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
jsquy...@cisco.com
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