Re: [petsc-dev] circular dependencies SLEPc

[Smith, Barry F. via petsc-dev]

> On Jun 26, 2019, at 10:55 AM, Jed Brown <j...@jedbrown.org> wrote:
> 
> "Smith, Barry F. via petsc-dev" <petsc-dev@mcs.anl.gov> writes:
> 
>>> On Jun 26, 2019, at 9:56 AM, Balay, Satish via petsc-dev 
>>> <petsc-dev@mcs.anl.gov> wrote:
>>> 
>>> On Wed, 26 Jun 2019, Jakub Kruzik via petsc-dev wrote:
>>> 
>>>> Hello,
>>>> 
>>>> as I mentioned in PR #1819, I would like to use SLEPc in PETSc.
>>>> 
>>>> Currently when PETSc is configured with --download-slepc, it defines
>>>> PETSC_HAVE_SLEPC and each compilation of PETSc recompiles SLEPc.
>>> 
>>> yes - slepc uses petsc, so when petsc is updated - its best to rebuild slepc
>>> 
>>> You can ignore PETSC_HAVE_SLEPC flag [its just a build tool thingy]
>>> PETSc code does not use this flag - and there is no circular
>>> dependency.
>>> 
>>>> The first way to use SLEPc is from an example. That should be easy, all we
>>>> need is to add -lslepc when compiling an example.
>>> 
>>> Its best to use slepc examples as templates - and slepc makefiles [as 
>>> examples].
>>> 
>>> --download-slepc is a convinence feature to install petsc and slepc in
>>> a single go. It does not change how you would use slepc.
>>> 
>>> Satish
>>> 
>>> 
>>>> 
>>>> The other option is to use SLEPc inside PETSc code. I do not know how to
>>>> achieve this. One way could be to define PETSC_HAVE_SLEPC after the
>>>> compilation of SLEPc and again compile PETSc but this time linking with 
>>>> SLEPc.
>>>> Although, even if it works, it is ugly.
>> 
>>   If you make SLEPc calls from PETSc source you should only need the SLEPc 
>> header files to compile the PETSc source; not the SLEPc library. So one way 
>> to accomplish this would be to do a "partial" install of SLEPc, build PETSc 
>> (that uses SLEPc) and then complete the SLEPc install. When --download-slepc 
>> is used this would mean during the SLEPc.py script it would copy over the 
>> SLEPc include files to the prefix location and after PETSc is built it would 
>> build the SLEPc libraries and move them to the prefix location.  The on iffy 
>> thing is that SLEPc include files may depend on generated PETSc include 
>> files (which are not fully generated until configure is done). Thus instead 
>> of having SLEPc.py move the SLEPc include to the prefix location it would 
>> need to post-pone that until just at the end of configure (we have other 
>> packages to do this). So when you ready to try this out let us know and we 
>> can help with the infrastructure. (it will avoid 2 builds of either PETSc or 
>> SLEPc).
> 
> That is disgusting.
> 
> If code in libpetsc.so depends on libslepc.so, then you'd have a circular 
> dependency.
> 
> 
> You can implement and register a PC in SLEPc (it would go in libslepc.so).

  It makes no sense to have a PC in SLEPc.