[petsc-dev] Request for comments: allow C99 internally

[Patrick Sanan]

Perhaps naively, I‘d assume that while there may well be someone out there
relying on compilers for which this would be a problem, that same person is
also less likely to be able to upgrade PETSc.

The benefits seem well worth it. It‘ll make things just that much easier to
work with.

+1 for the for-loop declarations.

No more need to police the use of // C++-style comments?



Jed Brown <jed at jedbrown.org> schrieb am Sa. 7. März 2020 um 06:48:

> I have a question for petsc-dev: Do you know anyone who needs to build
> PETSc with a compiler that doesn't support variadic macros and for-loop
> declarations?  (Both of these are in C99 and C++11, and supported by all
> tested configurations including compilers that don't fully implement
> these standards.)  Both MPICH and Open MPI use variable-length arrays
> and for-loop declarations, so you'd be hard-pressed building a modern
> stack with such a compiler.  I'm not proposing that we put these macros
> unguarded into the public headers, so a user of PETSc could still build
> with -std=c89 and the like.
>
>
> ## Background
>
> There is a common pattern in PETSc where we write
>
>   PetscInt some,several,variables;
>
>   // code
> #if defined(PETSC_HAVE_MAGIC)
>   function(several,&variables);
> #endif
>   use(some,variables);
>
>
> Of course this gives unused variable warnings, so we tear our code apart
> like
>
>   PetscInt some,variables;
> #if defined(PETSC_HAVE_MAGIC)
>   PetscInt several;
> #endif
>
>   // code
> #if defined(PETSC_HAVE_MAGIC)
>   function(several,&variables);
> #endif
>   use(some,variables);
>
>
> but the bigger problem is that we need different configurations just to
> check syntax of our compiled out blocks.  I propose allowing variadic
> macros (a C99 and C++11 feature) to allow code like
>
>   PetscInt some,several,variables;
>
>   // code
>   if (PetscDefined(HAVE_MAGIC)) {
>     function(several,&variables);
>   }
>   use(some,variables);
>
>
> This approach could also be used to avoid needing separate macros for
> every SETERRQ1-SETERRQ9, etc.  I have an example implementation in this
> MR, and it passes the full pipeline (after relaxing the -std=c89
> -pedantic build).
>
> https://gitlab.com/petsc/petsc/-/merge_requests/157/diffs
>
>
> We could also consider allowing for-loop declarations, which I believe
> leads to tighter and more understandable code because the reader doesn't
> have to wonder whether the variable is used after the loop.
>
>   for (PetscInt i=0; i<n; i++) { ... }
>
>
> Note that we cannot use variable-length arrays (VLA) because they are
> not in the intersection of C and C++.
>
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