[petsc-dev] Tons of errors with an unusual PETSc configuration

[Lisandro Dalcin]

On Wed, 17 Apr 2019 at 18:42, Smith, Barry F. <bsmith at mcs.anl.gov> wrote:

>
>     Moving to PETSc dev for more input
>
>     Lisandro,
>
>        I understand the rational for not wanting it on by default for
> random user code but the fact is that is is NEEDED to compile the PETSc
> source (and examples) and so seems wrong that those configurations can only
> be built if the user provides this obscure flag
> PETSC_USE_CXX_COMPLEX_FLOAT_WORKAROUND they won't know about.
>
>
I agree. My work to fix the issue is incomplete indeed, but the leftover is
rather small. This would probably involve modifying things in BuildSystem,
and I have a tendency to not mess with things I do not understand well.


>       We could have it on by default and have a flag
> PETSC_SKIP_CXX_COMPLEX_FLOAT_WORKAROUND that users put in before they
> include any PETSc includes but I suspect you won't like this?
>

It does not really matter to me, as long I'm not the one to be blamed by
taking that decision :-) . This change will not affect any of my own codes,
so I do not have any personal agenda here.

      Somehow have the gnu make system pass in the flag
> PETSC_USE_CXX_COMPLEX_FLOAT_WORKAROUND when compiling PETSc source and
> examples but not other code? How to organize that?
>

This is something we should definitely do. PETSc source code is our
codebase, we are always in control of fixing things around.


>        Define PETSC_USE_CXX_COMPLEX_FLOAT_WORKAROUND at the top of every
> PETSc source file that needs it (and examples that need it)? Ugly, but
> doable?
>
>
No, please!


>        "Fix" the PETSc source code so the issues it handles don't come up?
> Or reduce them and stick the PETSC_USE_CXX_COMPLEX_FLOAT_WORKAROUND just in
> the troublesome files?
>

No way, I already looked into that.

* Too much work to fix a broken the standard std::complex<float> template.
* Too many ugly (PetscReal) casts here and there in the codebase
* And it is not only the problem of fixing the current code, this issue
will continue to popup.


>       Any other way to have it on when needed but off otherwise?
>
>
 My final recommendation would be:

1) Put the relevant workaround code in a separate header users can #include
in their own code, let say #include <petsccomplexfloat.h>, we can even put
it under the private/ include folder.
2)  Keep the PETSC_USE_CXX_COMPLEX_FLOAT_WORKAROUND in "petscmat.h", and
make it #include <petsccomplexfloat.h>.
3) Modify our top level gmakefile to pass
-DPETSC_USE_CXX_COMPLEX_FLOAT_WORKAROUND, but only if we are building with
lang=C++, scalar=complex, precision=single. For example, we can make
configure write a
`PETSC_BUILD_CPPFLAGS=-DPETSC_USE_CXX_COMPLEX_FLOAT_WORKAROUND`, but we use
it ONLY in the toplevel gmakefile, NOT in
`$(PETSC_DIR)/lib/petsc/conf/variables`. Also gmakefile.tests?

This way users can use the workaround in two ways, either by #include
<petsccomplexfloat.h> or by defining PETSC_USE_CXX_COMPLEX_FLOAT_WORKAROUND
themselves (either on source before including petsc.h or via makefiles
variables). And we fix the issue when building PETSc in a rather
transparent and non-obtrusive way.

Now, Barry, seat in the Iron Throne and rule! And if someone complain, use
dragon fire... like in the good old days.

Regards,

-- 
Lisandro Dalcin
============
Research Scientist
Extreme Computing Research Center (ECRC)
King Abdullah University of Science and Technology (KAUST)
http://ecrc.kaust.edu.sa/
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