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

3 Mar 2018

      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 = 0.3333333333333333148296162562473909929394721984863281250000000000
third2 = 0.3333333333333333148296162562473909929394721984863281250000000000
v1 = 
(0.9999999999999998889776975374843459576368331909179687500000000000,1.0000000000000000000000000000000000000000000000000000000000000000)
v2 = 
(0.9999999999999998889776975374843459576368331909179687500000000000,1.0000000000000000000000000000000000000000000000000000000000000000)

$ g++-7 -O1 foo.cpp -o foo
./foo
third1 = 0.3333333333333333148296162562473909929394721984863281250000000000
third2 = 0.3333333333333333148296162562473909929394721984863281250000000000
v1 = 
(0.9999999999999998889776975374843459576368331909179687500000000000,1.0000000000000000000000000000000000000000000000000000000000000000)
v2 = 
(1.0000000000000000000000000000000000000000000000000000000000000000,1.0000000000000000000000000000000000000000000000000000000000000000)

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 
mailto: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 =
    0.3333333333333333148296162562473909929394721984863281250000000000
    third2 =
    0.3333333333333333148296162562473909929394721984863281250000000000
    v1 =
    (0.9999999999999998889776975374843459576368331909179687500000000000,1.0000000000000000000000000000000000000000000000000000000000000000)
    v2 =
    (0.9999999999999998889776975374843459576368331909179687500000000000,1.0000000000000000000000000000000000000000000000000000000000000000)
$ g++ -O2 foo.cpp -o foo
    $ ./foo
    third1 =
    0.3333333333333333148296162562473909929394721984863281250000000000
    third2 =
    0.3333333333333333148296162562473909929394721984863281250000000000
    v1 =
    (1.0000000000000000000000000000000000000000000000000000000000000000,1.0000000000000000000000000000000000000000000000000000000000000000)
    v2 =
    (1.0000000000000000000000000000000000000000000000000000000000000000,1.0000000000000000000000000000000000000000000000000000000000000000)
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 =
    0.3333333333333333148296162562473909929394721984863281250000000000
    third2 =
    0.3333333333333333148296162562473909929394721984863281250000000000
    v1 =
    (0.9999999999999998889776975374843459576368331909179687500000000000,1.0000000000000000000000000000000000000000000000000000000000000000)
    v2 =
    (0.9999999999999998889776975374843459576368331909179687500000000000,1.0000000000000000000000000000000000000000000000000000000000000000)
$ g++ -O2 foo.cpp -o foo
    $ ./foo
    third1 =
    0.3333333333333333148296162562473909929394721984863281250000000000
    third2 =
    0.3333333333333333148296162562473909929394721984863281250000000000
    v1 =
    (0.9999999999999998889776975374843459576368331909179687500000000000,1.0000000000000000000000000000000000000000000000000000000000000000)
    v2 =
    (1.0000000000000000000000000000000000000000000000000000000000000000,1.0000000000000000000000000000000000000000000000000000000000000000)
Can anyone explain what is going on?
Thanks in advance,
Tim
-- 
    Tim van Erven mailto:tim@timvanerven.nl
    www.timvanerven.nl http://www.timvanerven.nl
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