Nested stdpar algorithm calls

Accelerated Computing HPC Compilers nvc, nvc++ and nvfortran
1

Hi,

It looks like nesting stdpar algorithm calls is not possible when targeting GPUs; for example:

std::for_each(std::execution::par, /*...*/, [](int i) {
  // ...
  std::transform(std::execution::par, /*...*/);
  // ...
});

I’m getting the following error when compiling code like that:

NVC++-F-0155-Compiler failed to translate accelerator region (see -Minfo messages): Unsupported procedure

I’m using nvc++ 23.9-0 64-bit target on x86-64 Linux -tp alderlake, but I’ve also seen it with older versions.

Is that a structural limitation (if yes, do you know if it’s documented)? Are there any plans to support it?

Thanks,
Christos

2

Could you please try using par on the outer and unseq on the inner and see if that gets rid of your compilation failure?

3

Yes, that works, but only the outer algorithm runs in parallel this way, right?

4

Right, and that is a limitation currently. We have an open problem report on it internally but it is probably not high on the priority.

1 Like
5

Cool, thanks. It’d be exciting to have that one day.

The code snippet above is sometimes easily converted to a single parallel std::for_each (or std::transform).

However, this is not true when the inner algorithm is a scan (e.g. std::transform_inclusive_scan): currently, there’s no way you can run several parallel scans in parallel (I’m talking about GPU algorithms).