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: 2018-03-08 08:36:22

Hi Mário,

My (somewhat naive) view of compilers is that they can only do 
optimization that has no visible effects on the behavior of the program.

What I meant is:

double v1 = 1.0/3.0;
printf("%f", v1);
double computation = v1 * 3;

Then the print statement will show the user some value for v1 that fits 
in a double. So I would expect that it is then no longer free to treat 
v1 as exactly 1/3, because the use has already observed a different value.

Best,
   TIm

On 07/03/2018 18:43, Mário Costa wrote:
> I don't really understand what you mean.
>
> lets say you have
>
> double v1 = 1.0/3.0;
>
> double computation = v1 * 3;
>
> Using, O2, the compiler might resolve, by performing expression 
> analisys at compile time, that you have always a constant value of 1/3 
> * 3 = 1.
>
> Now using O0, it might not optimize, and compute the value at runtime 
> (just as an example).
>
> Because 1/3 does not fit into any number of bits without lossing 
> precision (unless you'd be using some symbolic library), multiplying 
> v1 * 3, will result in a value diferent from 1, that will be the rule.
>
> Another issue Degski mentions, its the error propagation associated 
> with the order the expressions are evaluated/executed ...
>
>
>
> On Wed, Mar 7, 2018 at 5:09 PM, Tim van Erven via Boost-users 
> <boost-users_at_[hidden] <mailto:boost-users_at_[hidden]>> wrote:
>
>     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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>>
>>
>>
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>
>     -- 
>     Tim van Erven<tim_at_[hidden]> <mailto:tim_at_[hidden]>
>     www.timvanerven.nl <http://www.timvanerven.nl>
>
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-- 
Tim van Erven <tim_at_[hidden]>
www.timvanerven.nl