[petsc-dev] need c++ expert help

[Smith, Barry F.]

> On Dec 31, 2017, at 7:05 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> "Smith, Barry F." <bsmith at mcs.anl.gov> writes:
> 
>>  I think it is just CXX.  The PCC business is the thing that magically changes from CC to CXX depending on --with-clanguage being set. But CXX is always the C++ linker.
> 
> That would make me happy, but in that case, let's get rid of PCC_LINKER,
> SL_LINKER, DYNAMICLINKER, and LD_SHARED because they're all redundant.

  I think they are absolutely needed in the legacy make system. 

   Since the gnumake system seems to have worked flawlessly for a long time I have no problem removing the legacy make system. Is that what you are proposing?

   Note you do have to have some mechanism to handle the --with-clanguage=cxx case 

   Barry

> 
>>  Barry
>> 
>> 
>>> On Dec 31, 2017, at 6:18 PM, Jed Brown <jed at jedbrown.org> wrote:
>>> 
>>> "Smith, Barry F." <bsmith at mcs.anl.gov> writes:
>>> 
>>>> There is always a C++ linker available so long as PETSc was built without --with-cxx=0 so just use it, why make a big deal out of it?
>>> 
>>> What is it called?  I would like to just use CXX, but we have PCC_LINKER
>>> written out independently.  I don't know any circumstance in which
>>> PCC_LINKER is different from PCC, but maybe there is.  We should get rid
>>> of this duplication unless there is a specific circumstance in which
>>> they are different.
>>> 
>>> $ grep mpicxx ompi/lib/petsc/conf/petscvariables                                                                                                              
>>> CXX = /home/jed/usr/ccache/ompi/bin/mpicxx                                                                                                                                                    
>>> CXXCPP = /home/jed/usr/ccache/ompi/bin/mpicxx -E
>>> 
>>> $ grep mpicc ompi/lib/petsc/conf/petscvariables                                                                                                              
>>> SL_LINKER = /home/jed/usr/ccache/ompi/bin/mpicc                                                                                                                                               
>>> PCC = /home/jed/usr/ccache/ompi/bin/mpicc
>>> PCC_LINKER = /home/jed/usr/ccache/ompi/bin/mpicc
>>> CC = /home/jed/usr/ccache/ompi/bin/mpicc
>>> DYNAMICLINKER = /home/jed/usr/ccache/ompi/bin/mpicc
>>> CPP = /home/jed/usr/ccache/ompi/bin/mpicc -E
>>> LD_SHARED = /home/jed/usr/ccache/ompi/bin/mpicc
>>> 
>>> 
>>>>  Barry
>>>> 
>>>>> On Dec 31, 2017, at 5:58 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>>> 
>>>>> "Smith, Barry F." <bsmith at mcs.anl.gov> writes:
>>>>> 
>>>>>>> On Dec 31, 2017, at 2:14 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>>>>> 
>>>>>>> Matthew Knepley <knepley at gmail.com> writes:
>>>>>>> 
>>>>>>>> On Sun, Dec 31, 2017 at 2:35 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>>>>>> 
>>>>>>>>> Matthew Knepley <knepley at gmail.com> writes:
>>>>>>>>> 
>>>>>>>>>> On Sun, Dec 31, 2017 at 1:55 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>>>>>>>> 
>>>>>>>>>>> These look like linker errors and that build uses
>>>>>>>>>>> --with-cxxlib-autodetect=0.  We either need a rule to *link* C++ using
>>>>>>>>>>> CXX (i.e., mpicxx) or add LIBS=-lstdc++.
>>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>>> I thought we were doing this (linking C++ mains with CXX).
>>>>>>>>> 
>>>>>>>>> After compiling to the object file (ex3.o) we don't know what language
>>>>>>>>> main was written in, and this doesn't solve the actual problem that C++
>>>>>>>>> linking is required if any object (not just main) depends on C++.
>>>>>>>>> 
>>>>>>>> 
>>>>>>>> My thinking here was the following:
>>>>>>>> 
>>>>>>>> 1) No C++ is allowed in PETSc, unless --with-clanguage=cxx, in which case
>>>>>>>> the linker is C++
>>>>>>>> 2) If C++ is in an external library, then that configure requires the C++
>>>>>>>> library
>>>>>>>> 3) The executable itself could be C++, in which case I proposed using the
>>>>>>>> C++ linker explicitly
>>>>>>>> 
>>>>>>>> About not knowing which objects come with C++ main: I thought we did. Don't
>>>>>>>> they go into a separate set?
>>>>>>> 
>>>>>>> Right, we do have that information for tests.  Note that a single test
>>>>>>> executable can depend on multiple source files, and the one containing
>>>>>>> main might not be C++ while another is.  I don't know if there are any
>>>>>>> such instances in PETSc.
>>>>>>> 
>>>>>>> I can change the build rules for tests with C++ sources later today.
>>>>>> 
>>>>>> Jed, are you indicating that you will resolve the original problem I reported? Where will you resolve it? The problem may come up in one of my branches.
>>>>> 
>>>>> I was going to put it in a new branch from 'master'.  There is
>>>>> disgusting duplication in 'maint' that has since been fixed so I'd
>>>>> rather not do it there.
>>>>> 
>>>>> Under what conditions is PCC_LINKER different from CC (or CXX when
>>>>> clanguage=C++)?  I need to find a C++ linker and would prefer to use
>>>>> $(CXX) rather than adding a new configure test.  I feel like the
>>>>> generated petscvariables has an inordinate amount of duplication all
>>>>> with non-standard names and no documentation.
>>>>> 
>>>>> The cheap way to fix your issue is to add LIBS=-lstdc++ to your
>>>>> configure.