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[libcu++] Implement P3793R2 Better Shifting (without SIMD)#9993

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davebayer:better_shifting
Jul 21, 2026
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[libcu++] Implement P3793R2 Better Shifting (without SIMD)#9993
davebayer merged 5 commits into
NVIDIA:mainfrom
davebayer:better_shifting

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@davebayer davebayer commented Jul 17, 2026 •
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This PR implement P3793R2 Better Shifting. I've omitted the simd support, should be done in a separate PR.

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davebayer requested a review from a team as a code owner July 17, 2026 12:56
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davebayer requested a review from wmaxey July 17, 2026 12:56
@github-project-automation github-project-automation Bot moved this to Todo in CCCL Jul 17, 2026
@cccl-authenticator-app cccl-authenticator-app Bot moved this from Todo to In Review in CCCL Jul 17, 2026
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📒 Files selected for processing (2)
  • libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shl.pass.cpp
  • libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shr.pass.cpp
🚧 Files skipped from review as they are similar to previous changes (2)
  • libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shl.pass.cpp
  • libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shr.pass.cpp
📝 Walkthrough

Summary by CodeRabbit

  • New Features
    • Added cuda::std::shl and cuda::std::shr constexpr integer shift utilities, including support for signed/unsigned operands and negative shift counts.
    • Included the new headers in the cuda/std/bit umbrella.
  • Tests
    • Added pass tests covering shl/shr across many integer types and edge-case shift values.
    • Refined the test DoNotOptimize barrier logic to use separate device vs. host handling.
  • Documentation
    • Updated the cuda/std/bit header copyright year.

Walkthrough

Changes

Adds cuda::std::shl and cuda::std::shr with constexpr integer shifting, signed-shift normalization, width clamping, and CUDA PTX dispatch. Exposes both APIs through cuda/std/bit and adds host/device tests.

Integer shift utilities

Layer / File(s) Summary
Shift API implementations
libcudacxx/include/cuda/std/__bit/*, libcudacxx/include/cuda/std/bit		 Adds constrained shl and shr functions with constexpr/noexcept behavior, signed-shift handling, out-of-range results, device PTX paths, and umbrella-header exports.
shl validation
libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shl.pass.cpp		 Tests positive and negative shifts, width boundaries, constant evaluation, noexcept/type properties, and signed, unsigned, and optional 128-bit integer combinations.
shr validation and device barriers
libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shr.pass.cpp, libcudacxx/test/support/test_macros.h		 Tests positive and negative shr behavior across integer types and architectures, while separating device and host DoNotOptimize barriers.

Possibly related PRs

  • NVIDIA/cccl#10037: Updates the same DoNotOptimize implementation with device/host-specific barriers.

Suggested labels: libcu++

Suggested reviewers: wmaxey, jacobfaib

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Actionable comments posted: 1

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📒 Files selected for processing (6)
  • libcudacxx/include/cuda/std/__bit/shl.h
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  • libcudacxx/include/cuda/std/bit
  • libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shl.pass.cpp
  • libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shr.pass.cpp
  • libcudacxx/test/support/test_macros.h

Comment thread libcudacxx/include/cuda/std/__bit/shl.h Outdated
@davebayer davebayer changed the title [libcu++] Implement P3793R2 Better Shifting [libcu++] Implement P3793R2 Better Shifting (without SIMD) Jul 17, 2026
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Actionable comments posted: 1

🧹 Nitpick comments (1)
libcudacxx/test/support/test_macros.h (1)

112-112: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

suggestion: Make the pointer object const. ptr is not reassigned; use a top-level const while retaining its volatile qualification for the assembly operand.

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  • libcudacxx/test/support/test_macros.h

Comment thread libcudacxx/test/support/test_macros.h
@coderabbitai coderabbitai Bot mentioned this pull request Jul 17, 2026
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fbusato
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Comment thread libcudacxx/include/cuda/std/__bit/shl.h
Comment thread libcudacxx/include/cuda/std/__bit/shl.h Outdated
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Comment thread libcudacxx/include/cuda/std/__bit/shr.h Outdated
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Comment thread libcudacxx/test/support/test_macros.h
Comment thread libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shl.pass.cpp
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Actionable comments posted: 1

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📥 Commits

Reviewing files that changed from the base of the PR and between bd57e62 and a446de5.

📒 Files selected for processing (6)
  • libcudacxx/include/cuda/std/__bit/shl.h
  • libcudacxx/include/cuda/std/__bit/shr.h
  • libcudacxx/include/cuda/std/bit
  • libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shl.pass.cpp
  • libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shr.pass.cpp
  • libcudacxx/test/support/test_macros.h
🚧 Files skipped from review as they are similar to previous changes (4)
  • libcudacxx/include/cuda/std/bit
  • libcudacxx/include/cuda/std/__bit/shr.h
  • libcudacxx/test/support/test_macros.h
  • libcudacxx/include/cuda/std/__bit/shl.h

Comment thread libcudacxx/test/libcudacxx/std/numerics/bit/bit.shift/shl.pass.cpp
@davebayer davebayer linked an issue Jul 21, 2026 that may be closed by this pull request
[FEA]: Non-SIMD part of P3793R2 #10040
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🥳 CI Workflow Results

🟩 Finished in 4h 36m: Pass: 100%/120 | Total: 4d 11h | Max: 3h 19m | Hits: 50%/2050801

See results here.

@davebayer
davebayer requested a review from fbusato July 21, 2026 17:41
@davebayer
davebayer merged commit f75bc70 into NVIDIA:main Jul 21, 2026
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This was referenced Jul 21, 2026
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@fbusato fbusato fbusato approved these changes
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[FEA]: Non-SIMD part of P3793R2

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