Use `llvm-bitcode-linker` as the default linker for `nvptx64-nvidia-cuda`

[kjetilkjeka]

Proposal

The current default linker for the nvptx64-nvidia-cuda target is rust-ptx-linker. However, the upstream rust-ptx-linker project has been broken since 2022 and is effectively unmaintained. In practice, this makes it difficult or impossible to use for most builds.

Since the introduction of the llvm-bitcode-linker it has become the recommended linker for nvptx64-nvidia-cuda in the official Rust documentation. The llvm-bitcode-linker provides a stable and maintainable path forward, and is in real world use for targeting Nvidia GPUs.

Given that:

• rust-ptx-linker is no longer functional or maintained,
• llvm-bitcode-linker is the officially recommended solution and actively maintained,
• and there are no known current users of rust-ptx-linker,

I propose to switch the default linker for the nvptx64-nvidia-cuda target to llvm-bitcode-linker.

This change will:

• Eliminate the need for explicitly passing -C linker-flavor=llbc.
• Simplify the build setup for CUDA targets.
• Lay the groundwork for potentially removing the dedicated “ptx” linker flavor in the future.

Mentors or Reviewers

The actual engineering work is trivial. I will "mentor" and create the MR if accepted

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
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• Once an MCP is seconded, the Final Comment Period begins.
  • Final Comment Period lasts for 10 days after all outstanding concerns are solved.
  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[rustbot]

Important

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cc @rust-lang/compiler

[Noratrieb]

@RDambrosio016 as our other maintainer

[davidtwco]

@rustbot second