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

I'm not really an expert in this space, but the one (rather tedious) exercise I could suggest is that for each differing option between -O0 and -O2, add the corresponding explicit option until you get the deviation you're exhibiting. I did do a diff on your settings, and I didn't really see anything that stood out to me, hence why I'm suggesting the switch-by-switch test. I'm not a boost contributor just a user and rare bug reporter. This does feel like a floating-point optimization switch, though, at least to me. Regards, Nate On Sat, Mar 3, 2018 at 2:08 PM, Tim van Erven wrote:

Hi Nate,

Thanks for your help.

Below is the output from gcc 7.2.0 on mac os. Apparently the difference already happens between -O0 and -O1. To summarize: - gcc on linux with boost 1.58 - clang and gcc 7.2.0 on mac os with boost 1.66 all give wrong (or at least inconsistent) output depending on optimization options. (Since third1 and third2 are both < 1/3, multiplying them by 3 should give an interval with lower end-point < 1, but it sometimes doesn't.)

I am attaching the outputs of g++-7 -Q -O0 --help=optimizers > o0.txt $ g++-7 -Q -O1 --help=optimizers > o1.txt

$ g++-7 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++-7 -O1 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)

Best, Tim

On 02/03/2018 18:23, Nathan Ernst wrote:

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@lists.boost.org> 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 = 0.3333333333333333148296162562473909929394721984863281250000 000000 third2 = 0.3333333333333333148296162562473909929394721984863281250000 000000 v1 = (0.999999999999999888977697537484345957636833190917968750000 0000000,1.00000000000000000000000000000000000000000000000000 00000000000000) v2 = (0.999999999999999888977697537484345957636833190917968750000 0000000,1.00000000000000000000000000000000000000000000000000 00000000000000)

$ g++ -O2 foo.cpp -o foo $ ./foo third1 = 0.3333333333333333148296162562473909929394721984863281250000 000000 third2 = 0.3333333333333333148296162562473909929394721984863281250000 000000 v1 = (1.000000000000000000000000000000000000000000000000000000000 0000000,1.00000000000000000000000000000000000000000000000000 00000000000000) v2 = (1.000000000000000000000000000000000000000000000000000000000 0000000,1.00000000000000000000000000000000000000000000000000 00000000000000)

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 = 0.3333333333333333148296162562473909929394721984863281250000 000000 third2 = 0.3333333333333333148296162562473909929394721984863281250000 000000 v1 = (0.999999999999999888977697537484345957636833190917968750000 0000000,1.00000000000000000000000000000000000000000000000000 00000000000000) v2 = (0.999999999999999888977697537484345957636833190917968750000 0000000,1.00000000000000000000000000000000000000000000000000 00000000000000)

$ g++ -O2 foo.cpp -o foo $ ./foo third1 = 0.3333333333333333148296162562473909929394721984863281250000 000000 third2 = 0.3333333333333333148296162562473909929394721984863281250000 000000 v1 = (0.999999999999999888977697537484345957636833190917968750000 0000000,1.00000000000000000000000000000000000000000000000000 00000000000000) v2 = (1.000000000000000000000000000000000000000000000000000000000 0000000,1.00000000000000000000000000000000000000000000000000 00000000000000)

Can anyone explain what is going on?

Thanks in advance,

Tim

-- Tim van Erven www.timvanerven.nl

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-- Tim van Erven www.timvanerven.nl