[cfe-dev] [Openmp-dev] [OpenMP] Parameter packs

David Greene via cfe-dev cfe-dev at lists.llvm.org
Mon Oct 15 08:08:49 PDT 2018


Good to hear.  Is it technically difficult to do in clang?  If not,
would an experimental implementation help the committee?

                               -David

Kelvin Li via Openmp-dev <openmp-dev at lists.llvm.org> writes:

> 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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>> Openmp-dev at lists.llvm.org
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>
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