Add checks for gpu-kernel calling conv #149991

Flakebi · 2025-12-14T14:30:37Z

The gpu-kernel calling convention has several restrictions that were not enforced by the compiler until now.
Add the following restrictions:

Cannot be async
Cannot be called
Cannot return values, return type must be () or !
Arguments should be simple, i.e. passed by value. More complicated types can work when you know what you are doing, but it is rather unintuitive, one needs to know ABI/compiler internals.
Export name should be unmangled, either through no_mangle or export_name. Kernels are searched by name on the CPU side, having a mangled name makes it hard to find and probably almost always unintentional.

Tracking issue: #135467
amdgpu target tracking issue: #135024

@workingjubilee, these should be all the restrictions we talked about a year ago.

cc @RDambrosio016 @kjetilkjeka for nvptx

rustbot · 2025-12-14T14:30:41Z

r? @WaffleLapkin

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rustbot · 2025-12-14T15:09:40Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

The `gpu-kernel` calling convention has several restrictions that were not enforced by the compiler until now. Add the following restrictions: 1. Cannot be async 2. Cannot be called 3. Cannot return values, return type must be `()` or `!` 4. Arguments should be simple, i.e. passed by value. More complicated types can work when you know what you are doing, but it is rather unintuitive, one needs to know ABI/compiler internals. 5. Export name should be unmangled, either through `no_mangle` or `export_name`. Kernels are searched by name on the CPU side, having a mangled name makes it hard to find and probably almost always unintentional.

WaffleLapkin · 2025-12-15T13:20:57Z

r? workingjubilee

As I'm completely missing context

rustbot · 2025-12-15T13:21:02Z

workingjubilee is currently at their maximum review capacity.
They may take a while to respond.

workingjubilee

The AST-level code looks good.

Some details on messaging here. I'm not committed to a precise message on these, which is why it's a bit "multiple choice" here, just wondering if these could be improved. In one or two cases it is a must-change.

Should we be enforcing a maximum number of arguments, also? Probably not if there's no cross-driver consensus on that, but maybe?

View changes since this review

workingjubilee · 2025-12-16T15:55:13Z

compiler/rustc_hir_typeck/messages.ftl

    functional record update syntax requires a struct

+hir_typeck_gpu_kernel_abi_cannot_be_called =
+    functions with the "gpu-kernel" ABI cannot be called


should we be more specific?

Suggested change

functions with the "gpu-kernel" ABI cannot be called

functions with the "gpu-kernel" ABI cannot be called from Rust

Suggested change

functions with the "gpu-kernel" ABI cannot be called

functions with the "gpu-kernel" ABI cannot be called, even in device code

workingjubilee · 2025-12-16T15:58:38Z

compiler/rustc_hir_typeck/messages.ftl


+hir_typeck_gpu_kernel_abi_cannot_be_called =
+    functions with the "gpu-kernel" ABI cannot be called
+    .note = an `extern "gpu-kernel"` function can only be launched on the GPU through an API


I'm not sure "can" is right? Maybe "should"? And "an API" is somewhat vague, what kind of API? Surely not a standard library one, for instance...

Suggested change

.note = an `extern "gpu-kernel"` function can only be launched on the GPU through an API

.note = an `extern "gpu-kernel"` function should only be launched on the GPU through a driver's API

Or "must"?

...I don't know if "kernel loader" is a real term for the thing that the GPU driver "does", it just came up intuitively while trying to grasp for words for the thing.

Suggested change

.note = an `extern "gpu-kernel"` function can only be launched on the GPU through an API

.note = an `extern "gpu-kernel"` function must only be launched on the GPU by the kernel loader

workingjubilee · 2025-12-16T16:00:12Z

compiler/rustc_lint/src/gpukernel_abi.rs

+declare_lint! {
+    /// The `missing_gpu_kernel_export_name` lint detects `gpu-kernel` functions that have a mangled name.


And we can't simply use an unmangled name by default because that would be unsafe, right.

workingjubilee · 2025-12-16T16:04:53Z

compiler/rustc_lint/src/gpukernel_abi.rs

+
+/// `ImproperGpuKernelLint` checks `gpu-kernel` function definitions:
+///
+/// - `extern "gpu-kernel" fn` arguments should be simple.


We should avoid "simple", even in a concise format like this: we mean "primitive types".

Suggested change

/// - `extern "gpu-kernel" fn` arguments should be simple.

/// - `extern "gpu-kernel" fn` arguments should be primitive types.

workingjubilee · 2025-12-16T16:09:21Z

compiler/rustc_lint/src/gpukernel_abi.rs

+    /// This lint is issued when it detects a probable mistake in a signature.
+    IMPROPER_GPU_KERNEL_ARG,
+    Warn,
+    "simple arguments of gpu-kernel functions"


This line should capture the reason for the lint, not what it is checking for, so something like

Suggested change

"simple arguments of gpu-kernel functions"

"GPU kernel entry points have a limited calling convention"

rustbot · 2025-12-16T16:14:20Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

rustbot assigned WaffleLapkin Dec 14, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Dec 14, 2025

This was referenced Dec 14, 2025

Tracking Issue for the gpu-kernel ABI #135467

Open

Tracking Issue for amdgcn target #135024

Open

Add inline asm support for amdgpu #149793

Open

Flakebi force-pushed the gpu-kernel-cc branch from b6cb0c2 to 0a9373c Compare December 14, 2025 15:09

Flakebi force-pushed the gpu-kernel-cc branch from 0a9373c to 1e9b1dc Compare December 14, 2025 15:10

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Flakebi force-pushed the gpu-kernel-cc branch from 10b32d6 to 88ad16c Compare December 15, 2025 13:17

rustbot assigned workingjubilee and unassigned WaffleLapkin Dec 15, 2025

workingjubilee requested changes Dec 16, 2025

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 16, 2025

	functions with the "gpu-kernel" ABI cannot be called
	functions with the "gpu-kernel" ABI cannot be called from Rust

	.note = an `extern "gpu-kernel"` function can only be launched on the GPU through an API
	.note = an `extern "gpu-kernel"` function should only be launched on the GPU through a driver's API

		declare_lint! {
		/// The `missing_gpu_kernel_export_name` lint detects `gpu-kernel` functions that have a mangled name.

	/// - `extern "gpu-kernel" fn` arguments should be simple.
	/// - `extern "gpu-kernel" fn` arguments should be primitive types.

	"simple arguments of gpu-kernel functions"
	"GPU kernel entry points have a limited calling convention"

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