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: 2018-03-12 09:08:46

Ilja, Paul, Degski,

Thanks for your help. I think that Ilja's comment below explains what's 
going on: compilers don't strictly respect IEEE floating point when they 
have optimizations turned on (see also the explanation here: 
https://stackoverflow.com/questions/7517588/different-floating-point-result-with-optimization-enabled-compiler-bug#7517877).

@Degski: I am not actually interested in fractions. The background is 
that I am using the interval library to compare the numerical accuracy 
of various statistical algorithms in a setting where we know that some 
of the methods will run out of precision for large data sets, and I am 
getting incorrect intervals (that do not contain the correct answer) 
when compiling with clang, but everything works when compiling with gcc. 
When trying to debug this I found that the numerical results diverged 
right from the first interval calculations and I could not understand 
why. I think I have a better handle on this now.

Best,
   Tim

On 08/03/2018 09:49, Ilja Honkonen wrote:
> Hello
>> My (somewhat naive) view of compilers is that they can only do 
>> optimization that has no visible effects on the behavior of the program.
>
> I wouldn't be surprised if ieee floating point is an exception to 
> that, especially with optimizations enabled.
>
>> 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.
>
> I think in order to show the number to the user it would have to be 
> copied to memory or at least a regular register but further 
> computation could continue to use the more accurate floating point 
> register. And the compiler might still optimize the computation away 
> and just show you the result of printf(..., 1.0/3.0). I don't know if 
> writing constexpr double v1 = 1.0/3.0 would change anything, at least 
> if particular optimizations are enabled and/or disabled.
>
> Ilja

-- 
Tim van Erven <tim_at_[hidden]>
www.timvanerven.nl