[Openmp-dev] [OpenMP] Parameter packs

[Kelvin Li via Openmp-dev]

The OpenMP specification has the following restriction:

"Variadic parameter packs cannot be expanded into a directive or its 
clauses except as part of an expression argument to be evaluated by the 
base language, such as into a function call inside an if clause."

The language committee has some discussions on it in the last face-to-face 
meeting.  We will look into relaxing this restriction to provide better 
usage with C++ templates.

Kelvin


"Openmp-dev" <openmp-dev-bounces at lists.llvm.org> wrote on 2018/10/15 
10:45:30 AM:

> From: David Greene via Openmp-dev <openmp-dev at lists.llvm.org>
> To: "cfe-dev at lists.llvm.org" <cfe-dev at lists.llvm.org>, "openmp-
> dev at lists.llvm.org" <openmp-dev at lists.llvm.org>
> Date: 2018/10/15 10:46 AM
> Subject: [Openmp-dev] [OpenMP] Parameter packs
> Sent by: "Openmp-dev" <openmp-dev-bounces at lists.llvm.org>
> 
> Should this compile?
> 
> #include <tuple>
> 
> constexpr std::size_t N = 100;
> 
> double A[N];
> 
> template<typename ...Args>
> void loop(Args ...args) {
> #pragma omp parallel firstprivate(args...)
>   for (std::size_t i = 0; i < N; ++i) {
>     A[i] = (i + ... + args);
>   }
> }
> 
> int main(void) {
>   std::apply([] (const auto & ...args) { loop(args...); },
>                  std::tuple<int, double, float>(10, 2.6, 3.4));
>   std::apply([] (const auto & ...args) { loop(args...); },
>                  std::tuple<int, long, char, double, short>(10, 100,
> 'b', 2.6, 3));
>   return 0;
> }
> 
> clang++ errors with:
> 
> firstprivate.cpp:10:39: error: expected ',' or ')' in 'firstprivate' 
clause
> #pragma omp parallel firstprivate(args...)
>                                       ^
> firstprivate.cpp:10:39: error: expected expression
> 2 errors generated.
> 
> g++ gives a similar error.  I was not terribly surprised by this.
> However, it sure would be nice if it worked.
> 
>                       -David
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