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 +156,17 @@
       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 +190,10 @@
       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 +203,18 @@
 #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)
-        return Loc.Hash < Other.Loc.Hash;
-      return Var < Other.Var;
+      return std::tie(Var, Kind, Loc.Hash) <
+             std::tie(Other.Var, Other.Kind, Other.Loc.Hash);
     }
   };
 
+  using DebugParamMap = SmallDenseMap<const DILocalVariable *, MachineInstr *>;
   using VarLocMap = UniqueVector<VarLoc>;
   using VarLocSet = SparseBitVector<>;
   using VarLocInMBB = SmallDenseMap<const MachineBasicBlock *, VarLocSet>;
@@ -282,15 +291,22 @@
                           VarLocMap &VarLocIDs);
   void transferSpillOrRestoreInst(MachineInstr &MI, OpenRangesSet &OpenRanges,
                                   VarLocMap &VarLocIDs, TransferMap &Transfers);
+  void emitEntryValues(MachineInstr &MI, OpenRangesSet &OpenRanges,
+                       VarLocMap &VarLocIDs, TransferMap &Transfers,
+                       DebugParamMap &DebugEntryVals,
+                       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,
+                           DebugParamMap &DebugEntryVals);
   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,
+               DebugParamMap &DebugEntryVals,
+               bool transferChanges);
 
   bool join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs, VarLocInMBB &InLocs,
             const VarLocMap &VarLocIDs,
@@ -408,14 +424,61 @@
   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 Kind = VarLoc::InvalidKind;
   if (isDbgValueDescribedByReg(MI)) {
-    VarLoc VL(MI, LS);
+    Kind = VarLoc::RegisterKind;
+    const DIExpression *Expr = MI.getDebugExpression();
+    if (Expr->isEntryValue())
+      Kind = VarLoc::EntryValueKind;
+  }
+
+  if (Kind != VarLoc::InvalidKind) {
+    VarLoc VL(MI, LS, Kind);
     unsigned ID = VarLocIDs.insert(VL);
     OpenRanges.insert(ID, VL.Var);
   }
 }
 
+void LiveDebugValues::emitEntryValues(MachineInstr &MI,
+                                      OpenRangesSet &OpenRanges,
+                                      VarLocMap &VarLocIDs,
+                                      TransferMap &Transfers,
+                                      DebugParamMap &DebugEntryVals,
+                                      SparseBitVector<> &KillSet) {
+  MachineFunction *MF = MI.getParent()->getParent();
+  for (unsigned ID : KillSet) {
+    if (!VarLocIDs[ID].Var.getVar()->isParameter())
+      continue;
+
+    const MachineInstr *CurrDebugInstr = &VarLocIDs[ID].MI;
+
+    // If parameter's DBG_VALUE is not in the map that means we can't
+    // generate parameter's entry value.
+    if (!DebugEntryVals.count(CurrDebugInstr->getDebugVariable()))
+      continue;
+
+    auto ParamDebugInstr = DebugEntryVals[CurrDebugInstr->getDebugVariable()];
+    DIExpression *NewExpr = DIExpression::prepend(
+        ParamDebugInstr->getDebugExpression(), DIExpression::EntryValue);
+    MachineInstr *EntryValDbgMI =
+        BuildMI(*MF, ParamDebugInstr->getDebugLoc(), ParamDebugInstr->getDesc(),
+                ParamDebugInstr->isIndirectDebugValue(),
+                ParamDebugInstr->getOperand(0).getReg(),
+                ParamDebugInstr->getDebugVariable(), NewExpr);
+
+    if (ParamDebugInstr->isIndirectDebugValue())
+      EntryValDbgMI->getOperand(1).setImm(
+          ParamDebugInstr->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
@@ -495,9 +558,9 @@
 }
 
 /// 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, DebugParamMap &DebugEntryVals) {
   MachineFunction *MF = MI.getMF();
   const TargetLowering *TLI = MF->getSubtarget().getTargetLowering();
   unsigned SP = TLI->getStackPointerRegisterToSaveRestore();
@@ -527,6 +590,13 @@
     }
   }
   OpenRanges.erase(KillSet, VarLocIDs);
+
+  if (auto *TPC = getAnalysisIfAvailable<TargetPassConfig>()) {
+    auto &TM = TPC->getTM<TargetMachine>();
+    if (TM.Options.EnableDebugEntryValues)
+      emitEntryValues(MI, OpenRanges, VarLocIDs, Transfers, DebugEntryVals,
+                      KillSet);
+  }
 }
 
 /// Decide if @MI is a spill instruction and return true if it is. We use 2
@@ -714,10 +784,13 @@
 /// This routine creates OpenRanges and OutLocs.
 bool LiveDebugValues::process(MachineInstr &MI, OpenRangesSet &OpenRanges,
                               VarLocInMBB &OutLocs, VarLocMap &VarLocIDs,
-                              TransferMap &Transfers, bool transferChanges) {
+                              TransferMap &Transfers,
+                              DebugParamMap &DebugEntryVals,
+                              bool transferChanges) {
   bool Changed = false;
   transferDebugValue(MI, OpenRanges, VarLocIDs);
-  transferRegisterDef(MI, OpenRanges, VarLocIDs);
+  transferRegisterDef(MI, OpenRanges, VarLocIDs, Transfers,
+                      DebugEntryVals);
   if (transferChanges) {
     transferRegisterCopy(MI, OpenRanges, VarLocIDs, Transfers);
     transferSpillOrRestoreInst(MI, OpenRanges, VarLocIDs, Transfers);
@@ -854,15 +927,57 @@
 
   enum : bool { dontTransferChanges = false, transferChanges = true };
 
+  // Besides parameter's modification, check whether a DBG_VALUE 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.
+  auto IsUnmodifiedFuncParam = [&](const MachineInstr &MI) {
+    auto *DIVar = MI.getDebugVariable();
+    return DIVar->isParameter() && DIVar->isNotModified() &&
+           !MI.getDebugLoc()->getInlinedAt();
+  };
+
+  const TargetLowering *TLI = MF.getSubtarget().getTargetLowering();
+  unsigned SP = TLI->getStackPointerRegisterToSaveRestore();
+  unsigned FP = TRI->getFrameRegister(MF);
+  auto IsRegOtherThanSPAndFP = [&](const MachineOperand &Op) -> bool {
+    return Op.isReg() && Op.getReg() != SP && Op.getReg() != FP;
+  };
+
+  // Working set of currently collected debug variables mapped to DBG_VALUEs
+  // representing candidates for production of debug entry values.
+  DebugParamMap DebugEntryVals;
+
+  MachineBasicBlock &First_MBB = *(MF.begin());
+  // Only in the case of entry MBB collect DBG_VALUEs representing
+  // function parameters in order to generate debug entry values for them.
+  // Currently, we generate debug entry values only for parameters that are
+  // unmodified throughout the function and located in a register.
+  // TODO: Add support for parameters that are described as fragments.
+  // TODO: Add support for modified arguments that can be expreseed
+  // by using its entry value.
+  for (auto &MI : First_MBB)
+    if (MI.isDebugValue() && IsUnmodifiedFuncParam(MI) &&
+        !MI.isIndirectDebugValue() && IsRegOtherThanSPAndFP(MI.getOperand(0)) &&
+        !DebugEntryVals.count(MI.getDebugVariable()) &&
+        !MI.getDebugExpression()->isFragment())
+      DebugEntryVals[MI.getDebugVariable()] = &MI;
+
   // 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);
+              DebugEntryVals, 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())
@@ -909,7 +1024,7 @@
         // correspond to user variables.
         for (auto &MI : *MBB)
           OLChanged |= process(MI, OpenRanges, OutLocs, VarLocIDs, Transfers,
-                               transferChanges);
+                               DebugEntryVals, 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 +1,79 @@
+# RUN: llc -debug-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"
+  
+  ; 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 !18
+    call void @llvm.dbg.value(metadata i32 7, metadata !17, metadata !DIExpression()), !dbg !18
+    tail call void @fn2(i32 7), !dbg !18
+    ret void, !dbg !18
+  }
+  
+  declare !dbg !4 dso_local void @fn2(i32) local_unnamed_addr
+  
+  ; Function Attrs: nounwind readnone speculatable
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+  
+  
+  !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 | DISPFlagDeclForCallSite, 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)
+
+...
+---
+name:            fn1
+alignment:       4
+tracksRegLiveness: true
+liveins:         []
+body:             |
+  bb.0.entry:
+    DBG_VALUE $edi, $noreg, !14, !DIExpression(), debug-location !18
+    DBG_VALUE $esi, $noreg, !15, !DIExpression(), debug-location !18
+    DBG_VALUE 7, $noreg, !17, !DIExpression(), debug-location !18
+    $edi = MOV32ri 7, debug-location !18
+    TAILJMPd64 @fn2, csr_64, implicit $rsp, implicit $ssp, implicit $rsp, implicit $ssp, implicit killed $edi, debug-location !18
+
+...
Index: test/DebugInfo/MIR/X86/multiple-param-dbg-value-entry.mir
===================================================================
--- /dev/null
+++ test/DebugInfo/MIR/X86/multiple-param-dbg-value-entry.mir
@@ -0,0 +1,83 @@
+# RUN: llc -debug-entry-values -run-pass=livedebugvalues -o - %s| FileCheck %s
+#
+#int global;
+#int foo(int p, int q, int r) {
+#  global = p + 1;
+#  asm __volatile("" : : : "edi", "esi", "edx");
+#  return 123;
+#}
+#
+# Verify that DW_OP_entry_values are generated for parameters with multiple
+# DBG_VALUEs at entry block.
+# CHECK: DBG_VALUE $edi, $noreg, !{{.*}}, !DIExpression(DW_OP_entry_value, 1), debug-location {{.*}}
+# CHECK: DBG_VALUE $edx, $noreg, !{{.*}}, !DIExpression(DW_OP_entry_value, 1), debug-location {{.*}}
+# CHECK: DBG_VALUE $esi, $noreg, !{{.*}}, !DIExpression(DW_OP_entry_value, 1), debug-location {{.*}}
+
+--- |
+  ; ModuleID = 'multiple-param-dbg-value-entry.ll'
+  source_filename = "multiple-param-dbg-value-entry.c"
+  target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+  
+  @global = common dso_local local_unnamed_addr global i32 0, align 4, !dbg !0
+  
+  define dso_local i32 @foo(i32 %p, i32 %q, i32 %r) local_unnamed_addr !dbg !11 {
+  entry:
+    call void @llvm.dbg.value(metadata i32 %p, metadata !15, metadata !DIExpression()), !dbg !18
+    call void @llvm.dbg.value(metadata i32 %q, metadata !16, metadata !DIExpression()), !dbg !18
+    call void @llvm.dbg.value(metadata i32 %r, metadata !17, metadata !DIExpression()), !dbg !18
+    %add = add nsw i32 %p, 1, !dbg !18
+    store i32 %add, i32* @global, align 4, !dbg !18
+    tail call void asm sideeffect "", "~{edi},~{esi},~{edx},~{dirflag},~{fpsr},~{flags}"(), !dbg !18, !srcloc !19
+    ret i32 123, !dbg !18
+  }
+  
+  ; Function Attrs: nounwind readnone speculatable
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+  
+  !llvm.dbg.cu = !{!2}
+  !llvm.module.flags = !{!7, !8, !9}
+  !llvm.ident = !{!10}
+  
+  !0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+  !1 = distinct !DIGlobalVariable(name: "global", scope: !2, file: !3, line: 8, type: !6, isLocal: false, isDefinition: true)
+  !2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 9.0.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
+  !3 = !DIFile(filename: "multiple-param-dbg-value-entry.c", directory: "/")
+  !4 = !{}
+  !5 = !{!0}
+  !6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !7 = !{i32 2, !"Dwarf Version", i32 4}
+  !8 = !{i32 2, !"Debug Info Version", i32 3}
+  !9 = !{i32 1, !"wchar_size", i32 4}
+  !10 = !{!"clang version 9.0.0 "}
+  !11 = distinct !DISubprogram(name: "foo", scope: !3, file: !3, line: 9, type: !12, scopeLine: 9, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !14)
+  !12 = !DISubroutineType(types: !13)
+  !13 = !{!6, !6, !6, !6}
+  !14 = !{!15, !16, !17}
+  !15 = !DILocalVariable(name: "p", arg: 1, scope: !11, file: !3, line: 9, type: !6, flags: DIFlagArgumentNotModified)
+  !16 = !DILocalVariable(name: "q", arg: 2, scope: !11, file: !3, line: 9, type: !6, flags: DIFlagArgumentNotModified)
+  !17 = !DILocalVariable(name: "r", arg: 3, scope: !11, file: !3, line: 9, type: !6, flags: DIFlagArgumentNotModified)
+  !18 = !DILocation(line: 9, column: 13, scope: !11)
+  !19 = !{i32 213}
+
+...
+---
+name:            foo
+alignment:       4
+tracksRegLiveness: true
+liveins:         
+  - { reg: '$edi' }
+body:             |
+  bb.0.entry:
+    liveins: $edi
+  
+    DBG_VALUE $edi, $noreg, !15, !DIExpression(), debug-location !18
+    DBG_VALUE $edi, $noreg, !15, !DIExpression(), debug-location !18
+    DBG_VALUE $esi, $noreg, !16, !DIExpression(), debug-location !18
+    DBG_VALUE $edx, $noreg, !17, !DIExpression(), debug-location !18
+    renamable $edi = nsw INC32r killed renamable $edi, implicit-def dead $eflags, debug-location !18
+    MOV32mr $rip, 1, $noreg, @global, $noreg, killed renamable $edi, debug-location !18 :: (store 4 into @global)
+    INLINEASM &"", 1, 12, implicit-def dead early-clobber $edi, 12, implicit-def dead early-clobber $esi, 12, implicit-def dead early-clobber $edx, 12, implicit-def dead early-clobber $df, 12, implicit-def dead early-clobber $fpsw, 12, implicit-def dead early-clobber $eflags, !19, debug-location !18
+    $eax = MOV32ri 123, debug-location !18
+    RETQ killed $eax, debug-location !18
+
+...