SHORT VERSION ============= Due to the possibility of silently introducing errors into user applications, the BigCount WG no longer thinks that C11 _Generic is a good idea. We are therefore dropping that from our proposal. The new proposal will therefore essentially just be the addition of a bunch of MPI_Count-enabled "_x" functions in C, combined with the addition of a bunch of polymorphic MPI_Count-enabled interfaces in Fortran. MORE DETAIL =========== Joseph Schuchart raised a very important point in a recent mailing thread: the following C/C++ code does not raise a compiler warning: ----- #include <stdio.h> static void foo(int j) { printf("foo(j) = %d\n", j); } int main(int argc, char *argv[]) { /* 8589934592LL == 2^33 */ long long i = 8589934592LL + 11; foo(i); return 0; } ----- If you compile and run this program on a commodity x86-64 platform, a) you won't get a warning from the compiler, and b) you'll see "11" printed out. I tried with gcc 9 and clang 8 -- both with the C and C++ compilers. I even tried with "-Wall -pedantic". No warnings. This is because casting from a larger int type to a smaller int type is perfectly valid C/C++. Because of this, there is a possibility that we could be silently introducing errors into user applications. Consider: 1. An application upgrades its "count" parameters to type MPI_Count for all calls to MPI_Send. --> Recall that "MPI_Count" already exists in MPI-3.1, and is likely of type (long long) on commodity x86-64 platforms 2. The application then uses values in that "count" parameter that are greater than 2^32. If the user's MPI implementation and compiler both support C11 _Generic, everything is great. But if either the MPI implementation or the compiler do not support C11 _Generic, ***the "count" value will be silently truncated at run time***. This seems like a very bad idea, from a design standpoint. We have therefore come full circle: we are back to adding a bunch of "_x" functions for C, and there will be no polymorphism (in C). Sorry, folks. Note that Fortran does not have similar problems: 1. Fortran compilers have supported polymorphism for 20+ years 2. Fortran does not automatically cast between INTEGER values of different sizes After much debate, the BigCount WG has decided that C11 _Generic just isn't worth it. That's no reason to penalize Fortran, though. -- Jeff Squyres [email protected]