Index: llvm/lib/Transforms/Scalar/Reassociate.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/Reassociate.cpp
+++ llvm/lib/Transforms/Scalar/Reassociate.cpp
@@ -1898,6 +1898,7 @@
   ValueRankMap.erase(I);
   Insts.remove(I);
   RedoInsts.remove(I);
+  llvm::salvageDebugInfoOrMarkUndef(*I);
   I->eraseFromParent();
   for (auto Op : Ops)
     if (Instruction *OpInst = dyn_cast<Instruction>(Op))
@@ -1914,6 +1915,7 @@
   // Erase the dead instruction.
   ValueRankMap.erase(I);
   RedoInsts.remove(I);
+  llvm::salvageDebugInfoOrMarkUndef(*I);
   I->eraseFromParent();
   // Optimize its operands.
   SmallPtrSet<Instruction *, 8> Visited; // Detect self-referential nodes.
Index: llvm/test/Transforms/Reassociate/reassociate_salvages_debug_info.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/Reassociate/reassociate_salvages_debug_info.ll
@@ -0,0 +1,50 @@
+; RUN: opt < %s -reassociate -S | FileCheck %s
+
+; Check that reassociate pass now salvages debug info when dropping instructions.
+
+define hidden i32 @main(i32 %argc, i8** %argv) {
+entry:
+  ; CHECK: call void @llvm.dbg.value(metadata i32 %argc, metadata [[VAR_B:![0-9]+]], metadata !DIExpression(DW_OP_plus_uconst, 1, DW_OP_stack_value))
+  %add = add nsw i32 %argc, 1, !dbg !26
+  call void @llvm.dbg.value(metadata i32 %add, metadata !22, metadata !DIExpression()), !dbg !25
+  %add1 = add nsw i32 %argc, %add, !dbg !27
+  ret i32 %add1, !dbg !28
+}
+
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 10.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, debugInfoForProfiling: true, nameTableKind: None)
+!1 = !DIFile(filename: "test2.cpp", directory: "C:\")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 2}
+!6 = !{i32 7, !"PIC Level", i32 2}
+!7 = !{!"clang version 10.0.0"}
+!8 = distinct !DISubprogram(name: "main", scope: !9, file: !9, line: 1, type: !10, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !18)
+!9 = !DIFile(filename: "./test2.cpp", directory: "C:\")
+!10 = !DISubroutineType(types: !11)
+!11 = !{!12, !13, !14}
+!12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!13 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !12)
+!14 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !15, size: 64)
+!15 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !16, size: 64)
+!16 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !17)
+!17 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+!18 = !{!19, !20, !21, !22, !23, !24}
+!19 = !DILocalVariable(name: "argc", arg: 1, scope: !8, file: !9, line: 1, type: !13)
+!20 = !DILocalVariable(name: "argv", arg: 2, scope: !8, file: !9, line: 1, type: !14)
+!21 = !DILocalVariable(name: "a", scope: !8, file: !9, line: 2, type: !12)
+; CHECK: [[VAR_B]] = !DILocalVariable(name: "b"
+!22 = !DILocalVariable(name: "b", scope: !8, file: !9, line: 3, type: !12)
+!23 = !DILocalVariable(name: "to_return", scope: !8, file: !9, line: 4, type: !12)
+!24 = !DILocalVariable(name: "result", scope: !8, file: !9, line: 5, type: !12)
+!25 = !DILocation(line: 0, scope: !8)
+!26 = !DILocation(line: 3, scope: !8)
+!27 = !DILocation(line: 4, scope: !8)
+!28 = !DILocation(line: 6, scope: !8)
Index: llvm/test/Transforms/Reassociate/undef_intrinsics_when_deleting_instructions.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/Reassociate/undef_intrinsics_when_deleting_instructions.ll
@@ -0,0 +1,95 @@
+; RUN: opt < %s -reassociate -S | FileCheck %s
+
+; Check that reassociate pass now undefs debug intrinsics that reference a value
+; that gets dropped and cannot be salvaged.
+
+define hidden i32 @main() local_unnamed_addr {
+entry:
+  %foo = alloca i32, align 4, !dbg !20
+  %foo.0.foo.0..sroa_cast = bitcast i32* %foo to i8*, !dbg !20
+  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %foo.0.foo.0..sroa_cast), !dbg !20
+  store volatile i32 4, i32* %foo, align 4, !dbg !20, !tbaa !21
+  %foo.0.foo.0. = load volatile i32, i32* %foo, align 4, !dbg !25, !tbaa !21
+  %foo.0.foo.0.15 = load volatile i32, i32* %foo, align 4, !dbg !27, !tbaa !21
+  %foo.0.foo.0.16 = load volatile i32, i32* %foo, align 4, !dbg !28, !tbaa !21
+  ; CHECK-NOT: %add = add nsw i32 %foo.0.foo.0., %foo.0.foo.0.15
+  %add = add nsw i32 %foo.0.foo.0., %foo.0.foo.0.15, !dbg !29
+  ; CHECK: call void @llvm.dbg.value(metadata i32 undef, metadata [[VAR_A:![0-9]+]], metadata !DIExpression())
+  call void @llvm.dbg.value(metadata i32 %add, metadata !19, metadata !DIExpression()), !dbg !26
+  %foo.0.foo.0.17 = load volatile i32, i32* %foo, align 4, !dbg !30, !tbaa !21
+  %cmp = icmp eq i32 %foo.0.foo.0.17, 4, !dbg !30
+  br i1 %cmp, label %if.then, label %if.end, !dbg !32
+
+  ; CHECK-LABEL: if.then:
+if.then:
+  ; CHECK-NOT: %add1 = add nsw i32 %add, %foo.0.foo.0.16
+  %add1 = add nsw i32 %add, %foo.0.foo.0.16, !dbg !33
+  ; CHECK: call void @llvm.dbg.value(metadata i32 undef, metadata [[VAR_A]], metadata !DIExpression())
+  call void @llvm.dbg.value(metadata i32 %add1, metadata !19, metadata !DIExpression()), !dbg !26
+  ; CHECK: call void @llvm.dbg.value(metadata i32 undef, metadata [[VAR_CHEESE:![0-9]+]], metadata !DIExpression())
+  call void @llvm.dbg.value(metadata i32 %add, metadata !18, metadata !DIExpression()), !dbg !26
+  %sub = add nsw i32 %add, -12, !dbg !34
+  %sub3 = sub nsw i32 %add1, %sub, !dbg !34
+  %mul = mul nsw i32 %sub3, 20, !dbg !36
+  %div = sdiv i32 %mul, 3, !dbg !37
+  br label %if.end, !dbg !38
+
+if.end:
+  %a.0 = phi i32 [ %div, %if.then ], [ 0, %entry ], !dbg !39
+  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %foo.0.foo.0..sroa_cast), !dbg !40
+  ret i32 %a.0, !dbg !41
+}
+
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #1
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #2
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture) #1
+declare void @llvm.dbg.value(metadata, metadata, metadata) #2
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 10.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, debugInfoForProfiling: true, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "F:\")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 2}
+!6 = !{i32 7, !"PIC Level", i32 2}
+!7 = !{!"clang version 10.0.0"}
+!8 = distinct !DISubprogram(name: "main", scope: !9, file: !9, line: 1, type: !10, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !13)
+!9 = !DIFile(filename: "./test.cpp", directory: "F:\")
+!10 = !DISubroutineType(types: !11)
+!11 = !{!12}
+!12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!13 = !{!14, !16, !17, !18, !19}
+!14 = !DILocalVariable(name: "foo", scope: !8, file: !9, line: 2, type: !15)
+!15 = !DIDerivedType(tag: DW_TAG_volatile_type, baseType: !12)
+!16 = !DILocalVariable(name: "read1", scope: !8, file: !9, line: 3, type: !12)
+!17 = !DILocalVariable(name: "read2", scope: !8, file: !9, line: 4, type: !12)
+; CHECK: [[VAR_CHEESE]] = !DILocalVariable(name: "cheese"
+!18 = !DILocalVariable(name: "cheese", scope: !8, file: !9, line: 6, type: !12)
+; CHECK: [[VAR_A]] = !DILocalVariable(name: "a"
+!19 = !DILocalVariable(name: "a", scope: !8, file: !9, line: 7, type: !12)
+!20 = !DILocation(line: 2, scope: !8)
+!21 = !{!22, !22, i64 0}
+!22 = !{!"int", !23, i64 0}
+!23 = !{!"omnipotent char", !24, i64 0}
+!24 = !{!"Simple C++ TBAA"}
+!25 = !DILocation(line: 3, scope: !8)
+!26 = !DILocation(line: 0, scope: !8)
+!27 = !DILocation(line: 4, scope: !8)
+!28 = !DILocation(line: 6, scope: !8)
+!29 = !DILocation(line: 7, scope: !8)
+!30 = !DILocation(line: 10, scope: !31)
+!31 = distinct !DILexicalBlock(scope: !8, file: !9, line: 10)
+!32 = !DILocation(line: 10, scope: !8)
+!33 = !DILocation(line: 8, scope: !8)
+!34 = !DILocation(line: 12, scope: !35)
+!35 = distinct !DILexicalBlock(scope: !31, file: !9, line: 10)
+!36 = !DILocation(line: 13, scope: !35)
+!37 = !DILocation(line: 14, scope: !35)
+!38 = !DILocation(line: 15, scope: !35)
+!39 = !DILocation(line: 0, scope: !31)
+!40 = !DILocation(line: 20, scope: !8)
+!41 = !DILocation(line: 19, scope: !8)