[NVPTX] Mark special registers as reserved

[asavonic]

Summary

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

llvm/lib/Target/NVPTX/NVPTXRegisterInfo.cpp

 const MCPhysReg *
 NVPTXRegisterInfo::getCalleeSavedRegs(const MachineFunction *) const {
   static const MCPhysReg CalleeSavedRegs[] = { 0 };
   return CalleeSavedRegs;
 }
 
 BitVector NVPTXRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
   BitVector Reserved(getNumRegs());
+  for (unsigned Reg = NVPTX::ENVREG0; Reg <= NVPTX::ENVREG31; ++Reg) {
+    markSuperRegs(Reserved, Reg);
+  }
+  markSuperRegs(Reserved, NVPTX::VRFrame32);
+  markSuperRegs(Reserved, NVPTX::VRFrameLocal32);
+  markSuperRegs(Reserved, NVPTX::VRFrame64);
+  markSuperRegs(Reserved, NVPTX::VRFrameLocal64);
+  markSuperRegs(Reserved, NVPTX::VRDepot);
   return Reserved;
 }
 
 void NVPTXRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
                                             int SPAdj, unsigned FIOperandNum,
                                             RegScavenger *RS) const {
   assert(SPAdj == 0 && "Unexpected");
 

llvm/test/CodeGen/NVPTX/envreg.ll

-; RUN: llc < %s -march=nvptx -mcpu=sm_20 | FileCheck %s
+; RUN: llc < %s -march=nvptx -mcpu=sm_20 -verify-machineinstrs | FileCheck %s
 
 
 declare i32 @llvm.nvvm.read.ptx.sreg.envreg0()
 declare i32 @llvm.nvvm.read.ptx.sreg.envreg1()
 declare i32 @llvm.nvvm.read.ptx.sreg.envreg2()
 declare i32 @llvm.nvvm.read.ptx.sreg.envreg3()
 declare i32 @llvm.nvvm.read.ptx.sreg.envreg4()
 declare i32 @llvm.nvvm.read.ptx.sreg.envreg5()