Index: lib/CodeGen/AsmPrinter/DwarfDebug.cpp
===================================================================
--- lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -1362,8 +1362,8 @@
   if (auto *Scope = cast_or_null<DIScope>(S)) {
     Fn = Scope->getFilename();
     Dir = Scope->getDirectory();
-    if (auto *LBF = dyn_cast<DILexicalBlockFile>(Scope))
-      if (getDwarfVersion() >= 4)
+    if (Line != 0 && getDwarfVersion() >= 4)
+      if (auto *LBF = dyn_cast<DILexicalBlockFile>(Scope))
         Discriminator = LBF->getDiscriminator();
 
     unsigned CUID = Asm->OutStreamer->getContext().getDwarfCompileUnitID();
Index: test/CodeGen/Generic/dbg-line-0-no-discriminator.ll
===================================================================
--- test/CodeGen/Generic/dbg-line-0-no-discriminator.ll
+++ test/CodeGen/Generic/dbg-line-0-no-discriminator.ll
@@ -0,0 +1,100 @@
+; RUN: llc -O0 -filetype=asm %s -o - | FileCheck %s
+; Verify we got at least one discriminator, and line 0 never has one.
+; CHECK: .loc 1 {{[1-9]*}} {{.*}} discriminator
+; CHECK-NOT: .loc 1 0 {{.*}} discriminator
+;
+; Test source:
+; int main() {
+;   unsigned int x = 0U;
+;   static int const limit = 100000U;
+;   for (volatile unsigned int i = 0; i < limit; ++i) {
+;     if ((i & 0x40) == 0)
+;       x += i;
+;   }
+;   return int(x);
+; }
+; $ clang -c -gmlt -fdebug-info-for-profiling -S -emit-llvm
+; then remove the target triple (to make it generic) and the
+; function attributes (to simplify it a tiny bit).
+
+; ModuleID = 'bz183143.cpp'
+source_filename = "bz183143.cpp"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; Function Attrs: norecurse nounwind uwtable
+define i32 @main() local_unnamed_addr !dbg !7 {
+entry:
+  %i = alloca i32, align 4
+  %i.0.i.0..sroa_cast = bitcast i32* %i to i8*, !dbg !9
+  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %i.0.i.0..sroa_cast), !dbg !9
+  store volatile i32 0, i32* %i, align 4, !dbg !10
+  %i.0.i.0.9 = load volatile i32, i32* %i, align 4, !dbg !11
+  %cmp10 = icmp ult i32 %i.0.i.0.9, 100000, !dbg !13
+  br i1 %cmp10, label %for.body, label %for.cond.cleanup, !dbg !14
+
+for.cond.cleanup:                                 ; preds = %for.inc, %entry
+  %x.0.lcssa = phi i32 [ 0, %entry ], [ %x.1, %for.inc ]
+  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %i.0.i.0..sroa_cast), !dbg !15
+  ret i32 %x.0.lcssa, !dbg !16
+
+for.body:                                         ; preds = %entry, %for.inc
+  %x.011 = phi i32 [ %x.1, %for.inc ], [ 0, %entry ]
+  %i.0.i.0.2 = load volatile i32, i32* %i, align 4, !dbg !17
+  %and = and i32 %i.0.i.0.2, 64, !dbg !18
+  %cmp1 = icmp eq i32 %and, 0, !dbg !19
+  br i1 %cmp1, label %if.then, label %for.inc, !dbg !20
+
+if.then:                                          ; preds = %for.body
+  %i.0.i.0.3 = load volatile i32, i32* %i, align 4, !dbg !21
+  %add = add i32 %i.0.i.0.3, %x.011, !dbg !22
+  br label %for.inc, !dbg !23
+
+for.inc:                                          ; preds = %for.body, %if.then
+  %x.1 = phi i32 [ %add, %if.then ], [ %x.011, %for.body ]
+  %i.0.i.0.4 = load volatile i32, i32* %i, align 4, !dbg !24
+  %inc = add i32 %i.0.i.0.4, 1, !dbg !24
+  store volatile i32 %inc, i32* %i, align 4, !dbg !24
+  %i.0.i.0. = load volatile i32, i32* %i, align 4, !dbg !11
+  %cmp = icmp ult i32 %i.0.i.0., 100000, !dbg !13
+  br i1 %cmp, label %for.body, label %for.cond.cleanup, !dbg !14, !llvm.loop !26
+}
+
+; Function Attrs: argmemonly nounwind
+declare void @llvm.lifetime.start.p0i8(i64, i8* nocapture)
+
+; Function Attrs: argmemonly nounwind
+declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture)
+
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 5.0.0 (trunk 307600)", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2, debugInfoForProfiling: true)
+!1 = !DIFile(filename: "bz183143.cpp", directory: "/home/probinson/projects/scratch")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 5.0.0 (trunk 307600)"}
+!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !8, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !2)
+!8 = !DISubroutineType(types: !2)
+!9 = !DILocation(line: 4, column: 8, scope: !7)
+!10 = !DILocation(line: 4, column: 30, scope: !7)
+!11 = !DILocation(line: 4, column: 37, scope: !12)
+!12 = !DILexicalBlockFile(scope: !7, file: !1, discriminator: 2)
+!13 = !DILocation(line: 4, column: 39, scope: !12)
+!14 = !DILocation(line: 4, column: 3, scope: !12)
+!15 = !DILocation(line: 4, column: 3, scope: !7)
+!16 = !DILocation(line: 8, column: 3, scope: !7)
+!17 = !DILocation(line: 5, column: 10, scope: !7)
+!18 = !DILocation(line: 5, column: 12, scope: !7)
+!19 = !DILocation(line: 5, column: 20, scope: !7)
+!20 = !DILocation(line: 5, column: 9, scope: !7)
+!21 = !DILocation(line: 6, column: 12, scope: !7)
+!22 = !DILocation(line: 6, column: 9, scope: !7)
+!23 = !DILocation(line: 6, column: 7, scope: !7)
+!24 = !DILocation(line: 4, column: 48, scope: !25)
+!25 = !DILexicalBlockFile(scope: !7, file: !1, discriminator: 6)
+!26 = distinct !{!26, !15, !27}
+!27 = !DILocation(line: 7, column: 3, scope: !7)