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

[Satish Balay]

On Fri, 6 Mar 2020, Jed Brown wrote:

> 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);

One minor issue: we haven't yet fixed up clang analyzer build. Likely this will be listed as 'dead code' block or something like that.
 
> 
> 
> 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).

These are in public includes. So apps using -std=c89 won't have access to this part of the API?

Satish

> 
> 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++.
>