Bug#888220: ignition-math4: FTBFS on mips(el) and hppa: UNIT_Helpers_TEST fails badly

[Jose Luis Rivero]

Thanks very much James for the research and the patch.

I've sent it upstream:
https://bitbucket.org/ignitionrobotics/ign-math/pull-requests/232/bug-in-pairing-function-insde-helperscc/diff

On Wed, Jan 24, 2018 at 9:14 AM, James Cowgill <jcowgill at debian.org> wrote:

> Control: tags -1 patch
>
> Hi,
>
> On 23/01/18 23:48, Aaron M. Ucko wrote:
> > Source: ignition-math4
> > Version: 4.0.0+dfsg1-1
> > Severity: important
> > Tags: upstream
> > Justification: fails to build from source
> > User: debian-mips at lists.debian.org
> > Usertags: mips mipsel
> >
> > Builds of ignition-math4 for mips, mipsel, and the non-release
> > architecture hppa have been failing as detailed at
> >
> > https://buildd.debian.org/status/fetch.php?pkg=ignition-
> math4&arch=mips&ver=4.0.0%2Bdfsg1-1&stamp=1516376024&raw=0
> > https://buildd.debian.org/status/fetch.php?pkg=ignition-
> math4&arch=mipsel&ver=4.0.0%2Bdfsg1-1&stamp=1516373688&raw=0
> > https://buildd.debian.org/status/fetch.php?pkg=ignition-
> math4&arch=hppa&ver=4.0.0%2Bdfsg1-1&stamp=1516373442&raw=0
> >
> > On mips, this test manages to make ctest itself to run out of memory:
> >
> >         Start 11: UNIT_Helpers_TEST
> >   terminate called after throwing an instance of 'std::bad_alloc'
> >     what():  std::bad_alloc
> >   Makefile:143: recipe for target 'test' failed
> >   make[2]: *** [test] Aborted
> >
> > On mipsel and hppa, HelpersTest.Pair spews millions(!) of errors and
> > then hits a timeout:
> >
> >   11/75 Test #11: UNIT_Helpers_TEST ......................***Timeout
> 240.12 sec
> >   [...]
> >   [ RUN      ] HelpersTest.Pair
> >   /<<BUILDDIR>>/ignition-math4-4.0.0+dfsg1/src/Helpers_TEST.cc:516:
> Failure
> >         Expected: a
> >         Which is: 4294962295
> >   To be equal to: c
> >         Which is: 4294962296
>
> This is caused by the pairing function in src/Helpers.cc
>
> Firstly, I am not sure I understand why anyone would need this pairing
> function exactly. The point of a pairing function is that the inputs can
> be infinitely big, but the inputs to these functions are 32-bit
> integers. As long as you don't need the specific algorithm, you could
> implement something similar using a single shift and an add.
>
> I think the issue is this code relies on infinite floating point
> precision in calculating the "sqrt" variable:
> >     std::tuple<PairInput, PairInput> Unpair(const PairOutput _key)
> >     {
> >       // Must explicitly cast so that the _key is not auto cast to a
> double
> >       uint64_t sqrt = static_cast<uint64_t>(
> >           std::floor(std::sqrt(static_cast<long double>(_key))));
> >       uint64_t sq = sqrt * sqrt;
> >
> >       return ((_key - sq) >= sqrt) ?
> >         std::make_tuple(static_cast<PairInput>(sqrt),
> >                         static_cast<PairInput>(_key - sq - sqrt)) :
> >         std::make_tuple(static_cast<PairInput>(_key - sq),
> >                         static_cast<PairInput>(sqrt));
> >     }
>
> On architectures with > 64-bit long doubles (like Linux x86_64) you get
> away with this. On other architectures where long double == double, the
> cast to long double and sqrt is too inaccurate to implement this properly.
>
> The attached patch is based on a more accurate integer square root I
> found on stack overflow. You should be able to drop your
> 0004_test_failures_in_non_x64_arches.patch after applying this and
> possibly some other #ifdefs I noticed that upstream have added.
>
> James
>
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