[BUG]: mismatch creates an object on a device after

[zkhatami]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this bug and that I agree to the Code of Conduct

Type of Bug

Compile-time Error

Component

Thrust

Describe the bug

For this example:

#include <execution>
#include <algorithm>

template <typename T>
class Wrapper
{
  public:
    Wrapper()
    {
        ++my_count;
    }

    bool
    operator==(const Wrapper& input) const
    {
        return true;
    }

    ~Wrapper()
    {
        --my_count;
    }

  private:
    static std::atomic<size_t> my_count;
    T dummy;
};


template <typename T>
void
test()
{
  std::vector<T> input1;
  std::vector<T> input2;
  const auto res1 = std::mismatch(std::execution::par, input1.begin(), input1.end(), input2.begin(), input2.end(), std::equal_to<T>());
}

int32_t
main()
{
    test<Wrapper<int32_t>>();
}

The NVC++ stdpar fails with:

"test.cpp", line 22: error: global or namespace scope variables such as "Wrapper<T>::my_count [with T=int32_t]" (declared at line 26) cannot be accessed from device code
            function "Wrapper<T>::~Wrapper [with T=int32_t]" is implicitly a device function because it is called from device function "cuda::std::__4::__tuple_leaf<_Ip, _Hp, cuda::std::__4::__tuple_leaf_specialization::__default>::~__tuple_leaf [with _Ip=0UL, _Hp=Wrapper<int32_t>]" (declared implicitly)
          --my_count;
            ^

This is a new failure that's being observed after this commit 35df3a9.
The mismatch algorithm should only inspect existing elements of a container, not create new elements. In this test, since Wrapper has a constructor/destructor accessing static storage object, it gets created on the device after this commitL and causes NVC++ stdpar to fail.
It looks like the recent change in this header file mismatch.h in the commit 35df3a9 could be potentially causing the creation of a cuda::std::tuple object, where one of the types inside the tuple is Wrapper<int32_t> in the smaller test case.

How to Reproduce

nvc++ -stdpar -Ofast --c++17 -c test.cpp

Expected behavior

"test.cpp", line 22: error: global or namespace scope variables such as "Wrapper::my_count [with T=int32_t]" (declared at line 26) cannot be accessed from device code
function "Wrapper::Wrapper [with T=int32_t]" is implicitly a device function because it is called from device function "cuda::std::__4::__tuple_leaf<_Ip, _Hp, cuda::std::__4::__tuple_leaf_specialization::__default>::__tuple_leaf [with _Ip=0UL, _Hp=Wrapper<int32_t>]" (declared implicitly)
--my_count;
^

Reproduction link

No response

Operating System

No response

nvidia-smi output

No response

NVCC version

No response

[bernhardmgruber]

I can reproduce the issue in a slightly different form (calling thrust::mismatch and compiling with nvcc), but the issue persists even if I revert 35df3a9 :/ In order to get this solved we need a larger re-engineering of thrust iterators to never pull a copy or construct a temporary anywhere.

[bernhardmgruber]

Using nvc++ 25.1 and your test.cpp from above inside a CCCL git clone of the current main (a1a73a8):

bgruber@concorde:~/dev/cccl$ /opt/nvidia/hpc_sdk/Linux_x86_64/25.1/compilers/bin/nvc++ -stdpar -Ofast --c++17 -c test.cpp -Ithrust -Ilibcudacxx/include -Icub
bgruber@concorde:~/dev/cccl$ 

shows successful compilation.

Using nvc++ 24.5, I got:

bgruber@concorde:~/dev/cccl$ /opt/nvidia/hpc_sdk/Linux_x86_64/2024/compilers/bin/nvc++ -stdpar -Ofast --c++17 -c test.cpp -Ithrust -Ilibcudacxx/include -Icub
NVC++-F-0000-Internal compiler error. size of unknown type       0  (bug.cpp)
NVC++/x86-64 Linux 24.5-1: compilation aborted

Can you please give me the exact CCCL git commit that you used to encounter this error?

[zkhatami]

I'm using nvc++ dev version, that tracks the CCCL main branch. In contrast, nvc++ release versions use CCCL release branches, so they don’t include this commit.

The exact CCCL commit causing the failure is:

35df3a9f79a21c65f3e317a0ab6d0fa8c491793f

Before this commit, everything worked fine with:

cebb54c232216f260d63db00352ae90b28fdfc72