Static linking for DXC via mach-dxcompiler

[DouglasDwyer]

Connections

• [Experiment]: bundled with dxc (as a feature?)
• Make DX12 the Default API on Windows

Description
This PR adds support for statically linking the DXC binaries from mach-dxcompiler. The mach-dxcompiler repo replaces Microsoft's DXC build system with Zig, which allows for building DXC as a static .lib file. I have created an upstream crate called mach-dxcompiler-rs that handles downloading a prebuilt mach-dxcompiler binary and linking it into a Rust project (ideally, it would be built from source, but that was too complicated for my purposes - it can be added later if desired).

This PR adds a dependency on mach-dxcompiler-rs, gated behind the mach-dxcompiler-rs feature flag. When enabled, it allows for specifying dx12_shader_compiler: Dx12Compiler::MachDxc in the InstanceDescriptor. This allows for using DXC in WGPU without having to ship any external .dlls alongside the main binary.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
  • (n/a) --target wasm32-unknown-unknown
  • (n/a) --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[DouglasDwyer]

Update on the current state of this PR:

The branch builds and I am able to successfully use static DXC in my own WGPU project on windows-msvc. However, the Windows WGPU unit tests don't compile due to a linking error: the mach-dxcompiler lib is built with MT_StaticRelease whereas another native library referenced by WGPU (nv-flip-sys) is built with MD_DynamicRelease. In general, I believe that this would be an error for any consumers of WGPU + Mach DXC that also dynamically link MSVCRT from other native libs.

From what I gather, the correct solution here is to offer both an MD_DynamicRelease and MT_StaticRelease binary for the the mach-dxcompiler Windows target. I played with the Zig build system for about four hours yesterday, and was unable to figure out how to emit a .lib file that dynamically links MSVCRT. It looks to me like the choice between static/dynamic CRT is hardcoded in the Zig build system, and is tied directly to whether you are building a .lib or .dll.

I will be asking around on the Zig forums (and maybe opening an issue on Zig's GitHub) about this. In the meantime, I will leave this PR open. If anyone has additional guidance or suggestions for how we can push this PR forward, I would be grateful. Otherwise, this PR is stagnant until we find a way to build static libs with MD_DynamicRelease using Zig.

[DouglasDwyer]

Well, after a good deal of more foraging, I managed to find the compiler flag that I was looking for: Zig accepts -fms-runtime-lib=dll to use the dynamic MSVCRT, just like Clang. With this, I was able to fix the linking issues and update mach-dxcompiler-rs to support either static/dynamic MSVCRT. All tests now build and I am able to build/run WGPU + Mach DX12 in my own project. This PR should be ready for review.

[Elabajaba]

I'm just curious, do you know if mach-dxcompiler is keeping up to date with DXC? From what I remember from the original blog post they didn't plan to try to track upstream at the time, but there's been some big features added in shader model 6.8 (workgraphs being the obvious headliner, but SV_StartVertex/InstanceLocation is also really nice especially for simplifying GPU driven rendering pipelines.

[DouglasDwyer]

@Elabajaba, that's a great question. The DXC branch that's actually being built is here - it hasn't been updated in about a year, so it doesn't support SM 6.8 at this time. In theory it should be possible to update it to latest, but I took a look and there are merge conflicts with DXC master. I don't know how easy it would be.