[Misc] Warn when CUDA PTX arch flags are ignored

[mehrdadxzaker]

Purpose

Addresses one item from #9129: warn when PTX code generation is requested via CUDA architecture flags but vLLM normalizes those global flags into per-source CUDA architecture settings.

Users can request PTX through TORCH_CUDA_ARCH_LIST values such as 8.0+PTX. vLLM strips the Torch-provided global -gencode flags and rebuilds per-source gencode flags, so the global PTX request is not preserved. This PR surfaces that behavior during CMake configuration instead of silently dropping the request.

Changes

• Add warn_if_ptx_arch_requested() in cmake/utils.cmake.
• Call it after clear_cuda_arches() captures and removes global CUDA arch flags.
• Detect both direct code=compute_XX and bracketed forms such as code=[sm_90a,compute_90a].

Testing

• git diff --check HEAD~1..HEAD
• Local regex smoke test for SASS-only and PTX-containing gencode flags
• cmake -P script validation with CMake 4.3.2:
  • no warning for -gencode arch=compute_80,code=sm_80
  • warning for -gencode arch=compute_80,code=compute_80
  • warning for -gencode arch=compute_90a,code=[sm_90a,compute_90a]

AI assistance disclosure

This PR was prepared with AI assistance. I reviewed the changed lines and validation results before opening the PR.

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[gemini-code-assist]

Code Review

This pull request introduces a warning mechanism in the CMake build system to alert users when PTX code generation is requested via global CUDA architecture flags. Since vLLM normalizes per-source CUDA architectures and does not preserve these global PTX requests, the new warn_if_ptx_arch_requested function in cmake/utils.cmake identifies such flags and provides guidance on how to resolve the issue. I have no feedback to provide.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[Harry-Chen]

Good catch. My further suggestions include:

• Rename the confusing clear_cuda_arches, since it only removes PTX-related options.
• Remove all +PTX options from our global build options, including the release pipeline, default build config, and wherever possible.
• Document this behavior somewhere.

CC @LucasWilkinson if you have any opinions

[github-actions]

This pull request has been automatically marked as stale because it has not had any activity within 90 days. It will be automatically closed if no further activity occurs within 30 days. Leave a comment if you feel this pull request should remain open. Thank you!

[github-actions]

This pull request has been automatically closed due to inactivity. Please feel free to reopen if you intend to continue working on it. Thank you!