Index: llvm/lib/CodeGen/LiveDebugVariables.cpp
===================================================================
--- llvm/lib/CodeGen/LiveDebugVariables.cpp
+++ llvm/lib/CodeGen/LiveDebugVariables.cpp
@@ -96,22 +96,24 @@
 
 enum : unsigned { UndefLocNo = ~0U };
 
-/// Describes a debug variable location by number.
+/// Describes a debug variable value by location number and expression.
 class DbgValueLocation {
 public:
   DbgValueLocation() = default;
-  DbgValueLocation(unsigned LocNo) : LocNo(LocNo) {}
+  DbgValueLocation(unsigned LocNo, const DIExpression &Expression)
+      : LocNo(LocNo), Expression(&Expression) {}
 
-  unsigned locNo() const { return LocNo; }
-  bool isUndef() const { return locNo() == UndefLocNo; }
+  unsigned getLocNo() const { return LocNo; }
+  const DIExpression *getExpression() const { return Expression; }
+  bool isUndef() const { return getLocNo() == UndefLocNo; }
 
   DbgValueLocation changeLocNo(unsigned NewLocNo) const {
-    return DbgValueLocation(NewLocNo);
+    return DbgValueLocation(NewLocNo, *Expression);
   }
 
   friend inline bool operator==(const DbgValueLocation &LHS,
                                 const DbgValueLocation &RHS) {
-    return LHS.LocNo == RHS.LocNo;
+    return LHS.LocNo == RHS.LocNo && LHS.Expression == RHS.Expression;
   }
 
   friend inline bool operator!=(const DbgValueLocation &LHS,
@@ -193,7 +195,9 @@
              const DILocation *IA) const {
     // FIXME: The fragment should be part of the equivalence class, but not
     // other things in the expression like stack values.
-    return Var == Variable && Expr == Expression && dl->getInlinedAt() == IA;
+    return Var == Variable &&
+           Expr->getFragmentInfo() == Expression->getFragmentInfo() &&
+           dl->getInlinedAt() == IA;
   }
 
   /// Merge equivalence classes.
@@ -271,8 +275,9 @@
   void mapVirtRegs(LDVImpl *LDV);
 
   /// Add a definition point to this value.
-  void addDef(SlotIndex Idx, const MachineOperand &LocMO) {
-    DbgValueLocation Loc(getLocationNo(LocMO));
+  void addDef(SlotIndex Idx, const MachineOperand &LocMO,
+              const DIExpression &Expr) {
+    DbgValueLocation Loc(getLocationNo(LocMO), Expr);
     // Add a singular (Idx,Idx) -> Loc mapping.
     LocMap::iterator I = locInts.find(Idx);
     if (!I.valid() || I.start() != Idx)
@@ -306,11 +311,11 @@
   /// possible.
   ///
   /// \param LI Scan for copies of the value in LI->reg.
-  /// \param LocNo Location number of LI->reg.
+  /// \param Loc Location number of LI->reg.
   /// \param Kills Points where the range of LocNo could be extended.
   /// \param [in,out] NewDefs Append (Idx, LocNo) of inserted defs here.
   void addDefsFromCopies(
-      LiveInterval *LI, unsigned LocNo,
+      LiveInterval *LI, DbgValueLocation Loc,
       const SmallVectorImpl<SlotIndex> &Kills,
       SmallVectorImpl<std::pair<SlotIndex, DbgValueLocation>> &NewDefs,
       MachineRegisterInfo &MRI, LiveIntervals &LIS);
@@ -645,11 +650,11 @@
   UserValue *UV =
       getUserValue(Var, Expr, MI.getDebugLoc());
   if (!Discard)
-    UV->addDef(Idx, MI.getOperand(0));
+    UV->addDef(Idx, MI.getOperand(0), *Expr);
   else {
     MachineOperand MO = MachineOperand::CreateReg(0U, false);
     MO.setIsDebug();
-    UV->addDef(Idx, MO);
+    UV->addDef(Idx, MO, *Expr);
   }
   return true;
 }
@@ -757,7 +762,7 @@
 }
 
 void UserValue::addDefsFromCopies(
-    LiveInterval *LI, unsigned LocNo,
+    LiveInterval *LI, DbgValueLocation Loc,
     const SmallVectorImpl<SlotIndex> &Kills,
     SmallVectorImpl<std::pair<SlotIndex, DbgValueLocation>> &NewDefs,
     MachineRegisterInfo &MRI, LiveIntervals &LIS) {
@@ -787,7 +792,7 @@
     // it, or we are looking at a wrong value of LI.
     SlotIndex Idx = LIS.getInstructionIndex(*MI);
     LocMap::iterator I = locInts.find(Idx.getRegSlot(true));
-    if (!I.valid() || I.value().locNo() != LocNo)
+    if (!I.valid() || I.value() != Loc)
       continue;
 
     if (!LIS.hasInterval(DstReg))
@@ -821,7 +826,7 @@
       MachineInstr *CopyMI = LIS.getInstructionFromIndex(DstVNI->def);
       assert(CopyMI && CopyMI->isCopy() && "Bad copy value");
       unsigned LocNo = getLocationNo(CopyMI->getOperand(0));
-      DbgValueLocation NewLoc(LocNo);
+      DbgValueLocation NewLoc = Loc.changeLocNo(LocNo);
       I.insert(Idx, Idx.getNextSlot(), NewLoc);
       NewDefs.push_back(std::make_pair(Idx, NewLoc));
       break;
@@ -869,7 +874,7 @@
       // sub-register in that regclass). For now, simply skip handling copies if
       // a sub-register is involved.
       if (LI && !LocMO.getSubReg())
-        addDefsFromCopies(LI, Loc.locNo(), Kills, Defs, MRI, LIS);
+        addDefsFromCopies(LI, Loc, Kills, Defs, MRI, LIS);
       continue;
     }
 
@@ -1297,7 +1302,7 @@
   // original DBG_VALUE was indirect, we need to add DW_OP_deref to indicate
   // that the original virtual register was a pointer. Also, add the stack slot
   // offset for the spilled register to the expression.
-  const DIExpression *Expr = Expression;
+  const DIExpression *Expr = Loc.getExpression();
   if (Spilled)
     Expr = DIExpression::prepend(Expr, DIExpression::ApplyOffset, SpillOffset);
 
Index: llvm/test/DebugInfo/X86/dbg-addr-dse.ll
===================================================================
--- llvm/test/DebugInfo/X86/dbg-addr-dse.ll
+++ llvm/test/DebugInfo/X86/dbg-addr-dse.ll
@@ -3,10 +3,6 @@
 ; RUN: FileCheck %s < %t.s --check-prefix=ASM
 ; RUN: llvm-dwarfdump %t.o | FileCheck %s --check-prefix=DWARF
 
-; XFAIL: *
-; See PR41992, the third and final dbg.value disappears after
-; LiveDebugVariables.
-
 ; In this example, the variable lives mostly in memory, but at the point of the
 ; assignment to global, it lives nowhere, and is described as the constant
 ; value 1.
Index: llvm/test/DebugInfo/X86/live-debug-vars-dse.mir
===================================================================
--- llvm/test/DebugInfo/X86/live-debug-vars-dse.mir
+++ llvm/test/DebugInfo/X86/live-debug-vars-dse.mir
@@ -1,9 +1,5 @@
 # RUN: llc -start-after=machine-scheduler %s -o - | FileCheck %s
 
-# XFAIL: *
-# See PR41992, the third and final dbg.value disappears after
-# LiveDebugVariables.
-
 # C source:
 # void escape(int *);
 # extern int global;
Index: llvm/test/DebugInfo/X86/live-debug-vars-intervals.mir
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/X86/live-debug-vars-intervals.mir
@@ -0,0 +1,154 @@
+# These tests check that DBG_VALUE intervals are correctly coalesced (or not) in
+# LiveDebugVariables.
+# NOTE: We do not currently handle partially overlapping fragments in LDV.
+#
+# The IR in this file has been modified by hand, generated from this source:
+# void escape(int *);
+# extern int global;
+# void f(int x) {
+#   escape(&x);
+#   x = 1;
+#   global = x;
+#   x = 2;
+#   escape(&x);
+# }
+
+# RUN: llc %s -start-before=machine-scheduler -stop-after=virtregrewriter -o - \
+# RUN:     | FileCheck %s --implicit-check-not=DBG_VALUE
+
+# Verify that DBG_VALUEs with same { Variable, Fragment } but different DIExpressions
+# are not coalesced.
+#
+# DV1 %0 "x" (deref) --+-- Do not coalesce these because DV2 refers to the same bits.
+# DV2 %0 "x" ()        |
+# DV3 %0 "x" (deref) --+
+#
+# CHECK: name: f1
+# CHECK: DBG_VALUE %stack.0.x.addr, $noreg, ![[VAR:[0-9]+]], !DIExpression(DW_OP_deref)
+# CHECK: DBG_VALUE %stack.0.x.addr, $noreg, ![[VAR]],        !DIExpression()
+# CHECK: DBG_VALUE %stack.0.x.addr, $noreg, ![[VAR]],        !DIExpression(DW_OP_deref)
+
+# Verify that DBG_VALUEs with same { Variable, Fragment } and { Location, DIExpression } are
+# coalesced if there are no intervening DBG_VALUEs with the same { Variable, Fragment }.
+#
+# DV1 %0 "x" (fragment 00 16) --+-- Coalesce these because DV2 refers to different bits.
+# DV2 %0 "x" (fragment 16 16)   |
+# DV3 %0 "x" (fragment 00 16) --+
+#
+# CHECK: name: f2
+# CHECK: DBG_VALUE %stack.0.x.addr, $noreg, ![[VAR:[0-9]+]], !DIExpression(DW_OP_deref, DW_OP_LLVM_fragment, 0, 16)
+# CHECK: DBG_VALUE %stack.0.x.addr, $noreg, ![[VAR]],        !DIExpression(DW_OP_LLVM_fragment, 16, 16)
+
+--- |
+  target triple = "x86_64-unknown-linux-gnu"
+
+  @global = external global i32, align 4
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+  declare void @escape(i32*)
+
+  define void @f1(i32 %x) !dbg !6 {
+  entry:
+    %x.addr = alloca i32, align 4
+    store i32 %x, i32* %x.addr, align 4
+    call void @llvm.dbg.value(metadata i32* %x.addr, metadata !11, metadata !DIExpression(DW_OP_deref)), !dbg !12
+    call void @escape(i32* %x.addr), !dbg !12
+    call void @llvm.dbg.value(metadata i32* %x.addr, metadata !11, metadata !DIExpression()), !dbg !12
+    store i32 1, i32* @global, align 4, !dbg !12
+    call void @llvm.dbg.value(metadata i32* %x.addr, metadata !11, metadata !DIExpression(DW_OP_deref)), !dbg !12
+    store i32 2, i32* %x.addr, align 4, !dbg !12
+    call void @escape(i32* %x.addr), !dbg !12
+    ret void, !dbg !12
+  }
+
+  define void @f2(i32 %x) !dbg !13 {
+  entry:
+    %x.addr = alloca i32, align 4
+    store i32 %x, i32* %x.addr, align 4
+    call void @llvm.dbg.value(metadata i32* %x.addr, metadata !14, metadata !DIExpression(DW_OP_deref, DW_OP_LLVM_fragment, 0, 16)), !dbg !15
+    call void @escape(i32* %x.addr)
+    call void @llvm.dbg.value(metadata i32* %x.addr, metadata !14, metadata !DIExpression(DW_OP_LLVM_fragment, 16, 16)), !dbg !15
+    store i32 1, i32* @global, align 4
+    call void @llvm.dbg.value(metadata i32* %x.addr, metadata !14, metadata !DIExpression(DW_OP_deref, DW_OP_LLVM_fragment, 0, 16)), !dbg !15
+    store i32 2, i32* %x.addr, align 4
+    call void @escape(i32* %x.addr)
+    ret void
+  }
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!3, !4}
+  !llvm.ident = !{!5}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 11.0.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+  !1 = !DIFile(filename: "test.c", directory: "")
+  !2 = !{}
+  !3 = !{i32 2, !"Dwarf Version", i32 4}
+  !4 = !{i32 2, !"Debug Info Version", i32 3}
+  !5 = !{!"clang version 11.0.0 "}
+  !6 = distinct !DISubprogram(name: "f1", scope: !1, file: !1, line: 3, type: !7, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !10)
+  !7 = !DISubroutineType(types: !8)
+  !8 = !{null, !9}
+  !9 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !10 = !{!11}
+  !11 = !DILocalVariable(name: "x", arg: 1, scope: !6, file: !1, line: 3, type: !9)
+  !12 = !DILocation(line: 3, column: 12, scope: !6)
+  !13 = distinct !DISubprogram(name: "f2", scope: !1, file: !1, line: 20, type: !7, scopeLine: 20, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !10)
+  !14 = !DILocalVariable(name: "x", arg: 1, scope: !13, file: !1, line: 21, type: !9)
+  !15 = !DILocation(line: 23, column: 12, scope: !13)
+
+...
+---
+name:            f1
+tracksRegLiveness: true
+stack:
+  - { id: 0, name: x.addr, type: default, offset: 0, size: 4, alignment: 4,
+      stack-id: default, callee-saved-register: '', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+body:             |
+  bb.0.entry:
+    liveins: $edi
+    %0:gr32 = COPY $edi
+    MOV32mr %stack.0.x.addr, 1, $noreg, 0, $noreg, %0 :: (store 4 into %ir.x.addr)
+    DBG_VALUE %stack.0.x.addr, $noreg, !11, !DIExpression(DW_OP_deref), debug-location !12
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !12
+    %1:gr64 = LEA64r %stack.0.x.addr, 1, $noreg, 0, $noreg
+    $rdi = COPY %1, debug-location !12
+    CALL64pcrel32 @escape, csr_64, implicit $rsp, implicit $ssp, implicit $rdi, implicit-def $rsp, implicit-def $ssp, debug-location !12
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !12
+    DBG_VALUE %stack.0.x.addr, $noreg, !11, !DIExpression(), debug-location !12
+    MOV32mi $rip, 1, $noreg, @global, $noreg, 1, debug-location !12 :: (store 4 into @global)
+    DBG_VALUE %stack.0.x.addr, $noreg, !11, !DIExpression(DW_OP_deref), debug-location !12
+    MOV32mi %stack.0.x.addr, 1, $noreg, 0, $noreg, 2, debug-location !12 :: (store 4 into %ir.x.addr)
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !12
+    $rdi = COPY %1, debug-location !12
+    CALL64pcrel32 @escape, csr_64, implicit $rsp, implicit $ssp, implicit $rdi, implicit-def $rsp, implicit-def $ssp, debug-location !12
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !12
+    RET 0, debug-location !12
+...
+---
+name:            f2
+tracksRegLiveness: true
+stack:
+  - { id: 0, name: x.addr, type: default, offset: 0, size: 4, alignment: 4,
+      stack-id: default, callee-saved-register: '', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+body:             |
+  bb.0.entry:
+    liveins: $edi
+    %0:gr32 = COPY $edi
+    MOV32mr %stack.0.x.addr, 1, $noreg, 0, $noreg, %0 :: (store 4 into %ir.x.addr)
+    DBG_VALUE %stack.0.x.addr, $noreg, !14, !DIExpression(DW_OP_deref, DW_OP_LLVM_fragment, 0, 16), debug-location !15
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %1:gr64 = LEA64r %stack.0.x.addr, 1, $noreg, 0, $noreg
+    $rdi = COPY %1
+    CALL64pcrel32 @escape, csr_64, implicit $rsp, implicit $ssp, implicit $rdi, implicit-def $rsp, implicit-def $ssp
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    DBG_VALUE %stack.0.x.addr, $noreg, !14, !DIExpression(DW_OP_LLVM_fragment, 16, 16), debug-location !15
+    MOV32mi $rip, 1, $noreg, @global, $noreg, 1 :: (store 4 into @global)
+    DBG_VALUE %stack.0.x.addr, $noreg, !14, !DIExpression(DW_OP_deref, DW_OP_LLVM_fragment, 0, 16), debug-location !15
+    MOV32mi %stack.0.x.addr, 1, $noreg, 0, $noreg, 2 :: (store 4 into %ir.x.addr)
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    $rdi = COPY %1
+    CALL64pcrel32 @escape, csr_64, implicit $rsp, implicit $ssp, implicit $rdi, implicit-def $rsp, implicit-def $ssp
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    RET 0
+...
Index: llvm/test/tools/llvm-locstats/locstats.ll
===================================================================
--- llvm/test/tools/llvm-locstats/locstats.ll
+++ llvm/test/tools/llvm-locstats/locstats.ll
@@ -11,9 +11,9 @@
 ; LOCSTATS: [30%,40%) 0 0%
 ; LOCSTATS: [40%,50%) 1 11%
 ; LOCSTATS: [50%,60%) 1 11%
-; LOCSTATS: [60%,70%) 0 0%
+; LOCSTATS: [60%,70%) 1 11%
 ; LOCSTATS: [70%,80%) 0 0%
-; LOCSTATS: [80%,90%) 3 33%
+; LOCSTATS: [80%,90%) 2 22%
 ; LOCSTATS: [90%,100%) 1 11%
 ; LOCSTATS: 100% 2 22%
 ;