[llvm] [DXIL] Add DXIL SubArch to correspond to version number (PR #89125)

[Damyan Pepper via llvm-commits]

damyanp wrote:

> > > > What's the plan for when a subarch isn't specified (ie, `dxil-pc-shadermodel-6.0`)?
> > > > I propose defaulting to version 1.0 when SubArch is not explicitly specified.
> > 
> > 
> > So if I specify `dxil-pc-shadermodel-6.8` then the SubArch will be `DXILSubArch_v10`? Will that work out ok everywhere?
> 
> On second thoughts, it appears that shader model version is embedded in the `-T <target profile>` option specified on the HLSL compiler command line (e.g., `ps_6_0`, `lib_6_8`). Since Shader Model version is being decoupled from DXIL version, the target DXIL version is also required to be explicitly specified in the target triple. This implies `dxil-pc-shadermodel-6.8` would be flagged as an error.

Something somewhere will need to validate that the dxil version is compatible with the SM version, right?

I think we're at a point now where we're speccing the feature in a PR for the implementation.  Sounds like we need some design discussion before continuing with this.

https://github.com/llvm/llvm-project/pull/89125