Index: lib/CodeGen/LiveDebugValues.cpp
===================================================================
--- lib/CodeGen/LiveDebugValues.cpp
+++ lib/CodeGen/LiveDebugValues.cpp
@@ -38,6 +38,7 @@
 #include "llvm/CodeGen/TargetFrameLowering.h"
 #include "llvm/CodeGen/TargetInstrInfo.h"
 #include "llvm/CodeGen/TargetLowering.h"
+#include "llvm/CodeGen/TargetPassConfig.h"
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 #include "llvm/Config/llvm-config.h"
@@ -143,7 +144,8 @@
     enum VarLocKind {
       InvalidKind = 0,
       RegisterKind,
-      SpillLocKind
+      SpillLocKind,
+      EntryValueKind
     } Kind = InvalidKind;
 
     /// The value location. Stored separately to avoid repeatedly
@@ -154,15 +156,16 @@
       uint64_t Hash;
     } Loc;
 
-    VarLoc(const MachineInstr &MI, LexicalScopes &LS)
+    VarLoc(const MachineInstr &MI, LexicalScopes &LS, VarLocKind K = InvalidKind)
         : Var(MI.getDebugVariable(), MI.getDebugLoc()->getInlinedAt()), MI(MI),
-          UVS(MI.getDebugLoc(), LS) {
+          UVS(MI.getDebugLoc(), LS), Kind(K) {
       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;
+        if (Kind != EntryValueKind)
+          Kind = RegisterKind;
         Loc.RegNo = RegNo;
       }
     }
@@ -186,6 +189,10 @@
       return 0;
     }
 
+    bool isEntryValueKind() const {
+      return Kind == EntryValueKind;
+    }
+
     /// Determine whether the lexical scope of this value's debug location
     /// dominates MBB.
     bool dominates(MachineBasicBlock &MBB) const { return UVS.dominates(&MBB); }
@@ -195,17 +202,21 @@
 #endif
 
     bool operator==(const VarLoc &Other) const {
-      return Var == Other.Var && Loc.Hash == Other.Loc.Hash;
+      return Var == Other.Var &&
+             Loc.Hash == Other.Loc.Hash && Kind == Other.Kind;
     }
 
     /// This operator guarantees that VarLocs are sorted by Variable first.
     bool operator<(const VarLoc &Other) const {
-      if (Var == Other.Var)
+      if (Var != Other.Var)
+        return Var < Other.Var;
+      if (Other.Kind == Kind)
         return Loc.Hash < Other.Loc.Hash;
-      return Var < Other.Var;
+      return Other.Kind < Kind;
     }
   };
 
+  using ParamSet = SmallVector<MachineInstr *, 6>;
   using VarLocMap = UniqueVector<VarLoc>;
   using VarLocSet = SparseBitVector<>;
   using VarLocInMBB = SmallDenseMap<const MachineBasicBlock *, VarLocSet>;
@@ -282,15 +293,22 @@
                           VarLocMap &VarLocIDs);
   void transferSpillOrRestoreInst(MachineInstr &MI, OpenRangesSet &OpenRanges,
                                   VarLocMap &VarLocIDs, TransferMap &Transfers);
+  void emitEntryValues(MachineInstr &MI, OpenRangesSet &OpenRanges,
+                       VarLocMap &VarLocIDs, TransferMap &Transfers,
+                       ParamSet &ParamEntryVals,
+                       SparseBitVector<> &KillSet);
   void transferRegisterCopy(MachineInstr &MI, OpenRangesSet &OpenRanges,
                             VarLocMap &VarLocIDs, TransferMap &Transfers);
   void transferRegisterDef(MachineInstr &MI, OpenRangesSet &OpenRanges,
-                           const VarLocMap &VarLocIDs);
+                           VarLocMap &VarLocIDs, TransferMap &Transfers,
+                           ParamSet &ParamEntryVals);
   bool transferTerminatorInst(MachineInstr &MI, OpenRangesSet &OpenRanges,
                               VarLocInMBB &OutLocs, const VarLocMap &VarLocIDs);
   bool process(MachineInstr &MI, OpenRangesSet &OpenRanges,
                VarLocInMBB &OutLocs, VarLocMap &VarLocIDs,
-               TransferMap &Transfers, bool transferChanges);
+               TransferMap &Transfers,
+               ParamSet &ParamEntryVals,
+               bool transferChanges);
 
   bool join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs, VarLocInMBB &InLocs,
             const VarLocMap &VarLocIDs,
@@ -408,14 +426,66 @@
   OpenRanges.erase(V);
 
   // Add the VarLoc to OpenRanges from this DBG_VALUE.
-  // TODO: Currently handles DBG_VALUE which has only reg as location.
+  // TODO: Currently handles DBG_VALUE which has only reg
+  // (with/out entry value) as location.
+  VarLoc::VarLocKind K = VarLoc::InvalidKind;
   if (isDbgValueDescribedByReg(MI)) {
-    VarLoc VL(MI, LS);
+    K = VarLoc::RegisterKind;
+    const DIExpression *Expr = MI.getDebugExpression();
+    if (Expr->isEntryValue())
+      K = VarLoc::EntryValueKind;
+  }
+
+  if (K != VarLoc::InvalidKind) {
+    VarLoc VL(MI, LS, K);
     unsigned ID = VarLocIDs.insert(VL);
     OpenRanges.insert(ID, VL.Var);
   }
 }
 
+void LiveDebugValues::emitEntryValues(MachineInstr &MI,
+                                      OpenRangesSet &OpenRanges,
+                                      VarLocMap &VarLocIDs,
+                                      TransferMap &Transfers,
+                                      ParamSet &ParamEntryVals,
+                                      SparseBitVector<> &KillSet) {
+  MachineFunction *MF = MI.getParent()->getParent();
+  for (unsigned ID : KillSet) {
+    if (!VarLocIDs[ID].Var.getVar()->isParameter())
+      continue;
+
+    const MachineInstr *CurrDMI = &VarLocIDs[ID].MI;
+    auto DMI = std::find_if(ParamEntryVals.begin(), ParamEntryVals.end(),
+                            [&CurrDMI](MachineInstr *MII) {
+                              return CurrDMI->getDebugVariable() ==
+                                     MII->getDebugVariable();
+                            });
+
+    // If parameter's DBG_VALUE is not in the vector that means that its value
+    // is changed troughout the function. That means that we can't
+    // use parameter's entry value.
+    if (DMI == ParamEntryVals.end())
+      continue;
+
+    auto &ParamDMI = **DMI;
+    DIExpression *NewExpr =
+        DIExpression::prepend(ParamDMI.getDebugExpression(), false, 0, false,
+                              false, DIExpression::WithEntryValue);
+    MachineInstr *EntryValDbgMI = BuildMI(
+        *MF, ParamDMI.getDebugLoc(), ParamDMI.getDesc(),
+        ParamDMI.isIndirectDebugValue(), ParamDMI.getOperand(0).getReg(),
+        ParamDMI.getDebugVariable(), NewExpr);
+
+    if (ParamDMI.isIndirectDebugValue())
+      EntryValDbgMI->getOperand(1).setImm(ParamDMI.getOperand(1).getImm());
+
+    Transfers.push_back({&MI, EntryValDbgMI});
+    VarLoc VL(*EntryValDbgMI, LS, VarLoc::EntryValueKind);
+    unsigned EntryValLocID = VarLocIDs.insert(VL);
+    OpenRanges.insert(EntryValLocID, VL.Var);
+  }
+}
+
 /// 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
@@ -492,9 +562,9 @@
 }
 
 /// A definition of a register may mark the end of a range.
-void LiveDebugValues::transferRegisterDef(MachineInstr &MI,
-                                          OpenRangesSet &OpenRanges,
-                                          const VarLocMap &VarLocIDs) {
+void LiveDebugValues::transferRegisterDef(
+    MachineInstr &MI, OpenRangesSet &OpenRanges, VarLocMap &VarLocIDs,
+    TransferMap &Transfers, ParamSet &ParamEntryVals) {
   MachineFunction *MF = MI.getMF();
   const TargetLowering *TLI = MF->getSubtarget().getTargetLowering();
   unsigned SP = TLI->getStackPointerRegisterToSaveRestore();
@@ -524,6 +594,12 @@
     }
   }
   OpenRanges.erase(KillSet, VarLocIDs);
+
+  auto *TPC = getAnalysisIfAvailable<TargetPassConfig>();
+  auto &TM = TPC->getTM<TargetMachine>();
+  if (TM.Options.EnableParamEntryValues)
+    emitEntryValues(MI, OpenRanges, VarLocIDs, Transfers, ParamEntryVals,
+                    KillSet);
 }
 
 /// Decide if @MI is a spill instruction and return true if it is. We use 2
@@ -711,10 +787,13 @@
 /// This routine creates OpenRanges and OutLocs.
 bool LiveDebugValues::process(MachineInstr &MI, OpenRangesSet &OpenRanges,
                               VarLocInMBB &OutLocs, VarLocMap &VarLocIDs,
-                              TransferMap &Transfers, bool transferChanges) {
+                              TransferMap &Transfers,
+                              ParamSet &ParamEntryVals,
+                              bool transferChanges) {
   bool Changed = false;
   transferDebugValue(MI, OpenRanges, VarLocIDs);
-  transferRegisterDef(MI, OpenRanges, VarLocIDs);
+  transferRegisterDef(MI, OpenRanges, VarLocIDs, Transfers,
+                      ParamEntryVals);
   if (transferChanges) {
     transferRegisterCopy(MI, OpenRanges, VarLocIDs, Transfers);
     transferSpillOrRestoreInst(MI, OpenRanges, VarLocIDs, Transfers);
@@ -850,15 +929,53 @@
 
   enum : bool { dontTransferChanges = false, transferChanges = true };
 
+  // Insert DEBUG_VALUE instructions that track parameters into ParamEntryVals.
+  // When we encounter a variable that we have already entered
+  // we assume that we have found all parameter's entry locations and stop.
+  // Note: We check whether a MI is inlined in order to deduce whether
+  // the variable that it tracks comes from a different function.
+  // If that is the case we can't track its entry value.
+  // TODO: Add support for modified arguments that can be expreseed
+  // by using its entry value.
+  ParamSet ParamEntryVals;
+  MachineBasicBlock &First_MBB = *(MF.begin());
+
+  auto IsNewParameter = [&ParamEntryVals](const MachineInstr &MI) {
+    for (auto MII : ParamEntryVals)
+      if (MII->getDebugVariable() == MI.getDebugVariable())
+        return false;
+    return true;
+  };
+
+  const TargetLowering *TLI = MF.getSubtarget().getTargetLowering();
+  unsigned SP = TLI->getStackPointerRegisterToSaveRestore();
+  for (auto &MI : First_MBB)
+    if (MI.isDebugValue() && MI.getDebugVariable()->isParameter() &&
+        MI.getDebugVariable()->isNotModified() &&
+        !MI.getDebugLoc()->getInlinedAt() && MI.getOperand(0).isReg() &&
+        MI.getOperand(0).getReg() && MI.getOperand(0).getReg() != SP) {
+      if (IsNewParameter(MI) && MI.getOperand(0).isReg())
+        ParamEntryVals.push_back(&MI);
+      else
+        break;
+    }
+
   // Initialize every mbb with OutLocs.
   // We are not looking at any spill instructions during the initial pass
   // over the BBs. The LiveDebugVariables pass has already created DBG_VALUE
   // instructions for spills of registers that are known to be user variables
   // within the BB in which the spill occurs.
-  for (auto &MBB : MF)
+  for (auto &MBB : MF) {
     for (auto &MI : MBB)
       process(MI, OpenRanges, OutLocs, VarLocIDs, Transfers,
-              dontTransferChanges);
+              ParamEntryVals, dontTransferChanges);
+    // Add any entry DBG_VALUE instructions necessitated by parameter
+    // clobbering.
+    for (auto &TR : Transfers)
+      MBB.insertAfter(MachineBasicBlock::iterator(*TR.TransferInst),
+                      TR.DebugInst);
+    Transfers.clear();
+  }
 
   auto hasNonArtificialLocation = [](const MachineInstr &MI) -> bool {
     if (const DebugLoc &DL = MI.getDebugLoc())
@@ -905,7 +1022,7 @@
         // correspond to user variables.
         for (auto &MI : *MBB)
           OLChanged |= process(MI, OpenRanges, OutLocs, VarLocIDs, Transfers,
-                               transferChanges);
+                               ParamEntryVals, transferChanges);
 
         // Add any DBG_VALUE instructions necessitated by spills.
         for (auto &TR : Transfers)
Index: test/DebugInfo/MIR/X86/dbginfo-entryvals.mir
===================================================================
--- /dev/null
+++ test/DebugInfo/MIR/X86/dbginfo-entryvals.mir
@@ -0,0 +1,115 @@
+# RUN: llc -param-entry-values -run-pass=livedebugvalues -march=x86-64 -o - %s | FileCheck %s
+#
+#extern void fn2(int);
+#
+#__attribute__((noinline))
+#void
+#fn1 (int x, int y) {
+#  int u = x + y;
+#  if (x > 1)
+#    u += 1;
+#  else
+#    u += 2;
+#  int a = 7;
+#  fn2 (a);
+#  u --;
+#}
+# CHECK: DBG_VALUE $edi, $noreg, !14, !DIExpression(DW_OP_entry_value, 1), debug-location {{.*}}
+# CHECK: DBG_VALUE $esi, $noreg, !15, !DIExpression(DW_OP_entry_value, 1), debug-location {{.*}}
+
+--- |
+  ; ModuleID = 'test.c'
+  source_filename = "test.c"
+  target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+  target triple = "x86_64-unknown-linux-gnu"
+  
+  ; Function Attrs: noinline nounwind uwtable
+  define dso_local void @fn1(i32 %x, i32 %y) local_unnamed_addr !dbg !9 {
+  entry:
+    call void @llvm.dbg.value(metadata i32 %x, metadata !14, metadata !DIExpression()), !dbg !18
+    call void @llvm.dbg.value(metadata i32 %y, metadata !15, metadata !DIExpression()), !dbg !19
+    call void @llvm.dbg.value(metadata i32 7, metadata !17, metadata !DIExpression()), !dbg !20
+    tail call void @fn2(i32 7) #3, !dbg !21
+    ret void, !dbg !22
+  }
+  
+  declare !dbg !4 dso_local void @fn2(i32) local_unnamed_addr
+  
+  ; Function Attrs: nounwind readnone speculatable
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+  
+  ; Function Attrs: nounwind
+  declare void @llvm.stackprotector(i8*, i8**)
+  
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!5, !6, !7}
+  !llvm.ident = !{!8}
+  
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 9.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !3, nameTableKind: None)
+  !1 = !DIFile(filename: "test.c", directory: "/dir")
+  !2 = !{}
+  !3 = !{!4}
+  !4 = !DISubprogram(name: "fn2", scope: !1, file: !1, line: 11, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !2)
+  !5 = !{i32 2, !"Dwarf Version", i32 4}
+  !6 = !{i32 2, !"Debug Info Version", i32 3}
+  !7 = !{i32 1, !"wchar_size", i32 4}
+  !8 = !{!"clang version 9.0.0"}
+  !9 = distinct !DISubprogram(name: "fn1", scope: !1, file: !1, line: 15, type: !10, scopeLine: 15, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !13)
+  !10 = !DISubroutineType(types: !11)
+  !11 = !{null, !12, !12}
+  !12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !13 = !{!14, !15, !16, !17}
+  !14 = !DILocalVariable(name: "x", arg: 1, scope: !9, file: !1, line: 15, type: !12, flags: DIFlagArgumentNotModified)
+  !15 = !DILocalVariable(name: "y", arg: 2, scope: !9, file: !1, line: 15, type: !12, flags: DIFlagArgumentNotModified)
+  !16 = !DILocalVariable(name: "u", scope: !9, file: !1, line: 16, type: !12, flags: DIFlagArgumentNotModified)
+  !17 = !DILocalVariable(name: "a", scope: !9, file: !1, line: 21, type: !12, flags: DIFlagArgumentNotModified)
+  !18 = !DILocation(line: 15, column: 10, scope: !9)
+  !19 = !DILocation(line: 15, column: 17, scope: !9)
+  !20 = !DILocation(line: 21, column: 7, scope: !9)
+  !21 = !DILocation(line: 22, column: 3, scope: !9)
+  !22 = !DILocation(line: 24, column: 1, scope: !9)
+
+...
+---
+name:            fn1
+alignment:       4
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+registers:       []
+liveins:         []
+frameInfo:       
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    0
+  adjustsStack:    false
+  hasCalls:        false
+  stackProtector:  ''
+  maxCallFrameSize: 0
+  cvBytesOfCalleeSavedRegisters: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:      []
+stack:           []
+constants:       []
+body:             |
+  bb.0.entry:
+    DBG_VALUE $edi, $noreg, !14, !DIExpression(), debug-location !18
+    DBG_VALUE $esi, $noreg, !15, !DIExpression(), debug-location !19
+    DBG_VALUE 7, $noreg, !17, !DIExpression(), debug-location !20
+    $edi = MOV32ri 7, debug-location !21
+    TAILJMPd64 @fn2, csr_64, implicit $rsp, implicit $ssp, implicit $rsp, implicit $ssp, implicit killed $edi, debug-location !21
+
+...