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

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On 7/12/17 3:07 AM, Cecil Westerhof wrote:
> The second code takes about 50% more time to execute. No surprise
> there. But the first code is aggressively optimised when run several
> times (the fifth run is about 2½ million times faster as the first
> run), while the second code is not optimised:

Fundamentally speaking, you need to execute about 2^32 additions, if you 
do the loop. That means you need to take a billion clock cycles, or 
something on that magnitude, to do the loop, and anything less than that 
is indicative of the loop not being done.

I don't know your quality of microbenchmark suite, but a result that 
takes around a thousand clock cycles or so strongly suggests that the 
optimizer did loop idiom detection to replace the loop with sum = 
f(start, end). That the second loop didn't suggests either that the loop 
didn't tickle the loop idiom detector or that it bailed on computing the 
result do to an overflow. Since my development computer is currently 
nonfunctional, I can't do any more detailed analysis.

-- 
Beware of bugs in the above code; I have only proved it correct, not 
tried it. -- Donald E. Knuth

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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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