On Wed, Jan 2, 2013 at 9:01 PM, Jed Brown <[email protected]> wrote:

On Wed, Jan 2, 2013 at 12:09 PM, Dmitri Gribenko <[email protected]> wrote:

...

Sorry if what I wrote was unclear. Here's a complete example:

$ cat t.cc #include <mpi.h>

enum MyEnum { Foo }; MPI_Datatype MyEnumDatatype MPICH_ATTR_TYPE_TAG(MyEnum);

Oh, fantastic. I didn't realize this could be applied to uninitialized variables (and somehow didn't recognize that you were saying as much in the first email). In light of this, can mpi.h change all the static declarations to extern

Unfortunately we can not, because user-defined datatypes are syntactically different from builtin datatypes in MPICH. MPI_INT is a macro, so what Clang sees is essentially a magic number. With user-defined datatypes there's an identifier (MyEnumDatatype) that actually shows up in the AST. There's a separate quirk in Clang for handling these magic numbers and corresponding 'static const' variables (not needed for OpenMPI, btw).

so that the symbols are guaranteed to only show up once in executables (rather than once per compilation unit)?

A smart enough compiler (and Clang is smart enough) will eliminate those even at -O0. Dmitri -- main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if (j){printf("%d\n",i);}}} /*Dmitri Gribenko <[email protected]>*/