Also Microsoft DirectX Shader Compiler[1] for HLSL. In fact, effort is ongoing to upstream HLSL support into Clang.[2][3][4]

To answer your other question—why LLVM instead of GCC:

- First-class support for non-Unix/Linux platforms. LLVM can be built on Windows and Visual Studio without ever needing a single GNU tool besides git[5]. Clang even has a MSVC-compatible interface that allows MSVC developers to switch to Clang without needing to change their command-line invocation[6].

- Written in C++ from the ground up, with a modular, first-class SSA IR-based interface.

- Permissive Apache 2.0 licence. As much as this might exasperate the open-source community, it allows for significantly faster iteration; things tend to be upstreamed when private/corporate developers realise it is hard to maintain separate forks.

All this allows LLVM to have a pretty mature infrastructure; some very large companies have contributed to its development.

[1]: https://github.com/microsoft/DirectXShaderCompiler

[2]: https://clang.llvm.org/docs/HLSL/HLSLDocs.html

[3]: https://github.com/microsoft/DirectXShaderCompiler/wiki/Cont...

[4]: https://discourse.llvm.org/t/rfc-adding-hlsl-and-directx-sup...

[5]: https://llvm.org/docs/GettingStartedVS.html

[6]: https://clang.llvm.org/docs/UsersManual.html#clang-cl