1885217 - (webgpu-fxc) WebGPU: shader compilation with FXC is slow, broken, and abandoned

Status: NEW

(Core :: Graphics: WebGPU, defect, P3)

Description

[Erich Gubler [:ErichDonGubler]]

Firefox currently uses Microsoft's FXC by default for its DX12 back end. FXC is a legacy component that has multiple significant issues:

• First and foremost, FXC is slow, easily taking over 10 seconds to compile relatively simple shaders.
• FXC is incorrect; it has some shader compilation bugs that have no roadmap for resolution.

We need to overcome both of these issues in order to have an objectively good experience with shader compilation in our implementation of WebGPU.

Comment 1

[Erich Gubler [:ErichDonGubler]]

Possible solutions we know of so far:

• Microsoft's DirectX Shader Compiler (AKA "DXC") is a successor to FXC that fixes these issues. We could consume it, and thereby resolve the above problems.
• Another approach might be to write a compilation backend for Firefox's shader transpiler (Naga) for DXIL (DXC's new intermediate representation), so we can emit it directly (viz., wgpu#4155).

:jgilbert has noted that our most likely path to success is to work with Microsoft directly, and ensure that DXC is suitable to ship for us if we run into obstacles.

Comment 2

[Erich Gubler [:ErichDonGubler]]

Currently planning on resolving this via bug 1903302.