glslang
glslang is Khronos's GLSL and ESSL reference frontend and SPIR-V generator with partial HLSL support.
Why use it
Engines use it for standards checks, Vulkan shader generation, and offline compiler tooling.
Where it fits
Find engines, frameworks, code libraries, version control, and production automation.
What to check
HLSL coverage is not identical to Microsoft's compiler, and version upgrades can change validation rules.