Index: llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp
+++ llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp
@@ -2362,6 +2362,8 @@
 
     /// Keep track of the original uses (pair Instruction, Index).
     SmallVector<InstructionAndIdx, 4> OriginalUses;
+    /// Keep track of the debug users.
+    SmallVector<DbgValueInst *, 1> DbgValues;
 
     using use_iterator = SmallVectorImpl<InstructionAndIdx>::iterator;
 
@@ -2375,6 +2377,10 @@
         Instruction *UserI = cast<Instruction>(U.getUser());
         OriginalUses.push_back(InstructionAndIdx(UserI, U.getOperandNo()));
       }
+      // Record the debug uses separately. They are not in the instruction's
+      // use list, but they are replaced by RAUW.
+      findDbgValues(DbgValues, Inst);
+
       // Now, we can replace the uses.
       Inst->replaceAllUsesWith(New);
     }
@@ -2387,6 +2393,15 @@
            UseIt != EndIt; ++UseIt) {
         UseIt->Inst->setOperand(UseIt->Idx, Inst);
       }
+      // RAUW has replaced all original uses with references to the new value,
+      // including the debug uses. Since we are undoing the replacements,
+      // the original debug uses must also be reinstated to maintain the
+      // correctness and utility of debug value instructions.
+      for (auto *DVI: DbgValues) {
+        LLVMContext &Ctx = Inst->getType()->getContext();
+        auto *MV = MetadataAsValue::get(Ctx, ValueAsMetadata::get(Inst));
+        DVI->setOperand(0, MV);
+      }
     }
   };
 
Index: llvm/trunk/test/DebugInfo/Generic/codegenprep-value.ll
===================================================================
--- llvm/trunk/test/DebugInfo/Generic/codegenprep-value.ll
+++ llvm/trunk/test/DebugInfo/Generic/codegenprep-value.ll
@@ -0,0 +1,75 @@
+; RUN: opt -codegenprepare -S %s | FileCheck %s
+;
+; Generated from the following source with:
+; clang -O2 -g -S -emit-llvm -mllvm -stop-after=indirectbr-expand test.cpp
+;
+; extern void use(int);
+; extern int foo(long long);
+; 
+; void test(int p)
+; {
+;   int i = p + 4;
+;   (void)foo(i);  // sign extension of i
+;   if (p)
+;     return;
+;   use(i);        // use of original i
+; }
+;
+; PR39845: Check that CodeGenPrepare does not drop the reference to a local when it is
+;          sign-extended and used later.
+;
+; CHECK: define{{.*}}test
+; CHECK: call{{.*}}dbg.value(metadata i32 %p
+; CHECK: call{{.*}}dbg.value(metadata i32 %add
+;
+define dso_local void @_Z4testi(i32 %p) local_unnamed_addr !dbg !7 {
+entry:
+  call void @llvm.dbg.value(metadata i32 %p, metadata !12, metadata !DIExpression()), !dbg !14
+  %add = add nsw i32 %p, 4, !dbg !15
+  call void @llvm.dbg.value(metadata i32 %add, metadata !13, metadata !DIExpression()), !dbg !16
+  %conv = sext i32 %add to i64, !dbg !17
+  %call = tail call i32 @_Z3foox(i64 %conv), !dbg !18
+  %tobool = icmp eq i32 %p, 0, !dbg !19
+  br i1 %tobool, label %if.end, label %cleanup, !dbg !21
+
+if.end:
+  tail call void @_Z3usei(i32 %add), !dbg !22
+  br label %cleanup, !dbg !23
+
+cleanup:
+  ret void, !dbg !23
+}
+
+declare dso_local i32 @_Z3foox(i64) local_unnamed_addr
+declare dso_local void @_Z3usei(i32) local_unnamed_addr
+
+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, file: !1, producer: "clang version 8.0.0 (trunk 348209)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "/home/test/src")
+!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 8.0.0 (trunk 348209)"}
+!7 = distinct !DISubprogram(name: "test", linkageName: "_Z4testi", scope: !1, file: !1, line: 4, type: !8, scopeLine: 5, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{null, !10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !{!12, !13}
+!12 = !DILocalVariable(name: "p", arg: 1, scope: !7, file: !1, line: 4, type: !10)
+!13 = !DILocalVariable(name: "i", scope: !7, file: !1, line: 6, type: !10)
+!14 = !DILocation(line: 4, column: 15, scope: !7)
+!15 = !DILocation(line: 6, column: 13, scope: !7)
+!16 = !DILocation(line: 6, column: 7, scope: !7)
+!17 = !DILocation(line: 7, column: 13, scope: !7)
+!18 = !DILocation(line: 7, column: 9, scope: !7)
+!19 = !DILocation(line: 8, column: 7, scope: !20)
+!20 = distinct !DILexicalBlock(scope: !7, file: !1, line: 8, column: 7)
+!21 = !DILocation(line: 8, column: 7, scope: !7)
+!22 = !DILocation(line: 10, column: 3, scope: !7)
+!23 = !DILocation(line: 11, column: 1, scope: !7)