[DebugInfo] Handle restore instructions in LiveDebugValues

[wolfgangp]

Summary

We recognize spill instructions in LiveDebugValues, i.e. we create DBG_VALUE instructions for variables that are spilled with the appropriate spill locations. We do not, however, recognize the corresponding restore instructions, which can cause large gaps in location information. For example: given a basic block BB with 2 or more predecessors, a variable is live in a register at the exit of the first predecessor, but has been spilled in one or more of the other predecessors. The compiler of course restores the variable at the end of those predecessors, but at present we fail to recognize this. Consequently we fail to provide location information (i.e. we don't generate DBG_VALUE instructions) for the variable in BB because of seemingly conflicting locations.

This patch attempts to rectify this by recognizing restore instructions and folding them into the existing concept of 'transfers', similar to how copies are handled (see https://reviews.llvm.org/D44016).

In essence the patch consists of the following changes:

1. Enhancing the VarLoc structure to keep track of spill locations.
2. Introducing recognition of restore instructions analogous to recognition of spill instructions.
3. Refactoring 'insertTransferDebugPair()' to distinguish between copies, spills and restores. This causes new DBG_VALUE instructions to be generated in response to restores.

I haven't collected any data yet on improvements to location coverage, but will do so shortly.

[aprantl]

Thanks, looks generally reasonable!

lib/CodeGen/LiveDebugValues.cpp

432

as far as I can this lambda is called three times with the same arguments at the end of each if block. Why can't this code just remain at the end of the function?
Ah.. because VarLoc doesn't yet have an empty constructor?

444

I think with three cases, this qualifies for a switch statement :-)

591

This comment makes it sound as if this should be a method of MachineInstr used by both instead?

641

This is almost the same function as above; how about merging the two for the sake of code size?

[NikolaPrica]

LGTM!

[wolfgangp]

• Added methods to MachineInstr to recognize Spills and restores and have AsmPrinter and LIvedebugValues use them
• Unified tranferSpill and transferRestore functions.
• Addressed other minor review comments.

lib/CodeGen/LiveDebugValues.cpp

432

Right. Restructuring VarLoc a bit could help with this, but the lambda seemed to be slightly less awkward option. I could attempt this if you're interested, but perhaps a different patch would be better?

591

I gave MachineInstr a few more functions to identify spill and restore instructions. Could be done in a different patch as well.

[aprantl]

include/llvm/CodeGen/MachineInstr.h

1403

Might be better to call this getSpillInstructionSize if it isn't returning a bool. Naming multi-purpose functions i always awkward :-)

[aprantl]

include/llvm/CodeGen/MachineInstr.h

1403

Either:

Optional<unsigned> getSpillInstruction(const TargetInstrInfo *TII) const;

which makes the uses in AsmPrinter a bit odd.

Or one of
bool isSpillInstruction(const TargetInstrInfo *TII, unsigned &Size) const;
bool isSpillInstruction(const TargetInstrInfo *TII, unsigned *Size = nullptr) const;

which looks less elegant.

[wolfgangp]

include/llvm/CodeGen/MachineInstr.h

1403

How about

Optional<unsigned> getSpillSize(const TargetInstrInfo *TII) const;

etc.

This conveys retrieving the size as the main purpose of the function (we're not interested in the size of the instruction, only the size of the spilled/restored entity). If the instruction is not a spill/restore no meaningful size is returned (as opposed to size 0, which means we found a valid restore/spill but couldn't figure out what it's (re)storing).

[wolfgangp]

Changed the function names from isSpillInstruction/isRestoreInstruction to getSpillSize/getRestoreSize.

[aprantl]

include/llvm/CodeGen/MachineInstr.h

1403

Looks good!

lib/CodeGen/LiveDebugValues.cpp

542–543

I'd add something like: // This is not a spill instruction.

632

else if

632–639

else continue;
then we don't need Match any more.

[wolfgangp]

• Addressed review comments on code structure and clarifying comments.

include/llvm/CodeGen/MachineInstr.h

   bool hasUnmodeledSideEffects() const;
 
   /// Returns true if it is illegal to fold a load across this instruction.
   bool isLoadFoldBarrier() const;
 
   /// Return true if all the defs of this instruction are dead.
   bool allDefsAreDead() const;
 
+  /// Return a valid size if the instruction is a spill instruction.
+  Optional<unsigned> getSpillSize(const TargetInstrInfo *TII) const;
+
+  /// Return a valid size if the instruction is a folded spill instruction.
+  Optional<unsigned> getFoldedSpillSize(const TargetInstrInfo *TII) const;
+
+  /// Return a valid size if the instruction is a restore instruction.
+  Optional<unsigned> getRestoreSize(const TargetInstrInfo *TII) const;
+
+  /// Return a valid size if the instruction is a folded restore instruction.
+  Optional<unsigned>
+  getFoldedRestoreSize(const TargetInstrInfo *TII) const;
+
   /// Copy implicit register operands from specified
   /// instruction to this instruction.
   void copyImplicitOps(MachineFunction &MF, const MachineInstr &MI);
 
   /// Debugging support
   /// @{
   /// Determine the generic type to be printed (if needed) on uses and defs.
   LLT getTypeToPrint(unsigned OpIdx, SmallBitVector &PrintedTypes,

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

 /// It returns true iff the sched comment was emitted.
 ///   Otherwise it returns false.
 static bool emitComments(const MachineInstr &MI, raw_ostream &CommentOS,
                          AsmPrinter *AP) {
   const MachineFunction *MF = MI.getMF();
   const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
 
   // Check for spills and reloads
-  int FI;
-
-  const MachineFrameInfo &MFI = MF->getFrameInfo();
   bool Commented = false;
 
-  auto getSize =
-      [&MFI](const SmallVectorImpl<const MachineMemOperand *> &Accesses) {
-        unsigned Size = 0;
-        for (auto A : Accesses)
-          if (MFI.isSpillSlotObjectIndex(
-                  cast<FixedStackPseudoSourceValue>(A->getPseudoValue())
-                      ->getFrameIndex()))
-            Size += A->getSize();
-        return Size;
-      };
-
   // We assume a single instruction only has a spill or reload, not
   // both.
-  const MachineMemOperand *MMO;
-  SmallVector<const MachineMemOperand *, 2> Accesses;
-  if (TII->isLoadFromStackSlotPostFE(MI, FI)) {
-    if (MFI.isSpillSlotObjectIndex(FI)) {
-      MMO = *MI.memoperands_begin();
-      CommentOS << MMO->getSize() << "-byte Reload";
-      Commented = true;
-    }
-  } else if (TII->hasLoadFromStackSlot(MI, Accesses)) {
-    if (auto Size = getSize(Accesses)) {
-      CommentOS << Size << "-byte Folded Reload";
+  Optional<unsigned> Size;
+  if ((Size = MI.getRestoreSize(TII))) {
+    CommentOS << *Size << "-byte Reload";
+  } else if ((Size = MI.getFoldedRestoreSize(TII))) {
+    if (*Size) {
+      CommentOS << *Size << "-byte Folded Reload";
       Commented = true;
     }
-  } else if (TII->isStoreToStackSlotPostFE(MI, FI)) {
-    if (MFI.isSpillSlotObjectIndex(FI)) {
-      MMO = *MI.memoperands_begin();
-      CommentOS << MMO->getSize() << "-byte Spill";
-      Commented = true;
-    }
-  } else if (TII->hasStoreToStackSlot(MI, Accesses)) {
-    if (auto Size = getSize(Accesses)) {
-      CommentOS << Size << "-byte Folded Spill";
+  } else if ((Size = MI.getSpillSize(TII))) {
+    CommentOS << *Size << "-byte Spill";
+  } else if ((Size = MI.getFoldedSpillSize(TII))) {
+    if (*Size) {
+      CommentOS << *Size << "-byte Folded Spill";
       Commented = true;
     }
   }
 
   // Check for spill-induced copies
   if (MI.getAsmPrinterFlag(MachineInstr::ReloadReuse)) {
     Commented = true;
     CommentOS << " Reload Reuse";

lib/CodeGen/LiveDebugValues.cpp

 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineMemOperand.h"
 #include "llvm/CodeGen/MachineOperand.h"
 #include "llvm/CodeGen/PseudoSourceValue.h"
+#include "llvm/CodeGen/RegisterScavenging.h"
 #include "llvm/CodeGen/TargetFrameLowering.h"
 #include "llvm/CodeGen/TargetInstrInfo.h"
 #include "llvm/CodeGen/TargetLowering.h"
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
-#include "llvm/CodeGen/RegisterScavenging.h"
 #include "llvm/Config/llvm-config.h"
+#include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DebugLoc.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Module.h"
 #include "llvm/MC/MCRegisterInfo.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/Compiler.h"

 class LiveDebugValues : public MachineFunctionPass {
 private:
   const TargetRegisterInfo *TRI;
   const TargetInstrInfo *TII;
   const TargetFrameLowering *TFI;
   BitVector CalleeSavedRegs;
   LexicalScopes LS;
 
+  enum struct TransferKind { TransferCopy, TransferSpill, TransferRestore };
+
   /// Keeps track of lexical scopes associated with a user value's source
   /// location.
   class UserValueScopes {
     DebugLoc DL;
     LexicalScopes &LS;
     SmallPtrSet<const MachineBasicBlock *, 4> LBlocks;
 
   public:
bool operator<(const DebugVariable &DV) const {
       if (getVar() == DV.getVar())
         return getInlinedAt() < DV.getInlinedAt();
       return getVar() < DV.getVar();
     }
   };
 
   /// A pair of debug variable and value location.
   struct VarLoc {
+    // The location at which a spilled variable resides. It consists of a
+    // register and an offset.
+    struct SpillLoc {
+      unsigned SpillBase;
+      int SpillOffset;
+      bool operator==(const SpillLoc &Other) const {
+        return SpillBase == Other.SpillBase && SpillOffset == Other.SpillOffset;
+      }
+    };
+
     const DebugVariable Var;
     const MachineInstr &MI; ///< Only used for cloning a new DBG_VALUE.
     mutable UserValueScopes UVS;
-    enum { InvalidKind = 0, RegisterKind } Kind = InvalidKind;
+    enum VarLocKind {
+      InvalidKind = 0,
+      RegisterKind,
+      SpillLocKind
+    } Kind = InvalidKind;
 
     /// The value location. Stored separately to avoid repeatedly
     /// extracting it from MI.
     union {
       uint64_t RegNo;
+      SpillLoc SpillLocation;
       uint64_t Hash;
     } Loc;
 
     VarLoc(const MachineInstr &MI, LexicalScopes &LS)
         : Var(MI.getDebugVariable(), MI.getDebugLoc()->getInlinedAt()), MI(MI),
           UVS(MI.getDebugLoc(), LS) {
       static_assert((sizeof(Loc) == sizeof(uint64_t)),
                     "hash does not cover all members of Loc");
       assert(MI.isDebugValue() && "not a DBG_VALUE");
       assert(MI.getNumOperands() == 4 && "malformed DBG_VALUE");
       if (int RegNo = isDbgValueDescribedByReg(MI)) {
         Kind = RegisterKind;
         Loc.RegNo = RegNo;
       }
     }
 
+    /// The constructor for spill locations.
+    VarLoc(const MachineInstr &MI, unsigned SpillBase, int SpillOffset,
+           LexicalScopes &LS)
+        : Var(MI.getDebugVariable(), MI.getDebugLoc()->getInlinedAt()), MI(MI),
+          UVS(MI.getDebugLoc(), LS) {
+      assert(MI.isDebugValue() && "not a DBG_VALUE");
+      assert(MI.getNumOperands() == 4 && "malformed DBG_VALUE");
+      Kind = SpillLocKind;
+      Loc.SpillLocation = {SpillBase, SpillOffset};
+    }
+
     /// If this variable is described by a register, return it,
     /// otherwise return 0.
     unsigned isDescribedByReg() const {
       if (Kind == RegisterKind)
         return Loc.RegNo;
       return 0;
     }
 
public:
     bool empty() const {
       assert(Vars.empty() == VarLocs.empty() && "open ranges are inconsistent");
       return VarLocs.empty();
     }
   };
 
   bool isSpillInstruction(const MachineInstr &MI, MachineFunction *MF,
                           unsigned &Reg);
-  int extractSpillBaseRegAndOffset(const MachineInstr &MI, unsigned &Reg);
+  /// If a given instruction is identified as a spill, return the spill location
+  /// and set \p Reg to the spilled register.
+  Optional<VarLoc::SpillLoc> isRestoreInstruction(const MachineInstr &MI,
+                                                  MachineFunction *MF,
+                                                  unsigned &Reg);
+  /// Given a spill instruction, extract the register and offset used to
+  /// address the spill location in a target independent way.
+  VarLoc::SpillLoc extractSpillBaseRegAndOffset(const MachineInstr &MI);
   void insertTransferDebugPair(MachineInstr &MI, OpenRangesSet &OpenRanges,
                                TransferMap &Transfers, VarLocMap &VarLocIDs,
-                               unsigned OldVarID, unsigned NewReg = 0);
+                               unsigned OldVarID, TransferKind Kind,
+                               unsigned NewReg = 0);
 
   void transferDebugValue(const MachineInstr &MI, OpenRangesSet &OpenRanges,
                           VarLocMap &VarLocIDs);
-  void transferSpillInst(MachineInstr &MI, OpenRangesSet &OpenRanges,
+  void transferSpillOrRestoreInst(MachineInstr &MI, OpenRangesSet &OpenRanges,
                                   VarLocMap &VarLocIDs, TransferMap &Transfers);
   void transferRegisterCopy(MachineInstr &MI, OpenRangesSet &OpenRanges,
                             VarLocMap &VarLocIDs, TransferMap &Transfers);
   void transferRegisterDef(MachineInstr &MI, OpenRangesSet &OpenRanges,
                            const VarLocMap &VarLocIDs);
   bool transferTerminatorInst(MachineInstr &MI, OpenRangesSet &OpenRanges,
                               VarLocInMBB &OutLocs, const VarLocMap &VarLocIDs);
   bool process(MachineInstr &MI, OpenRangesSet &OpenRanges,
                VarLocInMBB &OutLocs, VarLocMap &VarLocIDs,
for (unsigned VLL : L) {
       Out << " MI: ";
       VL.dump();
     }
   }
   Out << "\n";
 }
 #endif
 
-/// Given a spill instruction, extract the register and offset used to
-/// address the spill location in a target independent way.
-int LiveDebugValues::extractSpillBaseRegAndOffset(const MachineInstr &MI,
-                                                  unsigned &Reg) {
+LiveDebugValues::VarLoc::SpillLoc
+LiveDebugValues::extractSpillBaseRegAndOffset(const MachineInstr &MI) {
   assert(MI.hasOneMemOperand() &&
          "Spill instruction does not have exactly one memory operand?");
   auto MMOI = MI.memoperands_begin();
   const PseudoSourceValue *PVal = (*MMOI)->getPseudoValue();
   assert(PVal->kind() == PseudoSourceValue::FixedStack &&
          "Inconsistent memory operand in spill instruction");
   int FI = cast<FixedStackPseudoSourceValue>(PVal)->getFrameIndex();
   const MachineBasicBlock *MBB = MI.getParent();
-  return TFI->getFrameIndexReference(*MBB->getParent(), FI, Reg);
+  unsigned Reg;
+  int Offset = TFI->getFrameIndexReference(*MBB->getParent(), FI, Reg);
+  return {Reg, Offset};
 }
 
 /// End all previous ranges related to @MI and start a new range from @MI
 /// if it is a DBG_VALUE instr.
 void LiveDebugValues::transferDebugValue(const MachineInstr &MI,
                                          OpenRangesSet &OpenRanges,
                                          VarLocMap &VarLocIDs) {
   if (!MI.isDebugValue())

 
 /// Create new TransferDebugPair and insert it in \p Transfers. The VarLoc
 /// with \p OldVarID should be deleted form \p OpenRanges and replaced with
 /// new VarLoc. If \p NewReg is different than default zero value then the
 /// new location will be register location created by the copy like instruction,
 /// otherwise it is variable's location on the stack.
 void LiveDebugValues::insertTransferDebugPair(
     MachineInstr &MI, OpenRangesSet &OpenRanges, TransferMap &Transfers,
-    VarLocMap &VarLocIDs, unsigned OldVarID, unsigned NewReg) {
+    VarLocMap &VarLocIDs, unsigned OldVarID, TransferKind Kind,
+    unsigned NewReg) {
   const MachineInstr *DMI = &VarLocIDs[OldVarID].MI;
   MachineFunction *MF = MI.getParent()->getParent();
   MachineInstr *NewDMI;
-  if (NewReg) {
+
+  auto ProcessVarLoc = [&MI, &OpenRanges, &Transfers,
+                        &VarLocIDs](VarLoc &VL, MachineInstr *NewDMI) {
+    unsigned LocId = VarLocIDs.insert(VL);
+    OpenRanges.insert(LocId, VL.Var);
+    // The newly created DBG_VALUE instruction NewDMI must be inserted after
+    // MI. Keep track of the pairing.
+    TransferDebugPair MIP = {&MI, NewDMI};
+    Transfers.push_back(MIP);
+  };
+
+  // End all previous ranges of Var.
+  OpenRanges.erase(VarLocIDs[OldVarID].Var);
+  switch (Kind) {
+  case TransferKind::TransferCopy: {
+    assert(NewReg &&
+           "No register supplied when handling a copy of a debug value");
     // Create a DBG_VALUE instruction to describe the Var in its new
     // register location.
     NewDMI = BuildMI(*MF, DMI->getDebugLoc(), DMI->getDesc(),
                      DMI->isIndirectDebugValue(), NewReg,
                      DMI->getDebugVariable(), DMI->getDebugExpression());
     if (DMI->isIndirectDebugValue())
       NewDMI->getOperand(1).setImm(DMI->getOperand(1).getImm());
+    VarLoc VL(*NewDMI, LS);
+    ProcessVarLoc(VL, NewDMI);
     LLVM_DEBUG(dbgs() << "Creating DBG_VALUE inst for register copy: ";
                NewDMI->print(dbgs(), false, false, false, TII));
-  } else {
+    return;
+  }
+  case TransferKind::TransferSpill: {
     // Create a DBG_VALUE instruction to describe the Var in its spilled
     // location.
-    unsigned SpillBase;
-    int SpillOffset = extractSpillBaseRegAndOffset(MI, SpillBase);
-    auto *SpillExpr = DIExpression::prepend(DMI->getDebugExpression(),
-                                            DIExpression::NoDeref, SpillOffset);
-    NewDMI = BuildMI(*MF, DMI->getDebugLoc(), DMI->getDesc(), true, SpillBase,
-                     DMI->getDebugVariable(), SpillExpr);
+    VarLoc::SpillLoc SpillLocation = extractSpillBaseRegAndOffset(MI);
+    auto *SpillExpr =
+        DIExpression::prepend(DMI->getDebugExpression(), DIExpression::NoDeref,
+                              SpillLocation.SpillOffset);
+    NewDMI =
+        BuildMI(*MF, DMI->getDebugLoc(), DMI->getDesc(), true,
+                SpillLocation.SpillBase, DMI->getDebugVariable(), SpillExpr);
+    VarLoc VL(*NewDMI, SpillLocation.SpillBase, SpillLocation.SpillOffset, LS);
+    ProcessVarLoc(VL, NewDMI);
     LLVM_DEBUG(dbgs() << "Creating DBG_VALUE inst for spill: ";
                NewDMI->print(dbgs(), false, false, false, TII));
+    return;
   }
-
-  // The newly created DBG_VALUE instruction NewDMI must be inserted after
-  // MI. Keep track of the pairing.
-  TransferDebugPair MIP = {&MI, NewDMI};
-  Transfers.push_back(MIP);
-
-  // End all previous ranges of Var.
-  OpenRanges.erase(VarLocIDs[OldVarID].Var);
-
-  // Add the VarLoc to OpenRanges.
+  case TransferKind::TransferRestore: {
+    assert(NewReg &&
+           "No register supplied when handling a restore of a debug value");
+    MachineFunction *MF = MI.getMF();
+    DIBuilder DIB(*const_cast<Function &>(MF->getFunction()).getParent());
+    NewDMI = BuildMI(*MF, DMI->getDebugLoc(), DMI->getDesc(), false, NewReg,
+                     DMI->getDebugVariable(), DIB.createExpression());
     VarLoc VL(*NewDMI, LS);
-  unsigned LocID = VarLocIDs.insert(VL);
-  OpenRanges.insert(LocID, VL.Var);
+    ProcessVarLoc(VL, NewDMI);
+    LLVM_DEBUG(dbgs() << "Creating DBG_VALUE inst for register restore: ";
+               NewDMI->print(dbgs(), false, false, false, TII));
+    return;
+  }
+  }
+  llvm_unreachable("Invalid transfer kind");
 }
 
 /// A definition of a register may mark the end of a range.
 void LiveDebugValues::transferRegisterDef(MachineInstr &MI,
                                           OpenRangesSet &OpenRanges,
                                           const VarLocMap &VarLocIDs) {
   MachineFunction *MF = MI.getMF();
   const TargetLowering *TLI = MF->getSubtarget().getTargetLowering();

 /// Decide if @MI is a spill instruction and return true if it is. We use 2
 /// criteria to make this decision:
 /// - Is this instruction a store to a spill slot?
 /// - Is there a register operand that is both used and killed?
 /// TODO: Store optimization can fold spills into other stores (including
 /// other spills). We do not handle this yet (more than one memory operand).
 bool LiveDebugValues::isSpillInstruction(const MachineInstr &MI,
                                          MachineFunction *MF, unsigned &Reg) {
-  const MachineFrameInfo &FrameInfo = MF->getFrameInfo();
-  int FI;
   SmallVector<const MachineMemOperand*, 1> Accesses;
 
   // TODO: Handle multiple stores folded into one.
   if (!MI.hasOneMemOperand())
     return false;
 
-  // To identify a spill instruction, use the same criteria as in AsmPrinter.
-  if (!((TII->isStoreToStackSlotPostFE(MI, FI) &&
-         FrameInfo.isSpillSlotObjectIndex(FI)) ||
-        (TII->hasStoreToStackSlot(MI, Accesses) &&
-         llvm::any_of(Accesses, [&FrameInfo](const MachineMemOperand *MMO) {
-           return FrameInfo.isSpillSlotObjectIndex(
-               cast<FixedStackPseudoSourceValue>(MMO->getPseudoValue())
-                   ->getFrameIndex());
-         }))))
-    return false;
+  if (!MI.getSpillSize(TII) && !MI.getFoldedSpillSize(TII))
+    return false; // This is not a spill instruction, since no valid size was
+                  // returned from either function.
 
   auto isKilledReg = [&](const MachineOperand MO, unsigned &Reg) {
     if (!MO.isReg() || !MO.isUse()) {
       Reg = 0;
       return false;
     }
     Reg = MO.getReg();
     return MO.isKill();
if (Reg != 0) {
           return true;
       }
     }
   }
   // Return false if we didn't find spilled register.
   return false;
 }
 
+Optional<LiveDebugValues::VarLoc::SpillLoc>
+LiveDebugValues::isRestoreInstruction(const MachineInstr &MI,
+                                      MachineFunction *MF, unsigned &Reg) {
+  if (!MI.hasOneMemOperand())
+    return None;
+
+  // FIXME: Handle folded restore instructions with more than one memory
+  // operand.
+  if (MI.getRestoreSize(TII)) {
+    Reg = MI.getOperand(0).getReg();
+    return extractSpillBaseRegAndOffset(MI);
+  }
+  return None;
+}
+
 /// A spilled register may indicate that we have to end the current range of
 /// a variable and create a new one for the spill location.
+/// A restored register may indicate the reverse situation.
 /// We don't want to insert any instructions in process(), so we just create
 /// the DBG_VALUE without inserting it and keep track of it in \p Transfers.
 /// It will be inserted into the BB when we're done iterating over the
 /// instructions.
-void LiveDebugValues::transferSpillInst(MachineInstr &MI,
+void LiveDebugValues::transferSpillOrRestoreInst(MachineInstr &MI,
                                                  OpenRangesSet &OpenRanges,
                                                  VarLocMap &VarLocIDs,
                                                  TransferMap &Transfers) {
-  unsigned Reg;
   MachineFunction *MF = MI.getMF();
-  if (!isSpillInstruction(MI, MF, Reg))
-    return;
+  TransferKind TKind;
+  unsigned Reg;
+  Optional<VarLoc::SpillLoc> Loc;
 
-  // Check if the register is the location of a debug value.
+  if (isSpillInstruction(MI, MF, Reg)) {
+    TKind = TransferKind::TransferSpill;
+    LLVM_DEBUG(dbgs() << "Recognized as spill: "; MI.dump(););
+    LLVM_DEBUG(dbgs() << "Register: " << Reg << " " << printReg(Reg, TRI)
+                      << "\n");
+  } else {
+    if (!(Loc = isRestoreInstruction(MI, MF, Reg)))
+      return;
+    TKind = TransferKind::TransferRestore;
+    LLVM_DEBUG(dbgs() << "Recognized as restore: "; MI.dump(););
+    LLVM_DEBUG(dbgs() << "Register: " << Reg << " " << printReg(Reg, TRI)
+                      << "\n");
+  }
+  // Check if the register or spill location is the location of a debug value.
   for (unsigned ID : OpenRanges.getVarLocs()) {
-    if (VarLocIDs[ID].isDescribedByReg() == Reg) {
+    if (TKind == TransferKind::TransferSpill &&
+        VarLocIDs[ID].isDescribedByReg() == Reg) {
       LLVM_DEBUG(dbgs() << "Spilling Register " << printReg(Reg, TRI) << '('
                         << VarLocIDs[ID].Var.getVar()->getName() << ")\n");
-      insertTransferDebugPair(MI, OpenRanges, Transfers, VarLocIDs, ID);
-      return;
-    }
+    } else if (TKind == TransferKind::TransferRestore &&
+               VarLocIDs[ID].Loc.SpillLocation == *Loc) {
+      LLVM_DEBUG(dbgs() << "Restoring Register " << printReg(Reg, TRI) << '('
+                        << VarLocIDs[ID].Var.getVar()->getName() << ")\n");
+    } else
+      continue;
+    insertTransferDebugPair(MI, OpenRanges, Transfers, VarLocIDs, ID, TKind,
+                            Reg);
   }
 }
 
 /// If \p MI is a register copy instruction, that copies a previously tracked
 /// value from one register to another register that is callee saved, we
 /// create new DBG_VALUE instruction  described with copy destination register.
 void LiveDebugValues::transferRegisterCopy(MachineInstr &MI,
                                            OpenRangesSet &OpenRanges,
                                            VarLocMap &VarLocIDs,
                                            TransferMap &Transfers) {
void LiveDebugValues::transferRegisterCopy(MachineInstr &MI,
   // soon. It is more likely that previous register location, which is callee
   // saved, is going to stay unclobbered longer, even if it is killed.
   if (!isCalleSavedReg(DestReg))
     return;
 
   for (unsigned ID : OpenRanges.getVarLocs()) {
     if (VarLocIDs[ID].isDescribedByReg() == SrcReg) {
       insertTransferDebugPair(MI, OpenRanges, Transfers, VarLocIDs, ID,
-                              DestReg);
+                              TransferKind::TransferCopy, DestReg);
       return;
     }
   }
 }
 
 /// Terminate all open ranges at the end of the current basic block.
 bool LiveDebugValues::transferTerminatorInst(MachineInstr &MI,
                                              OpenRangesSet &OpenRanges,

 bool LiveDebugValues::process(MachineInstr &MI, OpenRangesSet &OpenRanges,
                               VarLocInMBB &OutLocs, VarLocMap &VarLocIDs,
                               TransferMap &Transfers, bool transferChanges) {
   bool Changed = false;
   transferDebugValue(MI, OpenRanges, VarLocIDs);
   transferRegisterDef(MI, OpenRanges, VarLocIDs);
   if (transferChanges) {
     transferRegisterCopy(MI, OpenRanges, VarLocIDs, Transfers);
-    transferSpillInst(MI, OpenRanges, VarLocIDs, Transfers);
+    transferSpillOrRestoreInst(MI, OpenRanges, VarLocIDs, Transfers);
   }
   Changed = transferTerminatorInst(MI, OpenRanges, OutLocs, VarLocIDs);
   return Changed;
 }
 
 /// This routine joins the analysis results of all incoming edges in @MBB by
 /// inserting a new DBG_VALUE instruction at the start of the @MBB - if the same
 /// source variable in all the predecessors of @MBB reside in the same location.

lib/CodeGen/MachineInstr.cpp

 #include "llvm/ADT/SmallBitVector.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/Loads.h"
 #include "llvm/Analysis/MemoryLocation.h"
 #include "llvm/CodeGen/GlobalISel/RegisterBank.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineInstrBundle.h"
 #include "llvm/CodeGen/MachineMemOperand.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/MachineOperand.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/PseudoSourceValue.h"
 #include "llvm/CodeGen/TargetInstrInfo.h"
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DebugLoc.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/InlineAsm.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/ModuleSlotTracker.h"
+#include "llvm/IR/Operator.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/Value.h"
-#include "llvm/IR/Operator.h"
 #include "llvm/MC/MCInstrDesc.h"
 #include "llvm/MC/MCRegisterInfo.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
void MachineInstr::changeDebugValuesDefReg(unsigned Reg) {
   // Collect matching debug values.
   SmallVector<MachineInstr *, 2> DbgValues;
   collectDebugValues(DbgValues);
 
   // Propagate Reg to debug value instructions.
   for (auto *DBI : DbgValues)
     DBI->getOperand(0).setReg(Reg);
 }
+
+using MMOList = SmallVector<const MachineMemOperand *, 2>;
+
+static unsigned getSpillSlotSize(MMOList &Accesses,
+                                 const MachineFrameInfo &MFI) {
+  unsigned Size = 0;
+  for (auto A : Accesses)
+    if (MFI.isSpillSlotObjectIndex(
+            cast<FixedStackPseudoSourceValue>(A->getPseudoValue())
+                ->getFrameIndex()))
+      Size += A->getSize();
+  return Size;
+}
+
+Optional<unsigned>
+MachineInstr::getSpillSize(const TargetInstrInfo *TII) const {
+  int FI;
+  if (TII->isStoreToStackSlotPostFE(*this, FI)) {
+    const MachineFrameInfo &MFI = getMF()->getFrameInfo();
+    if (MFI.isSpillSlotObjectIndex(FI))
+      return (*memoperands_begin())->getSize();
+  }
+  return None;
+}
+
+Optional<unsigned>
+MachineInstr::getFoldedSpillSize(const TargetInstrInfo *TII) const {
+  MMOList Accesses;
+  if (TII->hasStoreToStackSlot(*this, Accesses))
+    return getSpillSlotSize(Accesses, getMF()->getFrameInfo());
+  return None;
+}
+
+Optional<unsigned>
+MachineInstr::getRestoreSize(const TargetInstrInfo *TII) const {
+  int FI;
+  if (TII->isLoadFromStackSlotPostFE(*this, FI)) {
+    const MachineFrameInfo &MFI = getMF()->getFrameInfo();
+    if (MFI.isSpillSlotObjectIndex(FI))
+      return (*memoperands_begin())->getSize();
+  }
+  return None;
+}
+
+Optional<unsigned>
+MachineInstr::getFoldedRestoreSize(const TargetInstrInfo *TII) const {
+  MMOList Accesses;
+  if (TII->hasLoadFromStackSlot(*this, Accesses))
+    return getSpillSlotSize(Accesses, getMF()->getFrameInfo());
+  return None;
+}

test/DebugInfo/MIR/X86/live-debug-values-restore.mir

+# RUN: llc -run-pass livedebugvalues -o - %s | FileCheck %s
+
+# Generated from the following source with:
+# clang -g -mllvm -stop-before=livedebugvalues -S -O2 test.c -o test.mir
+
+# #define FORCE_SPILL() \
+#   __asm volatile("" : : : \
+#                    "rax", "rbx", "rcx", "rdx", "rsi", "rdi", "rbp", "r8", \
+#                    "r9", "r10", "r11", "r12", "r13", "r14", "r15")
+# int f(int *p) {
+#   if (p) {
+#     FORCE_SPILL();
+#   }
+#   return *(p + 1);
+# }
+
+# Ascertain that the spill has been recognized and manifested in a DBG_VALUE.
+# CHECK: MOV64mr $rsp,{{.*-8.*}}killed{{.*}}$rdi :: (store 8 into %stack.0)
+# CHECK-NEXT: DBG_VALUE $rsp,{{.*}}![[MDIX:[0-9]+]],{{.*}}!DIExpression(DW_OP_constu, 8, DW_OP_minus)
+
+# Check for the restore.
+# CHECK: $rdi = MOV64rm $rsp,{{.*-8.*}}:: (load 8 from %stack.0)
+# CHECK-NEXT: DBG_VALUE $rdi,{{.*}}![[MDIX]], !DIExpression()
+
+--- |
+  define dso_local i32 @f(i32* readonly %p) local_unnamed_addr !dbg !7 {
+  entry:
+    call void @llvm.dbg.value(metadata i32* %p, metadata !13, metadata !DIExpression()), !dbg !14
+    %tobool = icmp eq i32* %p, null, !dbg !15
+    br i1 %tobool, label %if.end, label %if.then, !dbg !17
+  
+  if.then:                                          ; preds = %entry
+    tail call void asm sideeffect "", "~{rax},~{rbx},~{rcx},~{rdx},~{rsi},~{rdi},~{rbp},~{r8},~{r9},~{r10},~{r11},~{r12},~{r13},~{r14},~{r15},~{dirflag},~{fpsr},~{flags}"(), !dbg !18, !srcloc !20
+    br label %if.end, !dbg !21
+  
+  if.end:                                           ; preds = %entry, %if.then
+    %add.ptr = getelementptr inbounds i32, i32* %p, i64 1, !dbg !22
+    %0 = load i32, i32* %add.ptr, align 4, !dbg !23, !tbaa !24
+    ret i32 %0, !dbg !28
+  }
+  
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+  
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!3, !4, !5}
+  !llvm.ident = !{!6}
+  
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 9.0.0 (https://git.llvm.org/git/clang.git/ 57a6ce7ac318de98e3e777e09cb9ed8282b5cc03) (https://git.llvm.org/git/llvm.git/ ff54a19e4912d7f15cb02798a7f2048441bff751)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+  !1 = !DIFile(filename: "test2.c", directory: "/home/test")
+  !2 = !{}
+  !3 = !{i32 2, !"Dwarf Version", i32 4}
+  !4 = !{i32 2, !"Debug Info Version", i32 3}
+  !5 = !{i32 1, !"wchar_size", i32 4}
+  !6 = !{!"clang version 9.0.0 (https://git.llvm.org/git/clang.git/ 57a6ce7ac318de98e3e777e09cb9ed8282b5cc03) (https://git.llvm.org/git/llvm.git/ ff54a19e4912d7f15cb02798a7f2048441bff751)"}
+  !7 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 5, type: !8, scopeLine: 5, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+  !8 = !DISubroutineType(types: !9)
+  !9 = !{!10, !11}
+  !10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !11 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !10, size: 64)
+  !12 = !{!13}
+  !13 = !DILocalVariable(name: "p", arg: 1, scope: !7, file: !1, line: 5, type: !11)
+  !14 = !DILocation(line: 5, column: 12, scope: !7)
+  !15 = !DILocation(line: 6, column: 7, scope: !16)
+  !16 = distinct !DILexicalBlock(scope: !7, file: !1, line: 6, column: 7)
+  !17 = !DILocation(line: 6, column: 7, scope: !7)
+  !18 = !DILocation(line: 7, column: 5, scope: !19)
+  !19 = distinct !DILexicalBlock(scope: !16, file: !1, line: 6, column: 10)
+  !20 = !{i32 -2147471544}
+  !21 = !DILocation(line: 8, column: 3, scope: !19)
+  !22 = !DILocation(line: 9, column: 14, scope: !7)
+  !23 = !DILocation(line: 9, column: 10, scope: !7)
+  !24 = !{!25, !25, i64 0}
+  !25 = !{!"int", !26, i64 0}
+  !26 = !{!"omnipotent char", !27, i64 0}
+  !27 = !{!"Simple C/C++ TBAA"}
+  !28 = !DILocation(line: 9, column: 3, scope: !7)
+
+...
+---
+name:            f
+alignment:       4
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+registers:       []
+liveins:         
+  - { reg: '$rdi', virtual-reg: '' }
+frameInfo:       
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       48
+  offsetAdjustment: -48
+  maxAlignment:    8
+  adjustsStack:    false
+  hasCalls:        false
+  stackProtector:  ''
+  maxCallFrameSize: 0
+  cvBytesOfCalleeSavedRegisters: 48
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:      
+  - { id: 0, type: spill-slot, offset: -56, size: 8, alignment: 8, stack-id: 0, 
+      callee-saved-register: '$rbx', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+  - { id: 1, type: spill-slot, offset: -48, size: 8, alignment: 16, stack-id: 0, 
+      callee-saved-register: '$r12', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+  - { id: 2, type: spill-slot, offset: -40, size: 8, alignment: 8, stack-id: 0, 
+      callee-saved-register: '$r13', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+  - { id: 3, type: spill-slot, offset: -32, size: 8, alignment: 16, stack-id: 0, 
+      callee-saved-register: '$r14', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+  - { id: 4, type: spill-slot, offset: -24, size: 8, alignment: 8, stack-id: 0, 
+      callee-saved-register: '$r15', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+  - { id: 5, type: spill-slot, offset: -16, size: 8, alignment: 16, stack-id: 0, 
+      callee-saved-register: '$rbp', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+stack:           
+  - { id: 0, name: '', type: spill-slot, offset: -64, size: 8, alignment: 8, 
+      stack-id: 0, callee-saved-register: '', callee-saved-restored: true, 
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+constants:       []
+body:             |
+  bb.0.entry:
+    successors: %bb.2(0x30000000), %bb.1(0x50000000)
+    liveins: $rdi, $rbx, $r12, $r13, $r14, $r15, $rbp
+  
+    DBG_VALUE $rdi, $noreg, !13, !DIExpression(), debug-location !14
+    DBG_VALUE $rdi, $noreg, !13, !DIExpression(), debug-location !14
+    TEST64rr renamable $rdi, renamable $rdi, implicit-def $eflags, debug-location !15
+    JE_1 %bb.2, implicit $eflags, debug-location !17
+  
+  bb.1.if.then:
+    successors: %bb.2(0x80000000)
+    liveins: $rdi, $rbp, $r15, $r14, $r13, $r12, $rbx
+  
+    frame-setup PUSH64r killed $rbp, implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 16
+    frame-setup PUSH64r killed $r15, implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 24
+    frame-setup PUSH64r killed $r14, implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 32
+    frame-setup PUSH64r killed $r13, implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 40
+    frame-setup PUSH64r killed $r12, implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 48
+    frame-setup PUSH64r killed $rbx, implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 56
+    CFI_INSTRUCTION offset $rbx, -56
+    CFI_INSTRUCTION offset $r12, -48
+    CFI_INSTRUCTION offset $r13, -40
+    CFI_INSTRUCTION offset $r14, -32
+    CFI_INSTRUCTION offset $r15, -24
+    CFI_INSTRUCTION offset $rbp, -16
+    MOV64mr $rsp, 1, $noreg, -8, $noreg, killed renamable $rdi :: (store 8 into %stack.0)
+    INLINEASM &"", 1, 12, implicit-def dead early-clobber $rax, 12, implicit-def dead early-clobber $rbx, 12, implicit-def dead early-clobber $rcx, 12, implicit-def dead early-clobber $rdx, 12, implicit-def dead early-clobber $rsi, 12, implicit-def dead early-clobber $rdi, 12, implicit-def dead early-clobber $rbp, 12, implicit-def dead early-clobber $r8, 12, implicit-def dead early-clobber $r9, 12, implicit-def dead early-clobber $r10, 12, implicit-def dead early-clobber $r11, 12, implicit-def dead early-clobber $r12, 12, implicit-def dead early-clobber $r13, 12, implicit-def dead early-clobber $r14, 12, implicit-def dead early-clobber $r15, 12, implicit-def dead early-clobber $eflags, !20, debug-location !18
+    renamable $rdi = MOV64rm $rsp, 1, $noreg, -8, $noreg :: (load 8 from %stack.0)
+    $rbx = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 48
+    $r12 = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 40
+    $r13 = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 32
+    $r14 = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 24
+    $r15 = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 16
+    $rbp = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    CFI_INSTRUCTION def_cfa_offset 8
+  
+  bb.2.if.end:
+    liveins: $rdi, $rbx, $r12, $r13, $r14, $r15, $rbp
+  
+    renamable $eax = MOV32rm killed renamable $rdi, 1, $noreg, 4, $noreg, debug-location !23 :: (load 4 from %ir.add.ptr, !tbaa !24)
+    RETQ $eax, debug-location !28
+
+...