Index: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -134,6 +134,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/InjectTLIMappings.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/LoopSimplify.h"
 #include "llvm/Transforms/Utils/LoopUtils.h"
 #include "llvm/Transforms/Utils/LoopVersioning.h"
@@ -3701,6 +3702,15 @@
   // Remove redundant induction instructions.
   cse(LoopVectorBody);
 
+  // If there is an old induction variable we want it to have a meaningful value
+  // during the vector operations.
+  if (PHINode *OldInduction = Legal->getPrimaryInduction()) {
+    LLVM_DEBUG(dbgs() << "LV: Inspecting induction PHI node : " << *OldInduction
+                      << "\n");
+    // All debug users of the now undef induction variable must also be undef.
+    replaceDbgUsesWithUndef(OldInduction);
+  }
+
   // Set/update profile weights for the vector and remainder loops as original
   // loop iterations are now distributed among them. Note that original loop
   // represented by LoopScalarBody becomes remainder loop after vectorization.
@@ -7108,7 +7118,7 @@
   VPlans.front()->execute(&State);
 
   // 3. Fix the vectorized code: take care of header phi's, live-outs,
-  //    predication, updating analyses.
+  //    predication, updating analyses, debuginfo.
   ILV.fixVectorizedLoop();
 }
 
Index: llvm/test/Transforms/LoopVectorize/pr39018.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopVectorize/pr39018.ll
@@ -0,0 +1,103 @@
+;; PR39018
+;; Ensure that the loop vectorizer updates the debuginfo
+;; for the source induction variable.
+
+;; Source:
+;; #define BUFSZ 192
+
+;; int
+;; main()
+;; {
+;;   int foo[BUFSZ];
+;;   int sum = 5;
+;;   for (int i = 0; i < BUFSZ; i++)
+;;     sum += foo[i];
+;;   return sum;
+;; }
+
+; RUN: opt %s -loop-vectorize -S -o - | FileCheck %s
+
+; CHECK: call void @llvm.dbg.value(metadata i32 0, metadata ![[i:[0-9]+]]
+; CHECK: br i1 false,
+; CHECK: vector.body: 
+; CHECK: call void @llvm.dbg.value(metadata i64 undef, metadata ![[i]],
+
+target triple = "x86_64-pc-linux-gnu"
+
+define dso_local i32 @main() local_unnamed_addr !dbg !7 {
+  %1 = alloca [192 x i32], align 16
+  %2 = bitcast [192 x i32]* %1 to i8*, !dbg !20
+  call void @llvm.dbg.declare(metadata [192 x i32]* %1, metadata !13, metadata !DIExpression()), !dbg !21
+  call void @llvm.dbg.value(metadata i32 5, metadata !17, metadata !DIExpression()), !dbg !22
+  call void @llvm.dbg.value(metadata i32 0, metadata !18, metadata !DIExpression()), !dbg !23
+  br label %4, !dbg !24
+
+3:                                                ; preds = %4
+  call void @llvm.dbg.value(metadata i32 %9, metadata !17, metadata !DIExpression()), !dbg !22
+  ret i32 %9, !dbg !25
+
+4:                                                ; preds = %4, %0
+  %5 = phi i64 [ 0, %0 ], [ %10, %4 ]
+  %6 = phi i32 [ 5, %0 ], [ %9, %4 ]
+  call void @llvm.dbg.value(metadata i64 %5, metadata !18, metadata !DIExpression()), !dbg !23
+  call void @llvm.dbg.value(metadata i32 %6, metadata !17, metadata !DIExpression()), !dbg !22
+  %7 = getelementptr inbounds [192 x i32], [192 x i32]* %1, i64 0, i64 %5, !dbg !26
+  %8 = load i32, i32* %7, align 4, !dbg !26
+  %9 = add nsw i32 %8, %6, !dbg !28
+  call void @llvm.dbg.value(metadata i32 %9, metadata !17, metadata !DIExpression()), !dbg !22
+  %10 = add nuw nsw i64 %5, 1, !dbg !29
+  call void @llvm.dbg.value(metadata i64 %10, metadata !18, metadata !DIExpression()), !dbg !23
+  %11 = icmp eq i64 %10, 192, !dbg !30
+  br i1 %11, label %3, label %4, !dbg !24, !llvm.loop !31
+}
+
+; Function Attrs: argmemonly nofree nosync nounwind willreturn
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture)
+
+; Function Attrs: nofree nosync nounwind readnone speculatable willreturn
+declare void @llvm.dbg.declare(metadata, metadata, metadata)
+
+; Function Attrs: argmemonly nofree nosync nounwind willreturn
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture)
+
+; Function Attrs: nofree nosync nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 11.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "loopvec_1.cpp", directory: "/test")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"Ubuntu clang version 11.0.0 "}
+!7 = distinct !DISubprogram(name: "main", scope: !8, file: !8, line: 5, type: !9, scopeLine: 6, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+!8 = !DIFile(filename: "./loopvec1.cpp", directory: "/test")
+!9 = !DISubroutineType(types: !10)
+!10 = !{!11}
+!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!12 = !{!13, !17, !18}
+!13 = !DILocalVariable(name: "foo", scope: !7, file: !8, line: 7, type: !14)
+!14 = !DICompositeType(tag: DW_TAG_array_type, baseType: !11, size: 6144, elements: !15)
+!15 = !{!16}
+!16 = !DISubrange(count: 192)
+!17 = !DILocalVariable(name: "sum", scope: !7, file: !8, line: 8, type: !11)
+!18 = !DILocalVariable(name: "i", scope: !19, file: !8, line: 9, type: !11)
+!19 = distinct !DILexicalBlock(scope: !7, file: !8, line: 9, column: 3)
+!20 = !DILocation(line: 7, column: 3, scope: !7)
+!21 = !DILocation(line: 7, column: 7, scope: !7)
+!22 = !DILocation(line: 0, scope: !7)
+!23 = !DILocation(line: 0, scope: !19)
+!24 = !DILocation(line: 9, column: 3, scope: !19)
+!25 = !DILocation(line: 12, column: 3, scope: !7)
+!26 = !DILocation(line: 10, column: 12, scope: !27)
+!27 = distinct !DILexicalBlock(scope: !19, file: !8, line: 9, column: 3)
+!28 = !DILocation(line: 10, column: 9, scope: !27)
+!29 = !DILocation(line: 9, column: 31, scope: !27)
+!30 = !DILocation(line: 9, column: 21, scope: !27)
+!31 = distinct !{!31, !24, !32, !33}
+!32 = !DILocation(line: 10, column: 17, scope: !19)
+!33 = !{!"llvm.loop.unroll.disable"}