A reserved register:
- is not allocatable
- is considered always live
- is ignored by liveness tracking
NVPTX special registers match the criteria, and marking them as
reserved helps to avoid machine verifier error:
*** Bad machine code: Using an undefined physical register ***
- function: foo
- basic block: %bb.0 (0x557bb178b708)
- instruction: %0:int32regs = MOV_SPECIAL $envreg0
- operand 1: $envreg0
Differential Revision: https://reviews.llvm.org/D113008
123ad720f1