Make DX12 the Default API on Windows

[cwfitzgerald]

For a variety of reasons, we want to make DX12 the default API on windows:

• DXGI swapchains work a lot better on windows than vulkan WSI, particuarly AMD and Intel's. They are much more stable and more well behaved.
• Cleaner interop with native apis.
• Better debug layer and ease of bug reports as well as access to tools like PIX.

Before we make the switch, we need to get the DX12 backend tested and the major holdups fixed. This isn't a complete list and please add things as they are found.

Blocking Issues:

• Suballocate Buffers in DX12 #2720
• Creating a Window with DX12 and Immediate Mode Causes Error in ResizeBuffers #2712
• DXC Integration in DX12 Backend #2722
• Implement Push Constants for DX12 #2829
• Audit D3D12 Features and Limits wrt Resource Binding Tiers #3333
• DX12 Sampler Heap Requirements Overly Strict #3350
• [hlsl-out] Arrays in Structs Are Incorrectly Loaded When Loaded as A Whole Struct naga#2184
• [hlsl-out] Stores of vec2-based matrix inside struct tries to access raw matrix directly #4423
• dx12 panic with shared shader file using push constants #3624
• Crash on queue_submit #3617
• AMD on DX12 Renders Nothing on msaa-line Example #3838
• Resize error on DX12 backend in wgpu 0.17 #3967
• Investigate spirv_to_dxil Library #4155
• Enable DX12 GPU Based Validation #4291
• draw_indexed_indirect on DX12 doesn't handle non-zero base_instance #2471

There are some driver issues that we may not be able to fix, but we should keep in mind:

• WARP: Uniform Values Arriving As Zero #2296
• Queue::write_buffer() issue on Microsoft Basic Render Driver #2189

[ErichDonGubler]

We're interested in this in the Firefox org. Here's our own Bugzilla entry tracking this: bug 1836816

[ErichDonGubler]

@nical has made Firefox's usage of wgpu-core default to DX12 as the only backend on Windows: bug 1853140

This should indirectly help this issue, since fixing the most salient DX12 issues is our priority, for the time being.

[Elabajaba]

Fwiw I think making dx12 the default backend for firefox on windows at the moment is the wrong choice, since you don't enable the feature to allow suballocating dx12 resources (because of gpu-allocator's windows-rs dependency) which means it'll be significantly slower than the vulkan backend.

[nical]

It's a trade-off. With vulkan on windows we have to read every frame back and re-upload them on the dx11 device which renders the rest of the browser which affects performance to an even greater extent. Right now we are more focused on getting things to work reliably. We'll have to untangle the suballocation/windows-rs stuff eventually but switching to vulkan on windows does not bring us closer to where we want to be in the long run.

[jimblandy]

See also: Traverse-Research/gpu-allocator#181

They'd be cool with switching to windows-bindgen.