[petsc-dev] need c++ expert help

[Jed Brown]

"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.

>   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.