[llvm] [RISCV64] liveness analysis (PR #167454)

[via llvm-commits]

hiraditya wrote:

> * Not clear to me why RiscV needs another analysis when we already have `LiveIntervals`  (and we also have the old/deprecated `LiveVariables`; this would add a 3rd variant into the mix...)

LiveInterval is quite heavy and best suitable for reg-alloc (as it is being used today) but...

> * If you want to use this pass anyway: At least consider using the existing `LivePhysRegs` / `LiveRegUnits` helpers to perform liveness analysis within a basic block. They reduce the risk of getting tricky bits like early clobbers or the unintuitive/bad moddeling for return blocks ("pristine registers", ...); subregisters/aliasing, ...

... I'll look into this if these can be repurposed. For now, the reg alias set (MCRegAliasIterator) combined with  RISCVLiveVariables::isTrackableRegister should be taking care of the non-interesting registers.

https://github.com/llvm/llvm-project/pull/167454