Index: lib/Analysis/MemorySSAUpdater.cpp
===================================================================
--- lib/Analysis/MemorySSAUpdater.cpp
+++ lib/Analysis/MemorySSAUpdater.cpp
@@ -512,11 +512,11 @@
   }
 }
 
-static MemoryAccess *getNewDefiningAccessForClone(MemoryAccess *MA,
-                                                  const ValueToValueMapTy &VMap,
-                                                  PhiToDefMap &MPhiMap,
-                                                  bool CloneWasSimplified,
-                                                  MemorySSA *MSSA) {
+static MemoryAccess *
+getNewDefiningAccessForClone(MemoryAccess *MA, const ValueToValueMapTy &VMap,
+                             PhiToDefMap &MPhiMap, bool CloneWasSimplified,
+                             const MemoryUseOrDef *ClonedAccess,
+                             MemorySSA *MSSA) {
   MemoryAccess *InsnDefining = MA;
   if (MemoryDef *DefMUD = dyn_cast<MemoryDef>(InsnDefining)) {
     if (!MSSA->isLiveOnEntryDef(DefMUD)) {
@@ -529,15 +529,31 @@
           assert(InsnDefining && "Defining instruction cannot be nullptr.");
         else if (!InsnDefining || isa<MemoryUse>(InsnDefining)) {
           // The clone was simplified, it's no longer a MemoryDef, look up.
+          // Simplified clones only occur in single block cloning from
+          // loop header/body to preheader, and DefMUD must be in the same BB
+          // as ClonedAccess, otherwise NewDefMUDI would not be in the map.
+          // So a previous definition must exist (at least a phi).
+          assert(MA->getBlock() == ClonedAccess->getBlock() &&
+                 "Defining access expected to be in the same block");
           auto DefIt = DefMUD->getDefsIterator();
-          // Since simplified clones only occur in single block cloning, a
-          // previous definition must exist, otherwise NewDefMUDI would not
-          // have been found in VMap.
           assert(DefIt != MSSA->getBlockDefs(DefMUD->getBlock())->begin() &&
                  "Previous def must exist");
           InsnDefining = getNewDefiningAccessForClone(
-              &*(--DefIt), VMap, MPhiMap, CloneWasSimplified, MSSA);
+              &*(--DefIt), VMap, MPhiMap, CloneWasSimplified, ClonedAccess,
+              MSSA);
         }
+      } else if (CloneWasSimplified &&
+                 DefMUD->getBlock() == ClonedAccess->getBlock()) {
+        // Since simplified clones only occur in single block cloning, the
+        // defining access may be MA(DefMUD), as initialized (a definition
+        // preceding the preheader into which we are cloning), but it may also
+        // be an earlier definition if an access was not cloned (debug.value
+        // calls are simplified to no-ops). Hence we need to look up.
+        auto DefIt = DefMUD->getDefsIterator();
+        assert(DefIt != MSSA->getBlockDefs(DefMUD->getBlock())->begin() &&
+               "Previous def must exist");
+        InsnDefining = getNewDefiningAccessForClone(
+            &*(--DefIt), VMap, MPhiMap, CloneWasSimplified, ClonedAccess, MSSA);
       }
     }
   } else {
@@ -571,7 +587,8 @@
         MemoryAccess *NewUseOrDef = MSSA->createDefinedAccess(
             NewInsn,
             getNewDefiningAccessForClone(MUD->getDefiningAccess(), VMap,
-                                         MPhiMap, CloneWasSimplified, MSSA),
+                                         MPhiMap, CloneWasSimplified, MUD,
+                                         MSSA),
             /*Template=*/CloneWasSimplified ? nullptr : MUD,
             /*CreationMustSucceed=*/CloneWasSimplified ? false : true);
         if (NewUseOrDef)
Index: test/Analysis/MemorySSA/debugvalue2.ll
===================================================================
--- /dev/null
+++ test/Analysis/MemorySSA/debugvalue2.ll
@@ -0,0 +1,50 @@
+; RUN: opt -disable-basicaa -loop-rotate -enable-mssa-loop-dependency -verify-memoryssa -S %s | FileCheck %s
+; REQUIRES: asserts
+
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: @overflow_iter_var
+define void @overflow_iter_var() !dbg !11 {
+entry:
+  call void @llvm.dbg.value(metadata i16 0, metadata !16, metadata !DIExpression()), !dbg !18
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.body, %entry
+  call void @llvm.dbg.value(metadata i16 0, metadata !16, metadata !DIExpression()), !dbg !18
+  call void @llvm.dbg.value(metadata i16 undef, metadata !20, metadata !DIExpression()), !dbg !21
+  br i1 undef, label %for.end, label %for.body
+
+for.body:                                         ; preds = %for.cond
+  %0 = load i16, i16* undef, align 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+attributes #1 = { nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!10}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 10.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, globals: !3, nameTableKind: None)
+!1 = !DIFile(filename: "2_loops.c", directory: "/")
+!2 = !{}
+!3 = !{}
+!6 = !DICompositeType(tag: DW_TAG_array_type, baseType: !7, size: 4096, elements: !8)
+!7 = !DIBasicType(name: "int", size: 16, encoding: DW_ATE_signed)
+!8 = !{!9}
+!9 = !DISubrange(count: 256)
+!10 = !{i32 2, !"Debug Info Version", i32 3}
+!11 = distinct !DISubprogram(name: "overflow_iter_var", scope: !1, file: !1, line: 20, type: !12, scopeLine: 21, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!12 = !DISubroutineType(types: !13)
+!13 = !{null, !14, !14}
+!14 = !DIBasicType(name: "unsigned int", size: 16, encoding: DW_ATE_unsigned)
+!16 = !DILocalVariable(name: "i", scope: !17, file: !1, line: 23, type: !14)
+!17 = distinct !DILexicalBlock(scope: !11, file: !1, line: 23, column: 3)
+!18 = !DILocation(line: 0, scope: !17)
+!20 = !DILocalVariable(name: "stop1", arg: 1, scope: !11, file: !1, line: 20, type: !14)
+!21 = !DILocation(line: 0, scope: !11)