[clang] [compiler-rt] [llvm] [SystemZ] Add support for half (fp16) (PR #109164)

[Jonas Paulsson via cfe-commits]

JonPsson1 wrote:

Improved handling to utilize vector instructions when present. New VR16 regclass, but v8f16 *not* legal. It might make sense to have it as a legal type and e.g. do VL;VST when moving vectors in memory, and also set all vector ops to "Expand". Not sure how trivial that change would be, given some special handlings of vector nodes, so not done as of now: only the scalar f16 is legal.

Seems to work fine to add "16" versions for loads, stores and lzer/lcdfr in case of vector support.

Without vector support, it might make sense to have load/store pseudos with an extra GPR def operand, so that these loads/stores can be expanded as a PostRA pseudo. Then it would only need handling in one place, but OTOH having a second explicit def operand is also undesired, maybe.

f16 immediates handled like f32:
- Basic support added for fp 0.0/-0.0 and generation of vector constants (which should always work btw given their size with vrepih).
- Single-lane vector instructions like WFLCSB not used for fp16 (yet), even though it should be possible to add _16 variants. Doesn't seem important, so skipping.

Should fp16 inline asm operands also be supported at this point?

https://github.com/llvm/llvm-project/pull/109164