[libcu++][cuda::ptx] Add prefetch PTX wrappers

[gonidelis]

Second try to fix #9616

I took Fede's advice and tried to use the internal libcucxx_ptx tool and then added a test. Given that both Bernhard and Fede are out I'll ask @ahendriksen to pinpoint any wrongdoings he might notice.

[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Added CUDA PTX prefetch instruction support to the public cuda::ptx header set.
  • New prefetch-related declarations are now available through the main PTX include.

• Tests

  • Added a compile-time test covering the new prefetch PTX wrapper inclusion.

Walkthrough

Adds cuda/__ptx/instructions/prefetch.h, a new PTX instruction wrapper header with include guard, compiler system-header pragmas, and namespace wrappers delegating to a generated prefetch.h. Wires it into the cuda::ptx public header and adds a minimal compilation-pass test.

prefetch PTX wrapper

Layer / File(s)	Summary
prefetch header and public namespace wiring libcudacxx/include/cuda/__ptx/instructions/prefetch.h, libcudacxx/include/cuda/ptx	New prefetch.h with include guard, compiler system-header pragmas, and namespace prologue/epilogue including the generated definitions; added to cuda/ptx at line 97.
Compilation-pass test libcudacxx/test/libcudacxx/cuda/ptx/ptx.prefetch.compile.pass.cpp	New test including <cuda/ptx> and the generated prefetch header with a threads-unsupported annotation and a main returning 0.

Assessment against linked issues

Objective	Addressed	Explanation
Introduce cuda::ptx::prefetch() assembly wrapper [#9616]	❓	This PR adds the prefetch.h wrapper header and wiring, but the generated cuda/__ptx/instructions/generated/prefetch.h (containing the actual prefetch() declarations) is not included in this diff, so it is unclear whether the assembly wrapper itself is present and correct.

Out-of-scope changes

No out-of-scope changes found.

---

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

🧹 Nitpick comments (1)

libcudacxx/include/cuda/__ptx/instructions/prefetch.h (1)

12-13: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

suggestion: Rename the include guard to the path-derived form used in libcudacxx headers. _CUDA_PTX_PREFETCH_H_ is too generic for cuda/__ptx/instructions/prefetch.h and risks collisions with future headers. As per coding guidelines, "Headers must use include guards derived from the uppercase full path."

Also applies to: 40-40

Source: Coding guidelines

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 3f919381-8399-4ebf-9476-f350f32fafdc

📥 Commits

Reviewing files that changed from the base of the PR and between 7e585c9 and 01ce259.

⛔ Files ignored due to path filters (2)

• libcudacxx/include/cuda/__ptx/instructions/generated/prefetch.h is excluded by !**/generated/**
• libcudacxx/test/libcudacxx/cuda/ptx/generated/prefetch.h is excluded by !**/generated/**

📒 Files selected for processing (3)

• libcudacxx/include/cuda/__ptx/instructions/prefetch.h
• libcudacxx/include/cuda/ptx
• libcudacxx/test/libcudacxx/cuda/ptx/ptx.prefetch.compile.pass.cpp

[github-actions]

😬 CI Workflow Results

🟥 Finished in 1h 11m: Pass: 99%/120 | Total: 2d 04h | Max: 1h 03m | Hits: 78%/484549

See results here.