Index: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3211,6 +3211,10 @@
     // Check to see if we can DCE the instruction.
     if (isInstructionTriviallyDead(I, &TLI)) {
       LLVM_DEBUG(dbgs() << "IC: DCE: " << *I << '\n');
+
+      if (!salvageDebugInfo(*I))
+	replaceDbgUsesWithUndef(I);
+
       eraseInstFromFunction(*I);
       ++NumDeadInst;
       MadeIRChange = true;
Index: llvm/test/Transforms/InstCombine/pr43893.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/InstCombine/pr43893.ll
@@ -0,0 +1,61 @@
+; RUN: opt -instcombine -S -o - %s | FileCheck %s
+
+@a = common dso_local global i8 0, align 1, !dbg !0
+@b = common dso_local global i8 0, align 1, !dbg !6
+
+; Function Attrs: noinline nounwind uwtable
+define dso_local i32 @main() #0 !dbg !13 {
+entry:
+  %0 = load i8, i8* @a, align 1, !dbg !17
+  %dec = add i8 %0, -1, !dbg !17
+  store i8 %dec, i8* @a, align 1, !dbg !17
+;CHECK: call void @llvm.dbg.value(metadata i32 undef
+;CHECK: call void @llvm.dbg.value(metadata i32 -8
+;CHECK call void @llvm.dbg.value(metadata i32 undef
+  %conv = sext i8 %dec to i32, !dbg !17
+  call void @llvm.dbg.value(metadata i32 %conv, metadata !18, metadata !DIExpression()), !dbg !19
+  call void @llvm.dbg.value(metadata i32 -8, metadata !20, metadata !DIExpression()), !dbg !19
+  call void @llvm.dbg.value(metadata i32 %conv, metadata !20, metadata !DIExpression()), !dbg !19
+  store i8 0, i8* @b, align 1, !dbg !21
+  %cmp = icmp sgt i32 %conv, 0, !dbg !22
+  %conv1 = zext i1 %cmp to i32, !dbg !22
+  ret i32 0, !dbg !23
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+attributes #0 = { noinline nounwind uwtable }
+attributes #1 = { nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!9, !10, !11}
+!llvm.ident = !{!12}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "a", scope: !2, file: !3, line: 1, type: !8, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 10.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
+!3 = !DIFile(filename: "test", directory: "n")
+!4 = !{}
+!5 = !{!0, !6}
+!6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
+!7 = distinct !DIGlobalVariable(name: "b", scope: !2, file: !3, line: 1, type: !8, isLocal: false, isDefinition: true)
+!8 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+!9 = !{i32 2, !"Dwarf Version", i32 4}
+!10 = !{i32 2, !"Debug Info Version", i32 3}
+!11 = !{i32 1, !"wchar_size", i32 4}
+!12 = !{!"clang version 10.0.0"}
+!13 = distinct !DISubprogram(name: "main", scope: !3, file: !3, line: 2, type: !14, scopeLine: 2, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !4)
+!14 = !DISubroutineType(types: !15)
+!15 = !{!16}
+!16 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!17 = !DILocation(line: 4, column: 11, scope: !13)
+!18 = !DILocalVariable(name: "c", scope: !13, file: !3, line: 4, type: !16)
+!19 = !DILocation(line: 0, scope: !13)
+!20 = !DILocalVariable(name: "l_1240", scope: !13, file: !3, line: 6, type: !16)
+!21 = !DILocation(line: 10, column: 10, scope: !13)
+!22 = !DILocation(line: 10, column: 5, scope: !13)
+!23 = !DILocation(line: 12, column: 1, scope: !13)