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

Subject: Re: [Boost-users] [Interval] Help debugging compiler optimization
From: Nathan Ernst (nathan.ernst_at_[hidden])
Date: 2018-03-02 17:23:24

I'm not sure what's going on (seems like maybe a fast but error prone
floating point rounding), but comparing the optimization flags enabled
between -O0 and -O2 might help shed some light on it.

You can get the explicit optimization flags enabled for your compiler via:
 g++ -Q -O2 --help=optimizers
 g++ -Q -O0 --help=optimizers

Regards,
Nate

On Fri, Mar 2, 2018 at 10:17 AM, Tim van Erven via Boost-users <
boost-users_at_[hidden]> wrote:

> Dear all,
>
> I am trying to understand why I am getting different numerical results
> with the interval library depending on the optimization level of the
> compiler.
>
> I am attaching the smallest example I have been able to create:
>
> # On my Mac laptop
> Apple LLVM version 9.0.0 (clang-900.0.39.2)
> boost 1.66 (installed via homebrew)
> $ g++ foo.cpp -o foo
> $ ./foo
> third1 = 0.333333333333333314829616256247390992939472198486328125000000
> 0000
> third2 = 0.333333333333333314829616256247390992939472198486328125000000
> 0000
> v1 = (0.999999999999999888977697537484345957636833190917968750000000
> 0000,1.0000000000000000000000000000000000000000000000000000000000000000)
> v2 = (0.999999999999999888977697537484345957636833190917968750000000
> 0000,1.0000000000000000000000000000000000000000000000000000000000000000)
>
> $ g++ -O2 foo.cpp -o foo
> $ ./foo
> third1 = 0.333333333333333314829616256247390992939472198486328125000000
> 0000
> third2 = 0.333333333333333314829616256247390992939472198486328125000000
> 0000
> v1 = (1.000000000000000000000000000000000000000000000000000000000000
> 0000,1.0000000000000000000000000000000000000000000000000000000000000000)
> v2 = (1.000000000000000000000000000000000000000000000000000000000000
> 0000,1.0000000000000000000000000000000000000000000000000000000000000000)
>
> I would expect to get the same output in both cases, but the lower
> end-points are different in the second case, and seem wrong to me since
> third2 * 3.0 < 1.0.
>
> # On my Linux machine the effect is different:
> gcc version 5.4.0 20160609
> boost 1.58 on Ubuntu 16.04.9
>
> $ g++ foo.cpp -o foo
> $ ./foo
> third1 = 0.333333333333333314829616256247390992939472198486328125000000
> 0000
> third2 = 0.333333333333333314829616256247390992939472198486328125000000
> 0000
> v1 = (0.999999999999999888977697537484345957636833190917968750000000
> 0000,1.0000000000000000000000000000000000000000000000000000000000000000)
> v2 = (0.999999999999999888977697537484345957636833190917968750000000
> 0000,1.0000000000000000000000000000000000000000000000000000000000000000)
>
> $ g++ -O2 foo.cpp -o foo
> $ ./foo
> third1 = 0.333333333333333314829616256247390992939472198486328125000000
> 0000
> third2 = 0.333333333333333314829616256247390992939472198486328125000000
> 0000
> v1 = (0.999999999999999888977697537484345957636833190917968750000000
> 0000,1.0000000000000000000000000000000000000000000000000000000000000000)
> v2 = (1.000000000000000000000000000000000000000000000000000000000000
> 0000,1.0000000000000000000000000000000000000000000000000000000000000000)
>
> Can anyone explain what is going on?
>
> Thanks in advance,
>
> Tim
>
> --
> Tim van Erven <tim_at_[hidden]> <tim_at_[hidden]>www.timvanerven.nl
>
>
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