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# `nvptx64-nvidia-cuda`

Tier: 2

This is the target meant for deploying code for Nvidia® accelerators based on their CUDA platform.

## Target maintainers

@RDambrosio016 @kjetilkjeka

## Requirements

This target is `no_std` and will typically be built with crate-type `cdylib` and `-C linker-flavor=llbc`, which generates PTX. The necessary components for this workflow are:

- `rustup toolchain add nightly`
- `rustup component add llvm-tools --toolchain nightly`
- `rustup component add llvm-bitcode-linker --toolchain nightly`

There are two options for using the core library:

- `rustup component add rust-src --toolchain nightly` and build using `-Z build-std=core`.
- `rustup target add nvptx64-nvidia-cuda --toolchain nightly`

### Target and features

It is generally necessary to specify the target, such as `-C target-cpu=sm_89`, because the default is very old. This implies two target features: `sm_89` and `ptx78` (and all preceding features within `sm_*` and `ptx*`). Rust will default to using the oldest PTX version that supports the target processor (see this table), which maximizes driver compatibility. One can use `-C target-feature=+ptx80` to choose a later PTX version without changing the target (the default in this case, `ptx78`, requires CUDA driver version 11.8, while `ptx80` would require driver version 12.0). Later PTX versions may allow more efficient code generation.

Although Rust follows LLVM in representing `ptx*` and `sm_*` as target features, they should be thought of as having crate granularity, set via (either via `-Ctarget-cpu` and optionally `-Ctarget-feature`). While the compiler accepts `#[target_feature(enable = "ptx80", enable = "sm_89")]`, it is not supported, may not behave as intended, and may become erroneous in the future.

## Building Rust kernels

A `no_std` crate containing one or more functions with `extern "ptx-kernel"` can be compiled to PTX using a command like the following.

    $ RUSTFLAGS='-Ctarget-cpu=sm_89' cargo +nightly rustc --target=nvptx64-nvidia-cuda -Zbuild-std=core --crate-type=cdylib -- -Clinker-flavor=llbc -Zunstable-options

Intrinsics in `core::arch::nvptx` may use `#[cfg(target_feature = "...")]`, thus it's necessary to use `-Zbuild-std=core` with appropriate `RUSTFLAGS`. The following components are needed for this workflow:

    $ rustup component add rust-src --toolchain nightly
    $ rustup component add llvm-tools --toolchain nightly
    $ rustup component add llvm-bitcode-linker --toolchain nightly