[Statepoint] Add getters to StatepointOpers. NFC.

[dantrushin]

Summary

To simplify future work on statepoint representation, hide
direct access ti statepoint field indices and provide getters
for them. Add getters for couple more statepoint fields.

This also fixes two bugs in MachineVerifier for statepoint:
First, the break statement was falling out of if statement
scope, thus disabling following checks.
Second, it was incorrectly accessing some fields like CallingConv -
StatepointOpers gives index to their value directly, not to
preceeding field type encoding.

[skatkov]

it is not NFC as you fix verifier. Split this patch into two parts or remove NFC.

llvm/include/llvm/CodeGen/StackMaps.h

 ///   <gc base/derived pairs...> <gc allocas...>
 /// Note that the last two sets of arguments are not currently length
 ///   prefixed.
 class StatepointOpers {
   // TODO:: we should change the STATEPOINT representation so that CC and
   // Flags should be part of meta operands, with args and deopt operands, and
   // gc operands all prefixed by their length and a type code. This would be
   // much more consistent.
-public:
-  // These values are aboolute offsets into the operands of the statepoint
+
+  // These values are absolute offsets into the operands of the statepoint
   // instruction.
   enum { IDPos, NBytesPos, NCallArgsPos, CallTargetPos, MetaEnd };
 
-  // These values are relative offests from the start of the statepoint meta
+  // These values are relative offsets from the start of the statepoint meta
   // arguments (i.e. the end of the call arguments).
   enum { CCOffset = 1, FlagsOffset = 3, NumDeoptOperandsOffset = 5 };
 
+public:
   explicit StatepointOpers(const MachineInstr *MI) : MI(MI) {}
 
+  /// Get index of statepoint ID operand.
+  unsigned getIDPos() const { return IDPos; }
+
+  /// Get index of Num Patch Bytes operand.
+  unsigned getNBytesPos() const { return NBytesPos; }
+
+  /// Get index of Num Call Arguments operand.
+  unsigned getNCallArgsPos() const { return NCallArgsPos; }
+
   /// Get starting index of non call related arguments
   /// (calling convention, statepoint flags, vm state and gc state).
   unsigned getVarIdx() const {
     return MI->getOperand(NCallArgsPos).getImm() + MetaEnd;
   }
 
+  /// Get index of Calling Convention operand.
+  unsigned getCCIdx() const { return getVarIdx() + CCOffset; }
+
+  /// Get index of Flags operand.
+  unsigned getFlagsIdx() const { return getVarIdx() + FlagsOffset; }
+
+  /// Get index of Number Deopt Arguments operand.
+  unsigned getNumDeoptArgsIdx() const {
+    return getVarIdx() + NumDeoptOperandsOffset;
+  }
+
   /// Return the ID for the given statepoint.
   uint64_t getID() const { return MI->getOperand(IDPos).getImm(); }
 
   /// Return the number of patchable bytes the given statepoint should emit.
   uint32_t getNumPatchBytes() const {
     return MI->getOperand(NBytesPos).getImm();
   }
 
-  /// Returns the target of the underlying call.
+  /// Return the target of the underlying call.
   const MachineOperand &getCallTarget() const {
     return MI->getOperand(CallTargetPos);
   }
 
+  /// Return the calling convention.
+  CallingConv::ID getCallingConv() const {
+    return MI->getOperand(getCCIdx()).getImm();
+  }
+
+  /// Return the statepoint flags.
+  uint64_t getFlags() const { return MI->getOperand(getFlagsIdx()).getImm(); }
+
 private:
   const MachineInstr *MI;
 };
 
 class StackMaps {
 public:
   struct Location {
     enum LocationType {

llvm/lib/CodeGen/FixupStatepointCallerSaved.cpp

private:
   FrameIndexesCache CacheFI;
 
 public:
   StatepointProcessor(MachineFunction &MF)
       : MF(MF), TRI(*MF.getSubtarget().getRegisterInfo()),
         CacheFI(MF.getFrameInfo(), TRI) {}
 
   bool process(MachineInstr &MI) {
-    unsigned VarIdx = StatepointOpers(&MI).getVarIdx();
-    uint64_t Flags =
-        MI.getOperand(VarIdx + StatepointOpers::FlagsOffset).getImm();
+    StatepointOpers SO(&MI);
+    uint64_t Flags = SO.getFlags();
     // Do nothing for LiveIn, it supports all registers.
     if (Flags & (uint64_t)StatepointFlags::DeoptLiveIn)
       return false;
-    CallingConv::ID CC =
-        MI.getOperand(VarIdx + StatepointOpers::CCOffset).getImm();
+    CallingConv::ID CC = SO.getCallingConv();
     const uint32_t *Mask = TRI.getCallPreservedMask(MF, CC);
     CacheFI.reset();
     StatepointState SS(MI, Mask, CacheFI);
 
     if (!SS.findRegistersToSpill())
       return false;
 
     SS.spillRegisters();

llvm/lib/CodeGen/MachineVerifier.cpp

if (SrcTy.isValid() || DstTy.isValid()) {
         if (!DstOp.getSubReg() && !SrcOp.getSubReg()) {
           report("Copy Instruction is illegal with mismatching sizes", MI);
           errs() << "Def Size = " << DstSize << ", Src Size = " << SrcSize
                  << "\n";
         }
     }
     break;
   }
-  case TargetOpcode::STATEPOINT:
-    if (!MI->getOperand(StatepointOpers::IDPos).isImm() ||
-        !MI->getOperand(StatepointOpers::NBytesPos).isImm() ||
-        !MI->getOperand(StatepointOpers::NCallArgsPos).isImm())
+  case TargetOpcode::STATEPOINT: {
+    StatepointOpers SO(MI);
+    if (!MI->getOperand(SO.getIDPos()).isImm() ||
+        !MI->getOperand(SO.getNBytesPos()).isImm() ||
+        !MI->getOperand(SO.getNCallArgsPos()).isImm()) {
       report("meta operands to STATEPOINT not constant!", MI);
       break;
+    }
 
     auto VerifyStackMapConstant = [&](unsigned Offset) {
-      if (!MI->getOperand(Offset).isImm() ||
-          MI->getOperand(Offset).getImm() != StackMaps::ConstantOp ||
-          !MI->getOperand(Offset + 1).isImm())
+      if (!MI->getOperand(Offset - 1).isImm() ||
+          MI->getOperand(Offset - 1).getImm() != StackMaps::ConstantOp ||
+          !MI->getOperand(Offset).isImm())
         report("stack map constant to STATEPOINT not well formed!", MI);
     };
-    const unsigned VarStart = StatepointOpers(MI).getVarIdx();
-    VerifyStackMapConstant(VarStart + StatepointOpers::CCOffset);
-    VerifyStackMapConstant(VarStart + StatepointOpers::FlagsOffset);
-    VerifyStackMapConstant(VarStart + StatepointOpers::NumDeoptOperandsOffset);
+    VerifyStackMapConstant(SO.getCCIdx());
+    VerifyStackMapConstant(SO.getFlagsIdx());
+    VerifyStackMapConstant(SO.getNumDeoptArgsIdx());
 
     // TODO: verify we have properly encoded deopt arguments
-    break;
+  } break;
   }
 }
 
 void
 MachineVerifier::visitMachineOperand(const MachineOperand *MO, unsigned MONum) {
   const MachineInstr *MI = MO->getParent();
   const MCInstrDesc &MCID = MI->getDesc();
   unsigned NumDefs = MCID.getNumDefs();