne Vajhøj <arne@vajhoej.dk> - 2017-07-12 21:02 -0400

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