Interesting. Their previous roadmap was
to refactor the open-source DirectX Compiler as a HLSL frontend and DXIL backend
in the main branch of Clang/LLVM, instead of making yet another proprietary fork.
Though SPIR-V backend has been available for a while, the language of
the announcement blog post suggests SPIR-V for DirectX 12 would
replace DXIL bytecode with the release of shader model 7.0:
The Road to Replacing DXIL
As we look to the future, maintaining a proprietary IR format (even one based on an open-source project) is counter to our commitments to open technologies, so Shader Model 7.0 will adopt SPIR-V as its interchange format. Over the next few years, we will be working to define a SPIR-V environment for Direct3D, and a set of SPIR-V extensions to support all of Direct3D’s current and future shader programming features through SPIR-V.
Appendix: A Brief History of GPU Interchange Formats
LLVM’s bitcode format had some significant drawbacks. Notably it is not version stable. New versions of LLVM support a lossy upgrading of older LLVM IR modules, but new LLVM cannot write IR modules that can be read by older versions of LLVM. Additionally, LLVM bitcode is a bit-packed file format which has two big drawbacks (1) it compresses poorly, and (2) it is hard to read or write from tools that aren’t LLVM.
To solve these problems The Khronos Group developed SPIR-V as a successor to SPIR. SPIR-V is ideologically aligned with LLVM’s IR, but it supports a stable and simple binary serialization. This makes SPIR-V easy to read and write by simpler tools than LLVM IR.
It looks like formally specifying DXIL as a proprietary fork of LLVM 3.7 bitcode proved impractical - so the open-source DXIL backend will only be maintained for backward compatibility with shader model 6.0-6.9, much like DXBC bytecode which was limited to shader model 5.1.
This may also explain why
Wave MMA has been removed from DXIL bytecode. Though new DXIL frontend will still support shader model 6.8 and Work Graphs, shader model 6.9 will probably focus on minor updates like
C++ language features in HLSL 202x/202y, and the development of major new features like Wave MMA and
Graphics nodes will shift to shader model 7.0 and SPIR-V bytecode.
This transition to SPIR-V will surely take several years - or even more - to finalize. Microsoft hasn't even specified
shader model 6.9 yet - which was supposed to be developed during the Dilithium semester (25H1), but the latter didn't achieve much and has already been replaced by
Selenium (25H2).