This example is a bit more representative of how we would use this to
implement the MPI bindings (renamed "bar" macro to "foo" and shifted down
to enable the name aliasing):
#include <stdio.h>
static void foo(int j) {
printf("foo(j) = %d\n", j);
}
#define foo(j) foo(sizeof(j) > sizeof(int) ? -1 : j)
int main(int argc, char *argv[]) {
/* 8589934592LL == 2^33 */
long long i = 8589934592LL + 11;
foo(i);
return 0;
}
foo(j) = -1
~Jim.
On Wed, Aug 7, 2019 at 4:16 PM Jim Dinan <james.di...@gmail.com> wrote:
> Jeff,
>
> You can wrap the function in a macro that checks for truncation:
>
> #include <stdio.h>
>
>
>
> #define bar(j) foo(sizeof(j) > sizeof(int) ? -1 : j)
>
>
> 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);
>
> bar(i);
>
> return 0;
>
> }
>
> foo(j) = 11
>
> foo(j) = -1
>
> ~Jim.
>
> On Wed, Aug 7, 2019 at 9:59 AM Jeff Squyres (jsquyres) via mpi-forum <
> mpi-forum@lists.mpi-forum.org> wrote:
>
>> 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
>>
>> _______________________________________________
>> mpi-forum mailing list
>> mpi-forum@lists.mpi-forum.org
>> https://lists.mpi-forum.org/mailman/listinfo/mpi-forum
>>
>
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