Index: llvm/trunk/include/llvm/CodeGen/MachineInstr.h
===================================================================
--- llvm/trunk/include/llvm/CodeGen/MachineInstr.h
+++ llvm/trunk/include/llvm/CodeGen/MachineInstr.h
@@ -1005,6 +1005,12 @@
       && getOperand(1).isImm();
   }
 
+  /// Return true if the instruction is a debug value which describes a part of
+  /// a variable as unavailable.
+  bool isUndefDebugValue() const {
+    return isDebugValue() && getOperand(0).isReg() && !getOperand(0).getReg();
+  }
+
   bool isPHI() const {
     return getOpcode() == TargetOpcode::PHI ||
            getOpcode() == TargetOpcode::G_PHI;
Index: llvm/trunk/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
+++ llvm/trunk/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
@@ -246,8 +246,13 @@
                                        Pred.getInstr()->getDebugExpression());
                           }))
             break;
-          if (!IsDescribedByReg(I->getInstr()))
-            LabelsBeforeInsn[I->getInstr()] = Asm->getFunctionBegin();
+          // The code that generates location lists for DWARF assumes that the
+          // entries' start labels are monotonically increasing, and since we
+          // don't change the label for fragments that are described by
+          // registers, we must bail out when encountering such a fragment.
+          if (IsDescribedByReg(I->getInstr()))
+            break;
+          LabelsBeforeInsn[I->getInstr()] = Asm->getFunctionBegin();
         }
       }
     }
Index: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -1139,16 +1139,6 @@
   for (auto EB = Entries.begin(), EI = EB, EE = Entries.end(); EI != EE; ++EI) {
     const MachineInstr *Instr = EI->getInstr();
 
-    if (EI->isDbgValue()) {
-      // Check if a variable is inaccessible in this range.
-      // TODO: This should only truncate open ranges that are overlapping.
-      if (Instr->getNumOperands() > 1 &&
-          Instr->getOperand(0).isReg() && !Instr->getOperand(0).getReg()) {
-        OpenRanges.clear();
-        continue;
-      }
-    }
-
     // Remove all values that are no longer live.
     size_t Index = std::distance(EB, EI);
     auto Last =
@@ -1177,8 +1167,16 @@
     // If this history map entry has a debug value, add that to the list of
     // open ranges.
     if (EI->isDbgValue()) {
-      auto Value = getDebugLocValue(Instr);
-      OpenRanges.emplace_back(EI->getEndIndex(), Value);
+      // Do not add undef debug values, as they are redundant information in
+      // the location list entries. An undef debug results in an empty location
+      // description. If there are any non-undef fragments then padding pieces
+      // with empty location descriptions will automatically be inserted, and if
+      // all fragments are undef then the whole location list entry is
+      // redundant.
+      if (!Instr->isUndefDebugValue()) {
+        auto Value = getDebugLocValue(Instr);
+        OpenRanges.emplace_back(EI->getEndIndex(), Value);
+      }
     }
 
     // Location list entries with empty location descriptions are redundant
Index: llvm/trunk/test/DebugInfo/MIR/ARM/param-reg-const-mix.mir
===================================================================
--- llvm/trunk/test/DebugInfo/MIR/ARM/param-reg-const-mix.mir
+++ llvm/trunk/test/DebugInfo/MIR/ARM/param-reg-const-mix.mir
@@ -0,0 +1,96 @@
+# RUN: llc -mtriple=armv4t-unknown-unknown -start-after=livedebugvalues -filetype=obj -o - %s | llvm-dwarfdump - | FileCheck %s
+
+# This reproducer is based on the following C code:
+#
+# struct S0 { int f1; int f2; int f3; };
+#
+# int a;
+#
+# void fn1(struct S0 p1) {
+#   a = p1.f1 >= fn2(p1.f2);
+# }
+#
+# and was generated using the following commands:
+# $ clang -O1 -g --target=armv4t -S -emit-llvm
+# $ llc -O1 -stop-after=livedebugvalues
+
+--- |
+  target datalayout = "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64"
+  target triple = "armv4t-unknown-unknown"
+
+  ; Function Attrs: nounwind
+  define arm_aapcscc i32 @fn1([3 x i32] %p1.coerce) !dbg !7 {
+  entry:
+    %p1.coerce.fca.0.extract = extractvalue [3 x i32] %p1.coerce, 0
+    call void @llvm.dbg.value(metadata i32 %p1.coerce.fca.0.extract, metadata !17, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32)), !dbg !18
+    %p1.coerce.fca.1.extract = extractvalue [3 x i32] %p1.coerce, 1
+    call void @llvm.dbg.value(metadata i32 %p1.coerce.fca.1.extract, metadata !17, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32)), !dbg !18
+    call void @llvm.dbg.value(metadata i32 undef, metadata !17, metadata !DIExpression(DW_OP_LLVM_fragment, 64, 32)), !dbg !18
+    %call = tail call arm_aapcscc i32 bitcast (i32 (...)* @fn2 to i32 (i32)*)(i32 %p1.coerce.fca.1.extract), !dbg !19
+    %cmp = icmp sge i32 %p1.coerce.fca.0.extract, %call, !dbg !19
+    %conv = zext i1 %cmp to i32, !dbg !19
+    ret i32 %conv, !dbg !19
+  }
+
+  declare arm_aapcscc i32 @fn2(...)
+
+  ; Function Attrs: nounwind readnone speculatable
+  declare void @llvm.dbg.value(metadata, metadata, metadata) #0
+
+  attributes #0 = { nounwind readnone speculatable }
+
+  !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", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+  !1 = !DIFile(filename: "test.c", directory: "/")
+  !2 = !{}
+  !3 = !{i32 2, !"Dwarf Version", i32 4}
+  !4 = !{i32 2, !"Debug Info Version", i32 3}
+  !5 = !{i32 1, !"min_enum_size", i32 4}
+  !6 = !{!"clang version 9.0.0"}
+  !7 = distinct !DISubprogram(name: "fn1", scope: !1, file: !1, line: 3, type: !8, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !16)
+  !8 = !DISubroutineType(types: !9)
+  !9 = !{!10, !11}
+  !10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !11 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "S0", file: !1, line: 1, size: 96, elements: !12)
+  !12 = !{!13, !14, !15}
+  !13 = !DIDerivedType(tag: DW_TAG_member, name: "f1", scope: !11, file: !1, line: 1, baseType: !10, size: 32)
+  !14 = !DIDerivedType(tag: DW_TAG_member, name: "f2", scope: !11, file: !1, line: 1, baseType: !10, size: 32, offset: 32)
+  !15 = !DIDerivedType(tag: DW_TAG_member, name: "f3", scope: !11, file: !1, line: 1, baseType: !10, size: 32, offset: 64)
+  !16 = !{!17}
+  !17 = !DILocalVariable(name: "p1", arg: 1, scope: !7, file: !1, line: 3, type: !11)
+  !18 = !DILocation(line: 3, scope: !7)
+  !19 = !DILocation(line: 4, scope: !7)
+
+...
+---
+name:            fn1
+tracksRegLiveness: false
+body:             |
+  bb.0.entry:
+    $sp = frame-setup STMDB_UPD $sp, 14, $noreg, killed $r4, killed $lr
+    $r4 = MOVr $r0, 14, $noreg, $noreg
+    DBG_VALUE $r1, $noreg, !17, !DIExpression(DW_OP_LLVM_fragment, 32, 32), debug-location !18
+    DBG_VALUE $noreg, $noreg, !17, !DIExpression(DW_OP_LLVM_fragment, 64, 32), debug-location !18
+    DBG_VALUE $r4, $noreg, !17, !DIExpression(DW_OP_LLVM_fragment, 0, 32), debug-location !18
+    $r0 = MOVr killed $r1, 14, $noreg, $noreg, debug-location !19
+    BL @fn2, csr_aapcs, implicit-def dead $lr, implicit $sp, implicit $r0, implicit-def $sp, implicit-def $r0, debug-location !19
+    renamable $r1 = MOVi 0, 14, $noreg, $noreg
+    CMPrr killed renamable $r4, killed renamable $r0, 14, $noreg, implicit-def $cpsr, debug-location !19
+    $r1 = MOVi 1, 10, killed $cpsr, $noreg, implicit killed renamable $r1, debug-location !19
+    $r0 = MOVr killed $r1, 14, $noreg, $noreg, debug-location !19
+    $sp = LDMIA_UPD $sp, 14, $noreg, def $r4, def $lr, debug-location !19
+    BX_RET 14, $noreg, implicit $r0, debug-location !19
+
+...
+
+# Verify that the addresses in the location list for the parameter are
+# monotonically increasing, and that the undef debug value fragment does not
+# terminate the non-overlapping fragment that is described by $r1.
+
+# CHECK: DW_AT_location (0x00000000
+# CHECK-NEXT: [0x00000008, 0x00000010): DW_OP_reg4 R4, DW_OP_piece 0x4, DW_OP_reg1 R1, DW_OP_piece 0x4
+# CHECK-NEXT: [0x00000010, 0x00000024): DW_OP_reg4 R4, DW_OP_piece 0x4)
+# CHECK-NEXT: DW_AT_name ("p1")
Index: llvm/trunk/test/DebugInfo/X86/undef-fragment.ll
===================================================================
--- llvm/trunk/test/DebugInfo/X86/undef-fragment.ll
+++ llvm/trunk/test/DebugInfo/X86/undef-fragment.ll
@@ -0,0 +1,78 @@
+; RUN: llc -mtriple=x86_64-pc-linux-gnu -O2 -filetype=obj < %s | llvm-dwarfdump - | FileCheck %s
+
+; This reproducer is based on the following C code:
+;
+; typedef struct { int a; int b; } S;
+;
+; extern int ext(int);
+;
+; int foo() {
+;   S s = {123, 456};
+;   ext(1);
+;   s.a = ext(2);
+;   ext(3);
+;   s.a = 789;
+;   return s.b;
+; }
+;
+; and was generated using -O2 -g -fno-inline.
+;
+; As a small note, the third dbg.value's value has been changed from %call1 to
+; undef (it would have become undef either way, but this was done to make the
+; intention of the test a bit more clear).
+
+; Verify that a location list entry describing s.b is started at the
+; non-overlapping undef debug value.
+
+; CHECK: DW_AT_location (0x00000000
+; CHECK-NEXT: {{0x[0-9a-f]+}}, [[ADDR1:0x[0-9a-f]+]]): DW_OP_constu 0x7b, DW_OP_stack_value, DW_OP_piece 0x4, DW_OP_constu 0x1c8, DW_OP_stack_value, DW_OP_piece 0x4
+; CHECK-NEXT: [[ADDR1]], [[ADDR2:0x[0-9a-f]+]]): DW_OP_piece 0x4, DW_OP_constu 0x1c8, DW_OP_stack_value, DW_OP_piece 0x4
+; CHECK-NEXT: [[ADDR2]], {{0x[0-9a-f]+}}): DW_OP_constu 0x315, DW_OP_stack_value, DW_OP_piece 0x4, DW_OP_constu 0x1c8, DW_OP_stack_value, DW_OP_piece 0x4)
+; CHECK-NEXT: DW_AT_name    ("s")
+
+; Function Attrs: noinline nounwind uwtable
+define i32 @main() !dbg !7 {
+entry:
+  call void @llvm.dbg.value(metadata i32 123, metadata !12, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32)), !dbg !18
+  call void @llvm.dbg.value(metadata i32 456, metadata !12, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32)), !dbg !18
+  %call = tail call i32 @ext(i32 1), !dbg !19
+  %call1 = tail call i32 @ext(i32 2), !dbg !20
+  call void @llvm.dbg.value(metadata i32 undef, metadata !12, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32)), !dbg !18
+  %call2 = tail call i32 @ext(i32 3), !dbg !21
+  call void @llvm.dbg.value(metadata i32 789, metadata !12, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32)), !dbg !18
+  ret i32 456, !dbg !22
+}
+
+declare i32 @ext(i32)
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #0
+
+attributes #0 = { nounwind readnone speculatable }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 9.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "undef.c", directory: "/")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = !{!"clang version 9.0.0"}
+!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 5, type: !8, scopeLine: 5, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !{!12}
+!12 = !DILocalVariable(name: "s", scope: !7, file: !1, line: 6, type: !13)
+!13 = !DIDerivedType(tag: DW_TAG_typedef, name: "S", file: !1, line: 1, baseType: !14)
+!14 = distinct !DICompositeType(tag: DW_TAG_structure_type, file: !1, line: 1, size: 64, elements: !15)
+!15 = !{!16, !17}
+!16 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !14, file: !1, line: 1, baseType: !10, size: 32)
+!17 = !DIDerivedType(tag: DW_TAG_member, name: "b", scope: !14, file: !1, line: 1, baseType: !10, size: 32, offset: 32)
+!18 = !DILocation(line: 6, scope: !7)
+!19 = !DILocation(line: 7, scope: !7)
+!20 = !DILocation(line: 8, scope: !7)
+!21 = !DILocation(line: 9, scope: !7)
+!22 = !DILocation(line: 11, scope: !7)