MIRParser: Rewrite register info initialization; mostly NFC

[MatzeB]

Summary

This changes MachineRegisterInfo to be initializes after parsing all
instructions. This is in preparation for upcoming commits that allow the
register class specification on the operand or deduce them from the
MCInstrDesc.

This commit removes the unused feature of having nonsequential register
numbers. This was confusing anyway as the vreg numbers would be
different after parsing when you had "holes" in your numbering.

This patch also introduces the concept of an incomplete virtual
register. An incomplete virtual register may be used during .mir parsing
to construct MachineOperands without knowing the exact register class
(or register bank) yet.

NFC except for some error messages.

[MatzeB]

ping

[qcolombet]

lib/CodeGen/MIRParser/MIParser.cpp

52

Could we maintain a map of "virtual ID” to actual vreg number instead of creating a bunch of useless ones?
Although, that’s probably cheap, I don’t like the idea of a bunch of undef registers in the middle.

Also, that should reduce the number of incomplete virtual registers, since we would create registers only at use/def points and we will know the register class there for the most part.

lib/CodeGen/MIRParser/MIParser.h

39

explicitly

lib/CodeGen/MachineFunctionAnalysis.cpp

58 ↗

(On Diff #64084)

Unrelated to the patch (+ I pushed a fix for that yesterday ;))

test/CodeGen/MIR/X86/undefined-virtual-register.mir

22–23

For that specific case, I would prefer a different message. Like %1 never defined.
For cases where the register is properly defined, but we can’t deduce the type with the use/def (like here with copy), I believe we shouldn’t call that a type. We should use register class/bank.

What I am saying is that we should reserve the word “type” for instruction types like sN, unsized, etc.

test/CodeGen/MIR/X86/unused-virtual-register.mir

14 ↗

(On Diff #64084)

I would get rid of that test. I basically don’t care if we don’t use dense numbering.

[MatzeB]

Thanks for the review! New patch is coming.

lib/CodeGen/MIRParser/MIParser.cpp

52

I went without a map to give the user control over the exact vreg numbers he will end up with.
Thinking about this some more it is probably not an important feature to have and having a map does indeed simplify writing down some tests.

lib/CodeGen/MachineFunctionAnalysis.cpp

58 ↗

(On Diff #64084)

that went in by accident.

test/CodeGen/MIR/X86/unused-virtual-register.mir

14 ↗

(On Diff #64084)

Obsolete and removed with the new patch.

[MatzeB]

Updated patch that maps vreg numbers in the .mir file to newly created vregs in the MachineFunction (as opposed to taking them literally as the previous version).

[MatzeB]

ping

[qcolombet]

LGTM.

include/llvm/CodeGen/MachineRegisterInfo.h

#endif
   /// Although the size should be set at build time, mir infrastructure
   /// is not yet able to do it.
   void setSize(unsigned VReg, unsigned Size);
 
   /// Create and return a new generic virtual register with a size of \p Size.
   /// \pre Size > 0.
   unsigned createGenericVirtualRegister(unsigned Size);
 
+  /// Creates a new virtual register that has no register class, register bank
+  /// or size assigned yet. This is only allowed to be used
+  /// temporarily while constructing machine instructions. Most operations are
+  /// undefined on a preliminary register until one of setRegClass(),
+  /// setRegBank() or setSize() has been called for it.
+  unsigned createIncompleteVirtualRegister();
+
   /// getNumVirtRegs - Return the number of virtual registers created.
   ///
   unsigned getNumVirtRegs() const { return VRegInfo.size(); }
 
   /// clearVirtRegs - Remove all virtual registers (after physreg assignment).
   void clearVirtRegs();
 
   /// setRegAllocationHint - Specify a register allocation hint for the

lib/CodeGen/MIRParser/MIParser.h

 
 class StringRef;
 class BasicBlock;
 class MachineBasicBlock;
 class MachineFunction;
 class MachineInstr;
 class MachineRegisterInfo;
 class MDNode;
+class RegisterBank;
 struct SlotMapping;
 class SMDiagnostic;
 class SourceMgr;
+class TargetRegisterClass;
+
+struct VRegInfo {
+  enum uint8_t {
+    UNKNOWN, NORMAL, GENERIC, REGBANK
+  } Kind = UNKNOWN;
+  bool Explicit = false; ///< VReg was explicitly specified in the .mir file.
+  union {
+    const TargetRegisterClass *RC;
+    const RegisterBank *RegBank;
+  } D;
+  unsigned VReg;
+  unsigned PreferredReg = 0;
+};
 
 struct PerFunctionMIParsingState {
+  BumpPtrAllocator Allocator;
   MachineFunction &MF;
   SourceMgr *SM;
   const SlotMapping &IRSlots;
 
   DenseMap<unsigned, MachineBasicBlock *> MBBSlots;
-  DenseMap<unsigned, unsigned> VirtualRegisterSlots;
+  DenseMap<unsigned, VRegInfo*> VRegInfos;
   DenseMap<unsigned, int> FixedStackObjectSlots;
   DenseMap<unsigned, int> StackObjectSlots;
   DenseMap<unsigned, unsigned> ConstantPoolSlots;
   DenseMap<unsigned, unsigned> JumpTableSlots;
-  /// Hold the generic virtual registers.
-  SmallSet<unsigned, 8> GenericVRegs;
 
   PerFunctionMIParsingState(MachineFunction &MF, SourceMgr &SM,
                             const SlotMapping &IRSlots);
+
+  VRegInfo &getVRegInfo(unsigned VReg);
 };
 
 /// Parse the machine basic block definitions, and skip the machine
 /// instructions.
 ///
 /// This function runs the first parsing pass on the machine function's body.
 /// It parses only the machine basic block definitions and creates the machine
 /// basic blocks in the given machine function.

 /// This function runs the second parsing pass on the machine function's body.
 /// It skips the machine basic block definitions and parses only the machine
 /// instructions and basic block attributes like liveins and successors.
 ///
 /// The second parsing pass assumes that the first parsing pass already ran
 /// on the given source string.
 ///
 /// Return true if an error occurred.
-bool parseMachineInstructions(const PerFunctionMIParsingState &PFS,
-                              StringRef Src, SMDiagnostic &Error);
+bool parseMachineInstructions(PerFunctionMIParsingState &PFS, StringRef Src,
+                              SMDiagnostic &Error);
 
-bool parseMBBReference(const PerFunctionMIParsingState &PFS,
+bool parseMBBReference(PerFunctionMIParsingState &PFS,
                        MachineBasicBlock *&MBB, StringRef Src,
                        SMDiagnostic &Error);
 
-bool parseNamedRegisterReference(const PerFunctionMIParsingState &PFS,
-                                 unsigned &Reg, StringRef Src,
-                                 SMDiagnostic &Error);
+bool parseNamedRegisterReference(PerFunctionMIParsingState &PFS, unsigned &Reg,
+                                 StringRef Src, SMDiagnostic &Error);
 
-bool parseVirtualRegisterReference(const PerFunctionMIParsingState &PFS,
-                                   unsigned &Reg, StringRef Src,
+bool parseVirtualRegisterReference(PerFunctionMIParsingState &PFS,
+                                   VRegInfo *&Info, StringRef Src,
                                    SMDiagnostic &Error);
 
-bool parseStackObjectReference(const PerFunctionMIParsingState &PFS,
-                               int &FI, StringRef Src, SMDiagnostic &Error);
-
-bool parseMDNode(const PerFunctionMIParsingState &PFS, MDNode *&Node,
+bool parseStackObjectReference(PerFunctionMIParsingState &PFS, int &FI,
                                StringRef Src, SMDiagnostic &Error);
 
+bool parseMDNode(PerFunctionMIParsingState &PFS, MDNode *&Node, StringRef Src,
+                 SMDiagnostic &Error);
+
 } // end namespace llvm
 
 #endif

lib/CodeGen/MIRParser/MIParser.cpp

 
 using namespace llvm;
 
 PerFunctionMIParsingState::PerFunctionMIParsingState(MachineFunction &MF,
     SourceMgr &SM, const SlotMapping &IRSlots)
   : MF(MF), SM(&SM), IRSlots(IRSlots) {
 }
 
+VRegInfo &PerFunctionMIParsingState::getVRegInfo(unsigned Num) {
+  auto I = VRegInfos.insert(std::make_pair(Num, nullptr));
+  if (I.second) {
+    MachineRegisterInfo &MRI = MF.getRegInfo();
+    VRegInfo *Info = new (Allocator) VRegInfo;
+    Info->VReg = MRI.createIncompleteVirtualRegister();
+    I.first->second = Info;
+  }
+  return *I.first->second;
+}
+
 namespace {
 
 /// A wrapper struct around the 'MachineOperand' struct that includes a source
 /// range and other attributes.
 struct ParsedMachineOperand {
   MachineOperand Operand;
   StringRef::iterator Begin;
   StringRef::iterator End;
   Optional<unsigned> TiedDefIdx;
 
   ParsedMachineOperand(const MachineOperand &Operand, StringRef::iterator Begin,
                        StringRef::iterator End, Optional<unsigned> &TiedDefIdx)
       : Operand(Operand), Begin(Begin), End(End), TiedDefIdx(TiedDefIdx) {
     if (TiedDefIdx)
       assert(Operand.isReg() && Operand.isUse() &&
              "Only used register operands can be tied");
   }
 };
 
 class MIParser {
   MachineFunction &MF;
   SMDiagnostic &Error;
   StringRef Source, CurrentSource;
   MIToken Token;
-  const PerFunctionMIParsingState &PFS;
+  PerFunctionMIParsingState &PFS;
   /// Maps from instruction names to op codes.
   StringMap<unsigned> Names2InstrOpCodes;
   /// Maps from register names to registers.
   StringMap<unsigned> Names2Regs;
   /// Maps from register mask names to register masks.
   StringMap<const uint32_t *> Names2RegMasks;
   /// Maps from subregister names to subregister indices.
   StringMap<unsigned> Names2SubRegIndices;
   /// Maps from slot numbers to function's unnamed basic blocks.
   DenseMap<unsigned, const BasicBlock *> Slots2BasicBlocks;
   /// Maps from slot numbers to function's unnamed values.
   DenseMap<unsigned, const Value *> Slots2Values;
   /// Maps from target index names to target indices.
   StringMap<int> Names2TargetIndices;
   /// Maps from direct target flag names to the direct target flag values.
   StringMap<unsigned> Names2DirectTargetFlags;
   /// Maps from direct target flag names to the bitmask target flag values.
   StringMap<unsigned> Names2BitmaskTargetFlags;
 
 public:
-  MIParser(const PerFunctionMIParsingState &PFS, SMDiagnostic &Error,
+  MIParser(PerFunctionMIParsingState &PFS, SMDiagnostic &Error,
            StringRef Source);
 
   /// \p SkipChar gives the number of characters to skip before looking
   /// for the next token.
   void lex(unsigned SkipChar = 0);
 
   /// Report an error at the current location with the given message.
   ///
   /// This function always return true.
   bool error(const Twine &Msg);
 
   /// Report an error at the given location with the given message.
   ///
   /// This function always return true.
   bool error(StringRef::iterator Loc, const Twine &Msg);
 
   bool
   parseBasicBlockDefinitions(DenseMap<unsigned, MachineBasicBlock *> &MBBSlots);
   bool parseBasicBlocks();
   bool parse(MachineInstr *&MI);
   bool parseStandaloneMBB(MachineBasicBlock *&MBB);
   bool parseStandaloneNamedRegister(unsigned &Reg);
-  bool parseStandaloneVirtualRegister(unsigned &Reg);
+  bool parseStandaloneVirtualRegister(VRegInfo *&Info);
   bool parseStandaloneStackObject(int &FI);
   bool parseStandaloneMDNode(MDNode *&Node);
 
   bool
   parseBasicBlockDefinition(DenseMap<unsigned, MachineBasicBlock *> &MBBSlots);
   bool parseBasicBlock(MachineBasicBlock &MBB);
   bool parseBasicBlockLiveins(MachineBasicBlock &MBB);
   bool parseBasicBlockSuccessors(MachineBasicBlock &MBB);
 
-  bool parseRegister(unsigned &Reg);
+  bool parseNamedRegister(unsigned &Reg);
+  bool parseVirtualRegister(VRegInfo *&Info);
+  bool parseRegister(unsigned &Reg, VRegInfo *&VRegInfo);
   bool parseRegisterFlag(unsigned &Flags);
   bool parseSubRegisterIndex(unsigned &SubReg);
   bool parseRegisterTiedDefIndex(unsigned &TiedDefIdx);
   bool parseSize(unsigned &Size);
   bool parseRegisterOperand(MachineOperand &Dest,
                             Optional<unsigned> &TiedDefIdx, bool IsDef = false);
   bool parseImmediateOperand(MachineOperand &Dest);
   bool parseIRConstant(StringRef::iterator Loc, StringRef Source,
private:
   /// target flag.
   ///
   /// Return true if the name isn't a name of a bitmask target flag.
   bool getBitmaskTargetFlag(StringRef Name, unsigned &Flag);
 };
 
 } // end anonymous namespace
 
-MIParser::MIParser(const PerFunctionMIParsingState &PFS, SMDiagnostic &Error,
+MIParser::MIParser(PerFunctionMIParsingState &PFS, SMDiagnostic &Error,
                    StringRef Source)
     : MF(PFS.MF), Error(Error), Source(Source), CurrentSource(Source), PFS(PFS)
 {}
 
 void MIParser::lex(unsigned SkipChar) {
   CurrentSource = lexMIToken(
       CurrentSource.data() + SkipChar, Token,
       [this](StringRef::iterator Loc, const Twine &Msg) { error(Loc, Msg); });
bool MIParser::parseBasicBlockLiveins(MachineBasicBlock &MBB) {
   if (expectAndConsume(MIToken::colon))
     return true;
   if (Token.isNewlineOrEOF()) // Allow an empty list of liveins.
     return false;
   do {
     if (Token.isNot(MIToken::NamedRegister))
       return error("expected a named register");
     unsigned Reg = 0;
-    if (parseRegister(Reg))
+    if (parseNamedRegister(Reg))
       return true;
     MBB.addLiveIn(Reg);
     lex();
   } while (consumeIfPresent(MIToken::comma));
   return false;
 }
 
 bool MIParser::parseBasicBlockSuccessors(MachineBasicBlock &MBB) {
return error(
         "expected end of string after the machine basic block reference");
   return false;
 }
 
 bool MIParser::parseStandaloneNamedRegister(unsigned &Reg) {
   lex();
   if (Token.isNot(MIToken::NamedRegister))
     return error("expected a named register");
-  if (parseRegister(Reg))
+  if (parseNamedRegister(Reg))
     return true;
   lex();
   if (Token.isNot(MIToken::Eof))
     return error("expected end of string after the register reference");
   return false;
 }
 
-bool MIParser::parseStandaloneVirtualRegister(unsigned &Reg) {
+bool MIParser::parseStandaloneVirtualRegister(VRegInfo *&Info) {
   lex();
   if (Token.isNot(MIToken::VirtualRegister))
     return error("expected a virtual register");
-  if (parseRegister(Reg))
+  if (parseVirtualRegister(Info))
     return true;
   lex();
   if (Token.isNot(MIToken::Eof))
     return error("expected end of string after the register reference");
   return false;
 }
 
 bool MIParser::parseStandaloneStackObject(int &FI) {
if (Token.isNot(MIToken::Identifier))
     return error("expected a machine instruction");
   StringRef InstrName = Token.stringValue();
   if (parseInstrName(InstrName, OpCode))
     return error(Twine("unknown machine instruction name '") + InstrName + "'");
   lex();
   return false;
 }
 
-bool MIParser::parseRegister(unsigned &Reg) {
-  switch (Token.kind()) {
-  case MIToken::underscore:
-    Reg = 0;
-    break;
-  case MIToken::NamedRegister: {
-    StringRef Name = Token.stringValue();
-    if (getRegisterByName(Name, Reg))
-      return error(Twine("unknown register name '") + Name + "'");
-    break;
-  }
-  case MIToken::VirtualRegister: {
-    unsigned ID;
-    if (getUnsigned(ID))
-      return true;
-    const auto RegInfo = PFS.VirtualRegisterSlots.find(ID);
-    if (RegInfo == PFS.VirtualRegisterSlots.end())
-      return error(Twine("use of undefined virtual register '%") + Twine(ID) +
-                   "'");
-    Reg = RegInfo->second;
-    break;
-  }
+bool MIParser::parseNamedRegister(unsigned &Reg) {
+  assert(Token.is(MIToken::NamedRegister) && "Needs NamedRegister token");
+  return false;
+
+bool MIParser::parseVirtualRegister(VRegInfo *&Info) {
+  assert(Token.is(MIToken::VirtualRegister) && "Needs VirtualRegister token");
+  Info = &PFS.getVRegInfo(ID);
+  return false;
+
+bool MIParser::parseRegister(unsigned &Reg, VRegInfo *&Info) {
+  switch (Token.kind()) {
+  case MIToken::underscore:
+    Reg = 0;
+    return false;
+  case MIToken::NamedRegister:
+    return parseNamedRegister(Reg);
+  case MIToken::VirtualRegister:
+    if (parseVirtualRegister(Info))
+      return true;
+    Reg = Info->VReg;
+    return false;
   // TODO: Parse other register kinds.
   default:
     llvm_unreachable("The current token should be a register");
   }
-  return false;
 }
 
 bool MIParser::parseRegisterFlag(unsigned &Flags) {
   const unsigned OldFlags = Flags;
   switch (Token.kind()) {
   case MIToken::kw_implicit:
     Flags |= RegState::Implicit;
     break;
bool MIParser::assignRegisterTies(MachineInstr &MI,
   for (const auto &TiedPair : TiedRegisterPairs)
     MI.tieOperands(TiedPair.first, TiedPair.second);
   return false;
 }
 
 bool MIParser::parseRegisterOperand(MachineOperand &Dest,
                                     Optional<unsigned> &TiedDefIdx,
                                     bool IsDef) {
-  unsigned Reg;
   unsigned Flags = IsDef ? RegState::Define : 0;
   while (Token.isRegisterFlag()) {
     if (parseRegisterFlag(Flags))
       return true;
   }
   if (!Token.isRegister())
     return error("expected a register after register flags");
-  if (parseRegister(Reg))
+  unsigned Reg;
+  VRegInfo *RegInfo;
+  if (parseRegister(Reg, RegInfo))
     return true;
   lex();
   unsigned SubReg = 0;
   if (Token.is(MIToken::dot)) {
     if (parseSubRegisterIndex(SubReg))
       return true;
     if (!TargetRegisterInfo::isVirtualRegister(Reg))
       return error("subregister index expects a virtual register");
if ((Flags & RegState::Define) == 0) {
     if (MRI.getRegClassOrRegBank(Reg).is<const TargetRegisterClass *>())
       return error("unexpected size on non-generic virtual register");
 
     unsigned Size;
     if (parseSize(Size))
       return true;
 
     MRI.setSize(Reg, Size);
-  } else if (PFS.GenericVRegs.count(Reg)) {
+  } else if (TargetRegisterInfo::isVirtualRegister(Reg)) {
     // Generic virtual registers must have a size.
     // If we end up here this means the size hasn't been specified and
     // this is bad!
+    if (RegInfo->Kind == VRegInfo::GENERIC ||
+        RegInfo->Kind == VRegInfo::REGBANK)
       return error("generic virtual registers must have a size");
   }
   Dest = MachineOperand::CreateReg(
       Reg, Flags & RegState::Define, Flags & RegState::Implicit,
       Flags & RegState::Kill, Flags & RegState::Dead, Flags & RegState::Undef,
       Flags & RegState::EarlyClobber, SubReg, Flags & RegState::Debug,
       Flags & RegState::InternalRead);
   return false;
 }
bool MIParser::parseCFIOffset(int &Offset) {
   lex();
   return false;
 }
 
 bool MIParser::parseCFIRegister(unsigned &Reg) {
   if (Token.isNot(MIToken::NamedRegister))
     return error("expected a cfi register");
   unsigned LLVMReg;
-  if (parseRegister(LLVMReg))
+  if (parseNamedRegister(LLVMReg))
     return true;
   const auto *TRI = MF.getSubtarget().getRegisterInfo();
   assert(TRI && "Expected target register info");
   int DwarfReg = TRI->getDwarfRegNum(LLVMReg, true);
   if (DwarfReg < 0)
     return error("invalid DWARF register");
   Reg = (unsigned)DwarfReg;
   lex();
bool MIParser::parseLiveoutRegisterMaskOperand(MachineOperand &Dest) {
   assert(TRI && "Expected target register info");
   uint32_t *Mask = MF.allocateRegisterMask(TRI->getNumRegs());
   lex();
   if (expectAndConsume(MIToken::lparen))
     return true;
   while (true) {
     if (Token.isNot(MIToken::NamedRegister))
       return error("expected a named register");
-    unsigned Reg = 0;
-    if (parseRegister(Reg))
+    unsigned Reg;
+    if (parseNamedRegister(Reg))
       return true;
     lex();
     Mask[Reg / 32] |= 1U << (Reg % 32);
     // TODO: Report an error if the same register is used more than once.
     if (Token.isNot(MIToken::comma))
       break;
     lex();
   }

 }
 
 bool llvm::parseMachineBasicBlockDefinitions(PerFunctionMIParsingState &PFS,
                                              StringRef Src,
                                              SMDiagnostic &Error) {
   return MIParser(PFS, Error, Src).parseBasicBlockDefinitions(PFS.MBBSlots);
 }
 
-bool llvm::parseMachineInstructions(const PerFunctionMIParsingState &PFS,
+bool llvm::parseMachineInstructions(PerFunctionMIParsingState &PFS,
                                     StringRef Src, SMDiagnostic &Error) {
   return MIParser(PFS, Error, Src).parseBasicBlocks();
 }
 
-bool llvm::parseMBBReference(const PerFunctionMIParsingState &PFS,
+bool llvm::parseMBBReference(PerFunctionMIParsingState &PFS,
                              MachineBasicBlock *&MBB, StringRef Src,
                              SMDiagnostic &Error) {
   return MIParser(PFS, Error, Src).parseStandaloneMBB(MBB);
 }
 
-bool llvm::parseNamedRegisterReference(const PerFunctionMIParsingState &PFS,
+bool llvm::parseNamedRegisterReference(PerFunctionMIParsingState &PFS,
                                        unsigned &Reg, StringRef Src,
                                        SMDiagnostic &Error) {
   return MIParser(PFS, Error, Src).parseStandaloneNamedRegister(Reg);
 }
 
-bool llvm::parseVirtualRegisterReference(const PerFunctionMIParsingState &PFS,
-                                         unsigned &Reg, StringRef Src,
+bool llvm::parseVirtualRegisterReference(PerFunctionMIParsingState &PFS,
+                                         VRegInfo *&Info, StringRef Src,
                                          SMDiagnostic &Error) {
-  return MIParser(PFS, Error, Src).parseStandaloneVirtualRegister(Reg);
+  return MIParser(PFS, Error, Src).parseStandaloneVirtualRegister(Info);
 }
 
-bool llvm::parseStackObjectReference(const PerFunctionMIParsingState &PFS,
+bool llvm::parseStackObjectReference(PerFunctionMIParsingState &PFS,
                                      int &FI, StringRef Src,
                                      SMDiagnostic &Error) {
   return MIParser(PFS, Error, Src).parseStandaloneStackObject(FI);
 }
 
-bool llvm::parseMDNode(const PerFunctionMIParsingState &PFS,
+bool llvm::parseMDNode(PerFunctionMIParsingState &PFS,
                        MDNode *&Node, StringRef Src, SMDiagnostic &Error) {
   return MIParser(PFS, Error, Src).parseStandaloneMDNode(Node);
 }

lib/CodeGen/MIRParser/MIRParser.cpp

public:
   bool parseMachineFunction(yaml::Input &In, Module &M, bool NoLLVMIR);
 
   /// Initialize the machine function to the state that's described in the MIR
   /// file.
   ///
   /// Return true if error occurred.
   bool initializeMachineFunction(MachineFunction &MF);
 
-  bool initializeRegisterInfo(PerFunctionMIParsingState &PFS,
-                              const yaml::MachineFunction &YamlMF);
+  bool parseRegisterInfo(PerFunctionMIParsingState &PFS,
 
-  void inferRegisterInfo(const PerFunctionMIParsingState &PFS,
+  bool setupRegisterInfo(const PerFunctionMIParsingState &PFS,
                          const yaml::MachineFunction &YamlMF);
 
   bool initializeFrameInfo(PerFunctionMIParsingState &PFS,
                            const yaml::MachineFunction &YamlMF);
 
   bool parseCalleeSavedRegister(PerFunctionMIParsingState &PFS,
                                 std::vector<CalleeSavedInfo> &CSIInfo,
                                 const yaml::StringValue &RegisterSource,
                                 int FrameIdx);
 
   bool parseStackObjectsDebugInfo(PerFunctionMIParsingState &PFS,
                                   const yaml::MachineStackObject &Object,
                                   int FrameIdx);
 
   bool initializeConstantPool(PerFunctionMIParsingState &PFS,
                               MachineConstantPool &ConstantPool,
                               const yaml::MachineFunction &YamlMF);
 
   bool initializeJumpTableInfo(PerFunctionMIParsingState &PFS,
                                const yaml::MachineJumpTable &YamlJTI);
 
 private:
-  bool parseMDNode(const PerFunctionMIParsingState &PFS, MDNode *&Node,
+  bool parseMDNode(PerFunctionMIParsingState &PFS, MDNode *&Node,
                    const yaml::StringValue &Source);
 
-  bool parseMBBReference(const PerFunctionMIParsingState &PFS,
+  bool parseMBBReference(PerFunctionMIParsingState &PFS,
                          MachineBasicBlock *&MBB,
                          const yaml::StringValue &Source);
 
   /// Return a MIR diagnostic converted from an MI string diagnostic.
   SMDiagnostic diagFromMIStringDiag(const SMDiagnostic &Error,
                                     SMRange SourceRange);
 
   /// Return a MIR diagnostic converted from a diagnostic located in a YAML
bool MIRParserImpl::initializeMachineFunction(MachineFunction &MF) {
   const yaml::MachineFunction &YamlMF = *It->getValue();
   if (YamlMF.Alignment)
     MF.setAlignment(YamlMF.Alignment);
   MF.setExposesReturnsTwice(YamlMF.ExposesReturnsTwice);
   MF.setHasInlineAsm(YamlMF.HasInlineAsm);
   if (YamlMF.AllVRegsAllocated)
     MF.getProperties().set(MachineFunctionProperties::Property::AllVRegsAllocated);
   PerFunctionMIParsingState PFS(MF, SM, IRSlots);
-  if (initializeRegisterInfo(PFS, YamlMF))
+  if (parseRegisterInfo(PFS, YamlMF))
     return true;
   if (!YamlMF.Constants.empty()) {
     auto *ConstantPool = MF.getConstantPool();
     assert(ConstantPool && "Constant pool must be created");
     if (initializeConstantPool(PFS, *ConstantPool, YamlMF))
       return true;
   }
 
bool MIRParserImpl::initializeMachineFunction(MachineFunction &MF) {
   PFS.SM = &InsnSM;
   if (parseMachineInstructions(PFS, InsnStr, Error)) {
     reportDiagnostic(
         diagFromBlockStringDiag(Error, YamlMF.Body.Value.SourceRange));
     return true;
   }
   PFS.SM = &SM;
 
-  inferRegisterInfo(PFS, YamlMF);
-  // FIXME: This is a temporary workaround until the reserved registers can be
-  // serialized.
-  MF.getRegInfo().freezeReservedRegs(MF);
+  if (setupRegisterInfo(PFS, YamlMF))
+    return true;
+
   MF.verify();
   return false;
 }
 
-bool MIRParserImpl::initializeRegisterInfo(PerFunctionMIParsingState &PFS,
+bool MIRParserImpl::parseRegisterInfo(PerFunctionMIParsingState &PFS,
                                       const yaml::MachineFunction &YamlMF) {
   MachineFunction &MF = PFS.MF;
   MachineRegisterInfo &RegInfo = MF.getRegInfo();
   assert(RegInfo.isSSA());
   if (!YamlMF.IsSSA)
     RegInfo.leaveSSA();
   assert(RegInfo.tracksLiveness());
   if (!YamlMF.TracksRegLiveness)
     RegInfo.invalidateLiveness();
   RegInfo.enableSubRegLiveness(YamlMF.TracksSubRegLiveness);
 
   SMDiagnostic Error;
   // Parse the virtual register information.
   for (const auto &VReg : YamlMF.VirtualRegisters) {
-    unsigned Reg;
+    VRegInfo &Info = PFS.getVRegInfo(VReg.ID.Value);
+    if (Info.Explicit)
+      return error(VReg.ID.SourceRange.Start,
+                   Twine("redefinition of virtual register '%") +
+                       Twine(VReg.ID.Value) + "'");
+    Info.Explicit = true;
+
     if (StringRef(VReg.Class.Value).equals("_")) {
-      // This is a generic virtual register.
-      // The size will be set appropriately when we reach the definition.
-      Reg = RegInfo.createGenericVirtualRegister(/*Size*/ 1);
-      PFS.GenericVRegs.insert(Reg);
+      Info.Kind = VRegInfo::GENERIC;
     } else {
       const auto *RC = getRegClass(MF, VReg.Class.Value);
       if (RC) {
-        Reg = RegInfo.createVirtualRegister(RC);
+        Info.Kind = VRegInfo::NORMAL;
+        Info.D.RC = RC;
       } else {
-        const auto *RegBank = getRegBank(MF, VReg.Class.Value);
+        const RegisterBank *RegBank = getRegBank(MF, VReg.Class.Value);
         if (!RegBank)
           return error(
               VReg.Class.SourceRange.Start,
               Twine("use of undefined register class or register bank '") +
                   VReg.Class.Value + "'");
-        Reg = RegInfo.createGenericVirtualRegister(/*Size*/ 1);
-        RegInfo.setRegBank(Reg, *RegBank);
-        PFS.GenericVRegs.insert(Reg);
+        Info.Kind = VRegInfo::REGBANK;
+        Info.D.RegBank = RegBank;
       }
     }
-    if (!PFS.VirtualRegisterSlots.insert(std::make_pair(VReg.ID.Value, Reg))
-             .second)
-      return error(VReg.ID.SourceRange.Start,
-                   Twine("redefinition of virtual register '%") +
-                       Twine(VReg.ID.Value) + "'");
+
     if (!VReg.PreferredRegister.Value.empty()) {
-      unsigned PreferredReg = 0;
-      if (parseNamedRegisterReference(PFS, PreferredReg,
+      if (Info.Kind != VRegInfo::NORMAL)
+        return error(VReg.Class.SourceRange.Start,
+              Twine("preferred register can only be set for normal vregs"));
+      if (parseNamedRegisterReference(PFS, Info.PreferredReg,
                                       VReg.PreferredRegister.Value, Error))
         return error(Error, VReg.PreferredRegister.SourceRange);
-      RegInfo.setSimpleHint(Reg, PreferredReg);
     }
   }
 
   // Parse the liveins.
   for (const auto &LiveIn : YamlMF.LiveIns) {
     unsigned Reg = 0;
     if (parseNamedRegisterReference(PFS, Reg, LiveIn.Register.Value, Error))
       return error(Error, LiveIn.Register.SourceRange);
     unsigned VReg = 0;
     if (!LiveIn.VirtualRegister.Value.empty()) {
-      if (parseVirtualRegisterReference(PFS, VReg, LiveIn.VirtualRegister.Value,
+      VRegInfo *Info;
+      if (parseVirtualRegisterReference(PFS, Info, LiveIn.VirtualRegister.Value,
                                         Error))
         return error(Error, LiveIn.VirtualRegister.SourceRange);
+      VReg = Info->VReg;
     }
     RegInfo.addLiveIn(Reg, VReg);
   }
 
   // Parse the callee saved register mask.
   BitVector CalleeSavedRegisterMask(RegInfo.getUsedPhysRegsMask().size());
   if (!YamlMF.CalleeSavedRegisters)
     return false;
   for (const auto &RegSource : YamlMF.CalleeSavedRegisters.getValue()) {
     unsigned Reg = 0;
     if (parseNamedRegisterReference(PFS, Reg, RegSource.Value, Error))
       return error(Error, RegSource.SourceRange);
     CalleeSavedRegisterMask[Reg] = true;
   }
   RegInfo.setUsedPhysRegMask(CalleeSavedRegisterMask.flip());
   return false;
 }
 
-void MIRParserImpl::inferRegisterInfo(const PerFunctionMIParsingState &PFS,
+bool MIRParserImpl::setupRegisterInfo(const PerFunctionMIParsingState &PFS,
                                       const yaml::MachineFunction &YamlMF) {
-  if (YamlMF.CalleeSavedRegisters)
-    return;
-  MachineRegisterInfo &MRI = PFS.MF.getRegInfo();
-  for (const MachineBasicBlock &MBB : PFS.MF) {
+  MachineFunction &MF = PFS.MF;
+  MachineRegisterInfo &MRI = MF.getRegInfo();
+  bool Error = false;
+  // Create VRegs
+  for (auto P : PFS.VRegInfos) {
+    const VRegInfo &Info = *P.second;
+    unsigned Reg = Info.VReg;
+    switch (Info.Kind) {
+    case VRegInfo::UNKNOWN:
+      error(Twine("Cannot determine class/bank of virtual register ") +
+            Twine(P.first) + " in function '" + MF.getName() + "'");
+      Error = true;
+      break;
+    case VRegInfo::NORMAL:
+      MRI.setRegClass(Reg, Info.D.RC);
+      if (Info.PreferredReg != 0)
+        MRI.setSimpleHint(Reg, Info.PreferredReg);
+      break;
+    case VRegInfo::GENERIC:
+      break;
+    case VRegInfo::REGBANK:
+      MRI.setRegBank(Reg, *Info.D.RegBank);
+      break;
+    }
+  }
+
+  // Compute MachineRegisterInfo::UsedPhysRegMask
+  if (!YamlMF.CalleeSavedRegisters) {
+    for (const MachineBasicBlock &MBB : MF) {
       for (const MachineInstr &MI : MBB) {
         for (const MachineOperand &MO : MI.operands()) {
           if (!MO.isRegMask())
             continue;
           MRI.addPhysRegsUsedFromRegMask(MO.getRegMask());
         }
       }
     }
   }
 
+  // FIXME: This is a temporary workaround until the reserved registers can be
+  // serialized.
+  MRI.freezeReservedRegs(MF);
+  return Error;
+}
+
 bool MIRParserImpl::initializeFrameInfo(PerFunctionMIParsingState &PFS,
                                         const yaml::MachineFunction &YamlMF) {
   MachineFunction &MF = PFS.MF;
   MachineFrameInfo &MFI = *MF.getFrameInfo();
   const Function &F = *MF.getFunction();
   const yaml::MachineFrameInfo &YamlMFI = YamlMF.FrameInfo;
   MFI.setFrameAddressIsTaken(YamlMFI.IsFrameAddressTaken);
   MFI.setReturnAddressIsTaken(YamlMFI.IsReturnAddressTaken);
bool MIRParserImpl::parseStackObjectsDebugInfo(PerFunctionMIParsingState &PFS,
   if (typecheckMDNode(DIVar, Var, Object.DebugVar, "DILocalVariable", *this) ||
       typecheckMDNode(DIExpr, Expr, Object.DebugExpr, "DIExpression", *this) ||
       typecheckMDNode(DILoc, Loc, Object.DebugLoc, "DILocation", *this))
     return true;
   PFS.MF.getMMI().setVariableDbgInfo(DIVar, DIExpr, unsigned(FrameIdx), DILoc);
   return false;
 }
 
-bool MIRParserImpl::parseMDNode(const PerFunctionMIParsingState &PFS,
+bool MIRParserImpl::parseMDNode(PerFunctionMIParsingState &PFS,
     MDNode *&Node, const yaml::StringValue &Source) {
   if (Source.Value.empty())
     return false;
   SMDiagnostic Error;
   if (llvm::parseMDNode(PFS, Node, Source.Value, Error))
     return error(Error, Source.SourceRange);
   return false;
 }
if (!PFS.JumpTableSlots.insert(std::make_pair(Entry.ID.Value, Index))
              .second)
       return error(Entry.ID.SourceRange.Start,
                    Twine("redefinition of jump table entry '%jump-table.") +
                        Twine(Entry.ID.Value) + "'");
   }
   return false;
 }
 
-bool MIRParserImpl::parseMBBReference(const PerFunctionMIParsingState &PFS,
+bool MIRParserImpl::parseMBBReference(PerFunctionMIParsingState &PFS,
                                       MachineBasicBlock *&MBB,
                                       const yaml::StringValue &Source) {
   SMDiagnostic Error;
   if (llvm::parseMBBReference(PFS, MBB, Source.Value, Error))
     return error(Error, Source.SourceRange);
   return false;
 }
 

lib/CodeGen/MachineRegisterInfo.cpp

NewRC = MI->getRegClassConstraintEffect(OpNo, NewRC, TII,
                                             getTargetRegisterInfo());
     if (!NewRC || NewRC == OldRC)
       return false;
   }
   setRegClass(Reg, NewRC);
   return true;
 }
 
+unsigned MachineRegisterInfo::createIncompleteVirtualRegister() {
+  unsigned Reg = TargetRegisterInfo::index2VirtReg(getNumVirtRegs());
+  VRegInfo.grow(Reg);
+  RegAllocHints.grow(Reg);
+  return Reg;
+}
+
 /// createVirtualRegister - Create and return a new virtual register in the
 /// function with the specified register class.
 ///
 unsigned
 MachineRegisterInfo::createVirtualRegister(const TargetRegisterClass *RegClass){
   assert(RegClass && "Cannot create register without RegClass!");
   assert(RegClass->isAllocatable() &&
          "Virtual register RegClass must be allocatable.");
 
   // New virtual register number.
-  unsigned Reg = TargetRegisterInfo::index2VirtReg(getNumVirtRegs());
-  VRegInfo.grow(Reg);
+  unsigned Reg = createIncompleteVirtualRegister();
   VRegInfo[Reg].first = RegClass;
-  RegAllocHints.grow(Reg);
   if (TheDelegate)
     TheDelegate->MRI_NoteNewVirtualRegister(Reg);
   return Reg;
 }
 
 unsigned
 MachineRegisterInfo::getSize(unsigned VReg) const {
   VRegToSizeMap::const_iterator SizeIt = getVRegToSize().find(VReg);
   return SizeIt != getVRegToSize().end() ? SizeIt->second : 0;
 }
 
 void MachineRegisterInfo::setSize(unsigned VReg, unsigned Size) {
   // Check that VReg doesn't have a class.
   assert(!getRegClassOrRegBank(VReg).is<const TargetRegisterClass *>() &&
          "Can't set the size of a non-generic virtual register");
   getVRegToSize()[VReg] = Size;
 }
 
 unsigned
 MachineRegisterInfo::createGenericVirtualRegister(unsigned Size) {
   assert(Size && "Cannot create empty virtual register");
 
   // New virtual register number.
-  unsigned Reg = TargetRegisterInfo::index2VirtReg(getNumVirtRegs());
-  VRegInfo.grow(Reg);
-  // FIXME: Should we use a dummy register bank?
+  unsigned Reg = createIncompleteVirtualRegister();
+  // FIXME: Should we use a dummy register class?
   VRegInfo[Reg].first = static_cast<RegisterBank *>(nullptr);
   getVRegToSize()[Reg] = Size;
-  RegAllocHints.grow(Reg);
   if (TheDelegate)
     TheDelegate->MRI_NoteNewVirtualRegister(Reg);
   return Reg;
 }
 
 /// clearVirtRegs - Remove all virtual registers (after physreg assignment).
 void MachineRegisterInfo::clearVirtRegs() {
 #ifndef NDEBUG

test/CodeGen/MIR/X86/undefined-virtual-register.mir

 name:            test
 isSSA:           true
 tracksRegLiveness: true
 registers:
   - { id: 0, class: gr32 }
 body: |
   bb.0.entry:
     %0 = COPY %edi
-    ; CHECK: [[@LINE+1]]:17: use of undefined virtual register '%10'
-    %eax = COPY %10
+    ; CHECK: Cannot determine class/bank of virtual register 1 in function 'test'
+    %eax = COPY %1
     RETQ %eax
 ...