[RFC] Let MVT generated and restore MVT into llvm/CodeGen

Code Generation Common CodeGen Infrastructure
3

My all diffs have been pushed.
Thanks @arsenm and @dblaikie for the reviews.

These are a few leftover issues as far as I know.
Feel free to open new topics for them.

Layout of CodeGenTypes

I have introduced LLVMCodeGenTypes as a sub component of LLVMCodeGen in ⚙ D148769 Split out `CodeGenTypes` from `CodeGen` for LLT/MVT. I wanted to isolate MVT.h from Support since I wanted to make MVT.h as a generated header.

I have got feedbacks from @dblaikie and @thakis.

My random thoughts for CodeGenTypes;

  • Don’t remain a part of Support. MVT.h is a generated header. I thought this might be a part of Core. It was pragmatic but less reasonable.
  • Should be available in TableGen. If this is a part of CodeGen (before D148769), llvm-tblgen would depend on too many components (includes Transforms) even if most of them were pruned by --gc-sections.
  • Both LLT.h and MVT.h are used in CodeGen and its dependents. I think CodeGen may be the right place theoretically (but inefficiently and not practically). It would not make sense if CodeGenTypes were a part of Core (or remained as a part of Support)

OTOH, I understand it is nasty for CodeGenTypes to be a sub component of CodeGen in the same directory with PARTIAL_SOURCES_INTENDED). I was too afraid if I made things more intrusive.

Could we move CodeGenTypes to elsewhere? So, could we move LLT.h and MVT.h as well? Could you suggest a better name than CodeGenTypes?

aarch64svcount

In ValueTypes.td, this is defined as a simple ValueType with bw=16 and no other attributes.
In MVT::getSizeInBits(), this should be treated as TypeSize::Scalable(16).

I committed a quick fix rGd091a19e58a2, or we saw TypeSize::Fixed(16)

LegalizeDAG.cpp:702: void {anonymous}::SelectionDAGLegalize::LegalizeLoadOps(llvm::SDNode*): Assertion `NVT.getSizeInBits() == VT.getSizeInBits() && "Can only promote loads to same size type"' failed.

I wonder how to handle this. I didn’t think good idea to define aarch64svcount as let isScalable=true in ValueTypes.td.