Index: lib/CodeGen/LexicalScopes.cpp
===================================================================
--- lib/CodeGen/LexicalScopes.cpp
+++ lib/CodeGen/LexicalScopes.cpp
@@ -47,11 +47,11 @@
 
 /// initialize - Scan machine function and constuct lexical scope nest.
 void LexicalScopes::initialize(const MachineFunction &Fn) {
+  reset();
   // Don't attempt any lexical scope creation for a NoDebug compile unit.
   if (Fn.getFunction()->getSubprogram()->getUnit()->getEmissionKind() ==
       DICompileUnit::NoDebug)
     return;
-  reset();
   MF = &Fn;
   SmallVector<InsnRange, 4> MIRanges;
   DenseMap<const MachineInstr *, LexicalScope *> MI2ScopeMap;
@@ -277,7 +277,9 @@
 /// DebugLoc.
 void LexicalScopes::getMachineBasicBlocks(
     const DILocation *DL, SmallPtrSetImpl<const MachineBasicBlock *> &MBBs) {
+  assert(MF && "Method called on a uninitialized LexicalScopes object!");
   MBBs.clear();
+
   LexicalScope *Scope = getOrCreateLexicalScope(DL);
   if (!Scope)
     return;
@@ -296,6 +298,7 @@
 /// dominates - Return true if DebugLoc's lexical scope dominates at least one
 /// machine instruction's lexical scope in a given machine basic block.
 bool LexicalScopes::dominates(const DILocation *DL, MachineBasicBlock *MBB) {
+  assert(MF && "Unexpected uninitialized LexicalScopes object!");
   LexicalScope *Scope = getOrCreateLexicalScope(DL);
   if (!Scope)
     return false;
Index: lib/CodeGen/LiveDebugValues.cpp
===================================================================
--- lib/CodeGen/LiveDebugValues.cpp
+++ lib/CodeGen/LiveDebugValues.cpp
@@ -702,6 +702,11 @@
     // LiveDebugValues will already have removed all DBG_VALUEs.
     return false;
 
+  // Skip functions from NoDebug compilation units.
+  if (MF.getFunction()->getSubprogram()->getUnit()->getEmissionKind() ==
+      DICompileUnit::NoDebug)
+    return false;
+
   TRI = MF.getSubtarget().getRegisterInfo();
   TII = MF.getSubtarget().getInstrInfo();
   TFI = MF.getSubtarget().getFrameLowering();
Index: test/CodeGen/X86/debug-nodebug-crash.ll
===================================================================
--- test/CodeGen/X86/debug-nodebug-crash.ll
+++ test/CodeGen/X86/debug-nodebug-crash.ll
@@ -0,0 +1,61 @@
+;RUN: llc < %s | FileCheck %s
+;
+; LexicalScope objects were not cleared when a nodebug function is handled in
+; LiveDebugValues. This may lead to an assertion in the constructor for LexicalScope,
+; triggered by LiveDebugValues when another (debug) function is handled later.
+;
+; This minimal example does not leave much to check for, so we just make sure we get
+; reasonable output, preserving function labels and a DBG_VALUE comment.
+;
+; CHECK-LABEL: foo:
+; CHECK-NEXT:  Lfunc_begin0:
+; CHECK:       Lfunc_end0:
+; CHECK-LABEL: bar:
+; CHECK-NEXT:  Lfunc_begin1:
+; CHECK:       #DEBUG_VALUE: foo:x <-
+; CHECK:       Lfunc_end1:
+
+define i32 @foo() {
+entry:
+  ret i32 0
+}
+
+define i32 @bar(i32 %x) !dbg !50 {
+entry:
+  tail call void @llvm.dbg.value(metadata i32 %x, i64 0, metadata !41, metadata !43), !dbg !52
+  %tobool.i = icmp eq i32 %x, 0
+  br i1 %tobool.i, label %foo.exit, label %if.then.i
+
+if.then.i:
+  br label %foo.exit
+
+foo.exit:
+  %x.addr.0.i = phi i32 [ 1, %if.then.i ], [ 0, %entry ]
+  ret i32 %x.addr.0.i
+}
+
+declare void @llvm.dbg.value(metadata, i64, metadata, metadata)
+
+!llvm.dbg.cu = !{!0, !3}
+!llvm.ident = !{!5, !5}
+!llvm.module.flags = !{!6, !7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "foo.cpp", directory: "c:\temp")
+!2 = !{}
+!3 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !4, producer: "", isOptimized: true, runtimeVersion: 0, emissionKind: NoDebug, enums: !2)
+!4 = !DIFile(filename: "bar.cpp", directory: "c:\temp")
+!5 = !{!"clang version 4.0.0"}
+!6 = !{i32 2, !"Dwarf Version", i32 4}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!36 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !37, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !40)
+!37 = !DISubroutineType(types: !38)
+!38 = !{!39, !39}
+!39 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!40 = !{!41}
+!41 = !DILocalVariable(name: "x", arg: 1, scope: !36, file: !1, line: 1, type: !39)
+!43 = !DIExpression()
+!50 = distinct !DISubprogram(name: "bar", scope: !4, file: !4, line: 3, type: !51, isLocal: false, isDefinition: true, scopeLine: 3, flags: DIFlagPrototyped, isOptimized: true, unit: !3, variables: !2)
+!51 = !DISubroutineType(types: !2)
+!52 = !DILocation(line: 1, scope: !36, inlinedAt: !53)
+!53 = distinct !DILocation(line: 5, scope: !50)