Mesh shaders MSL writer by inner-daemons · Pull Request #8739 · gfx-rs/wgpu

inner-daemons · 2025-12-16T08:42:26Z

Connections
Works towards #7197
Depends on #9099

Description
Adds support for mesh shaders to MSL writer.

Also adds a limit.

Testing
Snapshots & examples & other tests

Squash or Rebase?
Squash

Checklist

Run cargo fmt.
Run taplo format.
Run cargo clippy --tests. If applicable, add:
- --target wasm32-unknown-unknown
Run cargo xtask test to run tests.
If this contains user-facing changes, add a CHANGELOG.md entry.

Copilot

Pull request overview

This PR adds Metal Shading Language (MSL) writer support for mesh shaders, enabling WGSL mesh shaders to be automatically transpiled to MSL. This replaces the previous approach of using handwritten Metal shader files.

Changes:

Implements MSL code generation for task and mesh shader entry points with proper Metal 3.0 syntax
Refactors mesh shader detection into a shared uses_mesh_shaders() function used across multiple backends
Enables Metal as a target backend for mesh shader tests and examples

Reviewed changes

Copilot reviewed 14 out of 19 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/tests/wgpu-gpu/mesh_shader/shader.metal	Deleted handwritten Metal shader, replaced with auto-generated code
tests/tests/wgpu-gpu/mesh_shader/mod.rs	Removed compile_msl function, now uses WGSL compilation for Metal backend
naga/tests/out/msl/wgsl-policy-mix.msl	Fixed ordering of private variable initialization before workgroup barrier
naga/tests/out/msl/wgsl-mesh-shader*.msl	New generated MSL output files for mesh shader tests
naga/tests/in/wgsl/mesh-shader*.toml	Added Metal as target and MSL version 3.0 requirement
naga/src/proc/mod.rs	Added uses_mesh_shaders() utility function for detecting mesh shader usage
naga/src/back/wgsl/writer.rs	Refactored to use new uses_mesh_shaders() function
naga/src/back/spv/writer.rs	Refactored mesh shader checking, moved logic to use shared function
naga/src/back/spv/mesh_shader.rs	Removed require_mesh_shaders() helper, consolidated into writer
naga/src/back/msl/writer.rs	Main implementation of mesh shader MSL code generation
naga/src/back/msl/mod.rs	Added Payload binding type, MeshOutput location mode, and UnsupportedMeshShader error
examples/features/src/mesh_shader/shader.metal	Deleted handwritten Metal shader
examples/features/src/mesh_shader/mod.rs	Removed compile_msl function, now uses WGSL for Metal

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naga/src/back/msl/writer.rs

cwfitzgerald

Soooooooome comments

naga/src/back/msl/mod.rs

naga/src/back/msl/writer.rs

naga/src/back/spv/writer.rs

naga/src/back/msl/writer.rs

naga/tests/out/msl/wgsl-mesh-shader-empty.metal

naga/src/back/msl/writer.rs

inner-daemons · 2026-01-15T03:15:04Z

@cwfitzgerald Thanks for the review, will get to it today. I didn't make it fully clear, but when you reviewed this PR many parts were derived from #8756 which is designed to land before this and the HLSL PR. I've moved all changes from all 3 PRs that could could conflict into that PR and then back here. So thats why there are some changes related to spv

inner-daemons · 2026-02-22T22:12:54Z

Alright, I've cleaned this up a tiny bit, since I had left some changes from the SPV fixes PR.

Wumpf

frankly, changes to msl/writer.rs are just a nightmare to review. The file is so insanely large, even with an agent I can hardly make sense of it. As so often it seems this is no individual's fault, but we have to make sure it gets better, so anything we can do to improve readability & maintainability we have to do. I believe a lof of the steps could be factored more cleanly

Importantly, the outputs of the test shaders look good I believe, so I think after a little bit of polish we can move forward with this.

wgpu-types/src/limits.rs

naga/tests/out/msl/wgsl-mesh-shader.metal

naga/src/back/msl/writer.rs

naga/src/back/msl/mesh_shader.rs

inner-daemons · 2026-03-09T20:34:27Z

@Wumpf Mostly addressed

ChristopherBiscardi

While unfamiliar with a few of the intricacies of wgpu here, I have reviewed the code and not found anything obviously wrong.

Probably more interestingly, I ported Bevy to use this PR to run a couple of examples I've been working on using the current release's wgsl/vulkan support. The examples run as expected with no wgsl changes, using the wgsl I wrote targeting vulkan platforms, with this PR on an m1 max.

One example here is a visualization of a single task shader dispatching vec3u(u32(globals.time)) mesh shaders that draw a single cube at their grid position.

screenshot-2026-03-11-at-20.51.57-converted.mp4

The other is an in-progress version of this grass, which doesn't currently use task shaders.

screenshot-2026-03-11-at-20.54.34-converted.mp4

So this PR works as expected as far as my demo code/bevy is concerned.

…ding/msl-write

…rite

Wumpf

the extra barriers outside of mesh shaders worry me a bit, otherwise I believe this is good now

naga/tests/out/msl/wgsl-atomicOps-float32.metal

naga/src/back/msl/mesh_shader.rs

caelunshun · 2026-03-28T23:51:10Z

I've noticed that if my mesh shader outputs vertex data with the @builtin(clip_distance) attribute, I get a shader translation error:

Error log


Shader translation error for stage ShaderStages(MESH): Metal: program_source:195:51: warning: unused variable 'plane' [-Wunused-variable]
            package_shared_clipping_ClippingPlane plane = window.planes.inner[metal::min(unsigned(_e10), 4u)];
                                                  ^
program_source:212:32: error: 'clip_distance' attribute cannot be applied to types
    float clip_distances [5] [[clip_distance]];
                               ^
program_source:307:95: error: invalid type 'ms_mainVertexOutput' for mesh vertex type
  metal::mesh<ms_mainVertexOutput, ms_mainPrimitiveOutput, 64, 32, metal::topology::triangle> meshOutput
                                                                                              ^
program_source:307:95: note: in instantiation of template class 'metal::mesh<ms_mainVertexOutput, ms_mainPrimitiveOutput, 64, 32, metal::topology::triangle>' requested here
program_source:212:5: note: field of illegal type 'float[5]' declared here
    float clip_distances [5] [[clip_distance]];

Not sure if this is an inherent limitation or a bug in the shader translator. Let me know if making a minimal reproduction shader would help.

(This seems to work fine on the Vulkan backend.)

inner-daemons · 2026-03-29T06:18:27Z

@caelunshun Definitely a bug! I'd rather not block this PR any longer so if you file an issue I'll fix that and add testing in another PR. Appreciate the report!

…rite

inner-daemons · 2026-03-30T16:26:31Z

@Wumpf Can we please merge this soon? Every time theres a new review new stuff has been added and the diffs are gross. This PR is big enough that updating it every time is a huge hassle, and also comes with new issues. Like the clip distances thing, which was added a week or two ago and I guess conflicts, because this PR completely changes how input arguments are written. Then there's always something broken on DX12 which I have to wait for the CI with 4 thigns running at once to catch. Fix it. Wait another hour.

Can we keep any smaller issues to followup PRs. I have zero interest in continuing to maintain this much longer.

Moldy

# Conflicts: # CHANGELOG.md # naga/src/back/msl/mod.rs # naga/src/back/msl/writer.rs # wgpu-core/src/command/bundle.rs # wgpu-core/src/command/render.rs

inner-daemons · 2026-04-07T17:17:54Z

@Wumpf or @cwfitzgerald, if this is good to go, should we turn on automerge?

This was referenced Dec 16, 2025

Mesh Shaders Tracking Issue #7197

Open

Naga spir-v and msl output has divergent return statements in entry points #8738

Closed

inner-daemons marked this pull request as ready for review December 16, 2025 22:05

inner-daemons requested a review from cwfitzgerald December 16, 2025 22:07

inner-daemons assigned cwfitzgerald Dec 16, 2025

inner-daemons mentioned this pull request Dec 18, 2025

Mesh shader HLSL writer #8752

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cwfitzgerald requested a review from Copilot January 15, 2026 00:20

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inner-daemons requested a review from cwfitzgerald January 16, 2026 00:22

Wumpf self-assigned this Feb 22, 2026

inner-daemons mentioned this pull request Feb 23, 2026

Fix local_invocation_id and local_invocation_index handling #9099

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inner-daemons requested a review from Wumpf March 7, 2026 00:20

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inner-daemons requested a review from Wumpf March 9, 2026 20:34

Single squashed commit

a1fe019

inner-daemons force-pushed the mesh-shading/msl-write branch from 698ba38 to a1fe019 Compare March 10, 2026 21:35

cwfitzgerald removed their assignment Mar 11, 2026

ChristopherBiscardi approved these changes Mar 12, 2026

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inner-daemons and others added 6 commits March 16, 2026 11:55

Merge commit 'e682ce3f29afb3ff8fdc6f798ad9f6bfe097a252' into mesh-sha…

40e4b6d

…ding/msl-write

Merge commit '96b7872f7728526ced73fb2faa1f74cfc77b08d2' into mesh-sha…

066af69

…ding/msl-write

Merge remote-tracking branch 'upstream/trunk' into mesh-shading/msl-w…

153e381

…rite

Merge branch 'trunk' into mesh-shading/msl-write

621ff60

Updated changelog

55ddc38

Fixed stupid changelog thing

53b680c

Wumpf requested changes Mar 28, 2026

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naga/tests/out/msl/wgsl-atomicOps-float32.metal Show resolved Hide resolved

naga/src/back/msl/mesh_shader.rs Show resolved Hide resolved

inner-daemons added 3 commits March 30, 2026 11:16

Merge remote-tracking branch 'upstream/trunk' into mesh-shading/msl-w…

83b7b05

…rite

If I have to do this one more fucking time I'm gonna lose it

028529b

Removed some pub(super)'s

0226f76

Thing

bb25aa9

inner-daemons requested a review from Wumpf March 30, 2026 16:29

inner-daemons mentioned this pull request Mar 30, 2026

Mesh shaders and clip distances #9339

Open

Merge branch 'trunk' into mesh-shading/msl-write

c63f089

Wumpf approved these changes Apr 7, 2026

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Merge remote-tracking branch 'origin/trunk' into mesh-shading/msl-write

1452395

# Conflicts: # CHANGELOG.md # naga/src/back/msl/mod.rs # naga/src/back/msl/writer.rs # wgpu-core/src/command/bundle.rs # wgpu-core/src/command/render.rs

cwfitzgerald enabled auto-merge (squash) April 7, 2026 17:23

cwfitzgerald merged commit b5bd5ae into gfx-rs:trunk Apr 7, 2026
94 of 95 checks passed

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