Fix extracting CUDA stream in `cub::DeviceTransform` by bernhardmgruber · Pull Request #7239 · NVIDIA/cccl

bernhardmgruber · 2026-01-14T23:13:11Z

#6204 changed cub::DeviceTransform APIs from taking cudaStream_t to environments. Special handling preserved support for cudaStream_t. However, user-provided stream types with conversion operators to cudaStream_t were now queried as environments, failing to return a stream.

This PR treats any type that is convertible to a cudaStream_t not as an environment and extracts the underlying stream.

Fixes NVBug 5813928

bernhardmgruber · 2026-01-14T23:13:35Z

      num_items,
      ::cuda::std::move(transform_op),
-      get_stream(env));
+      ::cuda::std::move(env));


Drive-by fix

bernhardmgruber · 2026-01-14T23:19:45Z

+  auto run = [&](auto streamish) {
+    cub::DeviceTransform::Transform(cuda::std::make_tuple(a, b), result.begin(), num_items, _1 + _2, streamish);
+  };
  SECTION("raw stream")
  {
-    cub::DeviceTransform::Transform(cuda::std::make_tuple(a, b), result.begin(), num_items, _1 + _2, stream);
+    run(stream);
+  }
+  SECTION("custom stream")
+  {
+    run(custom_stream{stream});


I verified that the stream is extracted in the debugger, but I wonder if I could write the unit test in a way to detect if the default stream was taken anywhere. Does anybody know if I can query the stream whether something was really enqueued there?

One solution would be to start a graph capture on a stream and see if anything was captured, but that might have some limitations, not sure if its applicable here

miscco

I would love to better understand where the issue with the conversions from cudaStream_t lies so that we can extent our get_stream CPO

What was the exact case failing? from what I can see it the second and third branch in the if constexpr should already work

Maybe we just need another clause for get_stream

miscco · 2026-01-15T07:44:01Z

+    }
+    else
+    {
+      return ::cuda::std::execution::__query_or(env, ::cuda::get_stream, ::cuda::stream_ref{cudaStream_t{}}).get();


This is very close to what we have in get_stream

I believe the issue is that we are checking whether tis convertible to stream_ref and not cudaStream_t

But at the same time cudaStream_t should be convertible to stream_ref

copy-pr-bot · 2026-01-15T08:52:33Z

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bernhardmgruber · 2026-01-15T08:53:16Z

Discussed this PR with @miscco and we concluded that the environment query for cuda::get_stream misses a check whether the environment is convertible to a cudaStream_t. He will add this and then we can replace the get_stream in this PR by just the environment query.

miscco · 2026-01-15T09:26:07Z

/ok to test cc8f29e

miscco · 2026-01-15T09:27:04Z

I have updated our get_stream CPO to ensure that it also works with types that are convertible to ::cudaStream_t as apposed to ::cuda::stream_ref

bernhardmgruber · 2026-01-15T23:19:39Z

I started the backport already from the current state: #7263, since I need to rebase and change this PR again, since #6914 was merged and we have conflicts.

NVIDIA#6204 changed cub::DeviceTransform APIs from taking cudaStream_t to environments. Special handling preserved support for cudaStream_t. However, user-provided stream types with conversion operators to cudaStream_t were now queried as environments, failing to return a stream.

We should not have a special overload for the `get_stream` overload but should ensure that we can pass types that are convertible to `::cudaStream_t` and extract that stream I have expanded the get_stream CPO to accept a `::cudaStream_t __stream` so that we can work with those types

github-actions · 2026-01-16T16:06:49Z

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github-actions · 2026-01-16T16:07:16Z

Successfully created backport PR for branch/3.2.x:

[Backport branch/3.2.x] Fix extracting CUDA stream in cub::DeviceTransform #7272

* Fix extracting CUDA stream in cub::DeviceTransform #6204 changed cub::DeviceTransform APIs from taking cudaStream_t to environments. Special handling preserved support for cudaStream_t. However, user-provided stream types with conversion operators to cudaStream_t were now queried as environments, failing to return a stream. * Properly use `get_stream` in device transform We should not have a special overload for the `get_stream` overload but should ensure that we can pass types that are convertible to `::cudaStream_t` and extract that stream I have expanded the get_stream CPO to accept a `::cudaStream_t __stream` so that we can work with those types Co-authored-by: Michael Schellenberger Costa <miscco@nvidia.com> (cherry picked from commit 31f8a13)

bernhardmgruber requested a review from a team as a code owner January 14, 2026 23:13

bernhardmgruber requested a review from NaderAlAwar January 14, 2026 23:13

github-project-automation Bot added this to CCCL Jan 14, 2026

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bernhardmgruber commented Jan 14, 2026

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Comment thread cub/cub/device/device_transform.cuh Resolved

bernhardmgruber commented Jan 14, 2026

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bernhardmgruber marked this pull request as draft January 15, 2026 08:52

cccl-authenticator-app Bot moved this from In Review to In Progress in CCCL Jan 15, 2026

miscco marked this pull request as ready for review January 15, 2026 09:26

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[Backport to 3.2] Fix extracting CUDA stream in cub::DeviceTransform #7263

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bernhardmgruber and others added 2 commits January 16, 2026 00:28

bernhardmgruber force-pushed the fix_custom_stream_transform branch from cc8f29e to 8ff0cdd Compare January 15, 2026 23:29

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miscco added 2 commits January 16, 2026 09:16

Fix issue with __query_or

65d8ee9

Avoid unused variable warning

b87896f

bernhardmgruber merged commit 31f8a13 into NVIDIA:main Jan 16, 2026
139 checks passed

github-project-automation Bot moved this from In Review to Done in CCCL Jan 16, 2026

github-actions Bot mentioned this pull request Jan 16, 2026

[Backport branch/3.2.x] Fix extracting CUDA stream in cub::DeviceTransform #7272

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bernhardmgruber deleted the fix_custom_stream_transform branch January 16, 2026 17:46

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