Re: [Boost-users] [Interval] Help debugging compiler optimization

Subject: Re: [Boost-users] [Interval] Help debugging compiler optimization
From: Tim van Erven (tim_at_[hidden])
Date: 2018-03-07 17:09:04

Hi Degski,

But if I assign 1/3 to a double and then print that to the console, then 
I would expect the compiler to be committed to the value shown in the 
console, but apparently it isn't, and still does optimization.

Best,
   Tim

On 07/03/2018 15:53, Mário Costa via Boost-users wrote:
> Not sure this "random" aswer explains it:
>
> 1/3 it's a recurring decimal 
> (https://en.wikipedia.org/wiki/Repeating_decimal 
> <https://en.wikipedia.org/wiki/Repeating_decimal>), that means it is 
> not possible to represent it within a limited number of bits (even in 
> 64 bits).
>
> So your "bug" example, I would say that it's not a bug. Using 
> optimization in one of the cases the compiler can figure out, that the 
> value is 1, by analyzing constant values assigned to v2 and lazy 
> expanding the expression (template expansions, etc), (which is correct 
> math value).
> The other option v1, uses memory storage, and because of that, it 
> cannot lazy expand the expression at the function call, so because you 
> cannot store 1/3 perfectly in 64 bits, after multiplying it, it always 
> results in a number lower 1.
>
> On Wed, Mar 7, 2018 at 11:54 AM, degski via Boost-users 
> <boost-users_at_[hidden] <mailto:boost-users_at_[hidden]>> wrote:
>
>     On 6 March 2018 at 20:43, Nathan Ernst via Boost-users
>     <boost-users_at_[hidden] <mailto:boost-users_at_[hidden]>>
>     wrote:
>
>         This does feel like a floating-point optimization switch,
>         though, at least to me.
>
>
>     Additionally, the optimization level it self could interfere with
>     the order of calculation, leading to possibly different results.
>
>     degski
>
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-- 
Tim van Erven <tim_at_[hidden]>
www.timvanerven.nl