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"
@@ -3691,9 +3692,32 @@
   for (Instruction *PI : PredicatedInstructions)
     sinkScalarOperands(&*PI);
 
+
   // 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 (llvm::PHINode *OldInduction = Legal->getPrimaryInduction()) {
+    LLVM_DEBUG(dbgs() << "LV: Inspecting induction PHI node : " << *OldInduction
+                      << "\n");
+    // The induction PHINode's debug intrinsics, which will represent the source
+    // induction variable.
+    SmallVector<DbgVariableIntrinsic *, 2> DbgUsers;
+    findDbgUsers(DbgUsers, OldInduction);
+    // All debug users of the now undef induction variable must also be undef
+    for(auto &User: DbgUsers) {
+      LLVM_DEBUG(dbgs() << "LV: Update induction source variable or dependant: "
+                        << User->getVariable()->getName() << "\n ");
+      Instruction *NewDbgII = User->clone();
+      Value *Undef = UndefValue::get(OldInduction->getType());
+      NewDbgII->setOperand(0,
+                           MetadataAsValue::get(User->getContext(),
+                                                ValueAsMetadata::get(Undef)));
+      NewDbgII->insertBefore(&*LoopVectorBody->getFirstInsertionPt());
+    }
+  }
+
   // 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.
@@ -7079,7 +7103,7 @@
   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/DebugInfo/pr39018.ll
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/pr39018.ll
@@ -0,0 +1,176 @@
+;; PR39018
+;; Ensure that the loop vectorizer updates the debuginfo
+;; for the source induction variable.
+
+;; Source:
+;; #include <string.h>
+
+;; #define BUFSZ 192
+
+;; int
+;; main()
+;; {
+;;   int foo[BUFSZ];
+;;   int bar[BUFSZ];
+
+;;   memset(foo, 0, sizeof(foo));
+;;   memset(bar, 1, sizeof(bar));
+
+;;   for (int i = 0; i < BUFSZ; i++)
+;;     foo[i] += bar[i];
+
+;;   unsigned int sum = 0;
+;;   for (int j; j < BUFSZ; j++)
+;;     sum += foo[j];
+
+;;   return sum;
+;; }
+
+; RUN: opt %s -loop-vectorize -S -o - | FileCheck %s
+
+; CHECK: entry:
+; CHECK: call void @llvm.dbg.value(metadata i32 0, metadata ![[i:[0-9]+]]
+; CHECK-NEXT: br i1 false,
+; CHECK: vector.body: 
+; CHECK-NEXT: %index = phi i64
+; CHECK-NEXT: call void @llvm.dbg.value(metadata i64 undef, metadata ![[i]],
+
+; ModuleID = './demo_base.ll'
+source_filename = "demo.c"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: nounwind readnone uwtable
+define dso_local i32 @main() local_unnamed_addr !dbg !7 {
+entry:
+  %foo = alloca [192 x i32], align 16
+  %bar = alloca [192 x i32], align 16
+  %0 = bitcast [192 x i32]* %foo to i8*, !dbg !23
+  call void @llvm.lifetime.start.p0i8(i64 768, i8* nonnull %0) #4, !dbg !23
+  call void @llvm.dbg.declare(metadata [192 x i32]* %foo, metadata !12, metadata !DIExpression()), !dbg !24
+  %1 = bitcast [192 x i32]* %bar to i8*, !dbg !25
+  call void @llvm.lifetime.start.p0i8(i64 768, i8* nonnull %1) #4, !dbg !25
+  call void @llvm.dbg.declare(metadata [192 x i32]* %bar, metadata !16, metadata !DIExpression()), !dbg !26
+  call void @llvm.memset.p0i8.i64(i8* nonnull align 16 dereferenceable(768) %0, i8 0, i64 768, i1 false), !dbg !27
+  call void @llvm.memset.p0i8.i64(i8* nonnull align 16 dereferenceable(768) %1, i8 1, i64 768, i1 false), !dbg !28
+  call void @llvm.dbg.value(metadata i32 0, metadata !17, metadata !DIExpression()), !dbg !29
+  br label %for.body, !dbg !30
+
+for.body:                                         ; preds = %entry, %for.body
+  %indvars.iv13 = phi i64 [ 0, %entry ], [ %indvars.iv.next14, %for.body ]
+  call void @llvm.dbg.value(metadata i64 %indvars.iv13, metadata !17, metadata !DIExpression()), !dbg !29
+  %arrayidx = getelementptr inbounds [192 x i32], [192 x i32]* %bar, i64 0, i64 %indvars.iv13, !dbg !31
+  %2 = load i32, i32* %arrayidx, align 4, !dbg !31, !tbaa !33
+  %arrayidx3 = getelementptr inbounds [192 x i32], [192 x i32]* %foo, i64 0, i64 %indvars.iv13, !dbg !37
+  %3 = load i32, i32* %arrayidx3, align 4, !dbg !38, !tbaa !33
+  %add = add nsw i32 %3, %2, !dbg !38
+  store i32 %add, i32* %arrayidx3, align 4, !dbg !38, !tbaa !33
+  %indvars.iv.next14 = add nuw nsw i64 %indvars.iv13, 1, !dbg !39
+  call void @llvm.dbg.value(metadata i64 %indvars.iv.next14, metadata !17, metadata !DIExpression()), !dbg !29
+  %exitcond15.not = icmp eq i64 %indvars.iv.next14, 192, !dbg !40
+  br i1 %exitcond15.not, label %for.body7.preheader, label %for.body, !dbg !30, !llvm.loop !41
+
+for.body7.preheader:                              ; preds = %for.body
+  br label %for.body7, !dbg !44
+
+for.cond.cleanup6:                                ; preds = %for.body7
+  %add10.lcssa = phi i32 [ %add10, %for.body7 ], !dbg !45
+  call void @llvm.dbg.value(metadata i32 %add10.lcssa, metadata !19, metadata !DIExpression()), !dbg !47
+  call void @llvm.lifetime.end.p0i8(i64 768, i8* nonnull %1) #4, !dbg !48
+  call void @llvm.lifetime.end.p0i8(i64 768, i8* nonnull %0) #4, !dbg !48
+  ret i32 %add10.lcssa, !dbg !49
+
+for.body7:                                        ; preds = %for.body7.preheader, %for.body7
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body7 ], [ 0, %for.body7.preheader ]
+  %sum.010 = phi i32 [ %add10, %for.body7 ], [ 0, %for.body7.preheader ]
+  call void @llvm.dbg.value(metadata i64 %indvars.iv, metadata !21, metadata !DIExpression()), !dbg !50
+  call void @llvm.dbg.value(metadata i32 %sum.010, metadata !19, metadata !DIExpression()), !dbg !47
+  %arrayidx9 = getelementptr inbounds [192 x i32], [192 x i32]* %foo, i64 0, i64 %indvars.iv, !dbg !51
+  %4 = load i32, i32* %arrayidx9, align 4, !dbg !51, !tbaa !33
+  %add10 = add i32 %4, %sum.010, !dbg !45
+  call void @llvm.dbg.value(metadata i32 %add10, metadata !19, metadata !DIExpression()), !dbg !47
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1, !dbg !52
+  call void @llvm.dbg.value(metadata i64 %indvars.iv.next, metadata !21, metadata !DIExpression()), !dbg !50
+  %exitcond.not = icmp eq i64 %indvars.iv.next, 192, !dbg !53
+  br i1 %exitcond.not, label %for.cond.cleanup6, label %for.body7, !dbg !44, !llvm.loop !54
+}
+
+; Function Attrs: argmemonly nounwind willreturn
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture)
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #0
+
+; Function Attrs: argmemonly nounwind willreturn writeonly
+declare void @llvm.memset.p0i8.i64(i8* nocapture writeonly, i8, i64, i1 immarg)
+
+; Function Attrs: argmemonly nounwind willreturn
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture)
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #0
+
+
+attributes #0 = { nounwind readnone speculatable willreturn }
+
+
+!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 12.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "demo.c", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 12.0.0 "}
+!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 6, type: !8, scopeLine: 7, flags: DIFlagAllCallsDescribed, 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, !16, !17, !19, !21}
+!12 = !DILocalVariable(name: "foo", scope: !7, file: !1, line: 8, type: !13)
+!13 = !DICompositeType(tag: DW_TAG_array_type, baseType: !10, size: 6144, elements: !14)
+!14 = !{!15}
+!15 = !DISubrange(count: 192)
+!16 = !DILocalVariable(name: "bar", scope: !7, file: !1, line: 9, type: !13)
+!17 = !DILocalVariable(name: "i", scope: !18, file: !1, line: 14, type: !10)
+!18 = distinct !DILexicalBlock(scope: !7, file: !1, line: 14, column: 3)
+!19 = !DILocalVariable(name: "sum", scope: !7, file: !1, line: 17, type: !20)
+!20 = !DIBasicType(name: "unsigned int", size: 32, encoding: DW_ATE_unsigned)
+!21 = !DILocalVariable(name: "j", scope: !22, file: !1, line: 18, type: !10)
+!22 = distinct !DILexicalBlock(scope: !7, file: !1, line: 18, column: 3)
+!23 = !DILocation(line: 8, column: 3, scope: !7)
+!24 = !DILocation(line: 8, column: 7, scope: !7)
+!25 = !DILocation(line: 9, column: 3, scope: !7)
+!26 = !DILocation(line: 9, column: 7, scope: !7)
+!27 = !DILocation(line: 11, column: 3, scope: !7)
+!28 = !DILocation(line: 12, column: 3, scope: !7)
+!29 = !DILocation(line: 0, scope: !18)
+!30 = !DILocation(line: 14, column: 3, scope: !18)
+!31 = !DILocation(line: 15, column: 15, scope: !32)
+!32 = distinct !DILexicalBlock(scope: !18, file: !1, line: 14, column: 3)
+!33 = !{!34, !34, i64 0}
+!34 = !{!"int", !35, i64 0}
+!35 = !{!"omnipotent char", !36, i64 0}
+!36 = !{!"Simple C/C++ TBAA"}
+!37 = !DILocation(line: 15, column: 5, scope: !32)
+!38 = !DILocation(line: 15, column: 12, scope: !32)
+!39 = !DILocation(line: 14, column: 31, scope: !32)
+!40 = !DILocation(line: 14, column: 21, scope: !32)
+!41 = distinct !{!41, !30, !42, !43}
+!42 = !DILocation(line: 15, column: 20, scope: !18)
+!43 = !{!"llvm.loop.unroll.disable"}
+!44 = !DILocation(line: 18, column: 3, scope: !22)
+!45 = !DILocation(line: 19, column: 9, scope: !46)
+!46 = distinct !DILexicalBlock(scope: !22, file: !1, line: 18, column: 3)
+!47 = !DILocation(line: 0, scope: !7)
+!48 = !DILocation(line: 22, column: 1, scope: !7)
+!49 = !DILocation(line: 21, column: 3, scope: !7)
+!50 = !DILocation(line: 0, scope: !22)
+!51 = !DILocation(line: 19, column: 12, scope: !46)
+!52 = !DILocation(line: 18, column: 31, scope: !46)
+!53 = !DILocation(line: 18, column: 21, scope: !46)
+!54 = distinct !{!54, !44, !55, !43}
+!55 = !DILocation(line: 19, column: 17, scope: !22)