diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -1412,9 +1412,9 @@
     const Value *V = DI->getValue();
     assert(DI->getVariable()->isValidLocationForIntrinsic(DbgLoc) &&
            "Expected inlined-at fields to agree");
-    if (!V) {
+    if (!V || isa<UndefValue>(V)) {
       // Currently the optimizer can produce this; insert an undef to
-      // help debugging.  Probably the optimizer should not do this.
+      // help debugging.
       BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, II, false, 0U,
               DI->getVariable(), DI->getExpression());
     } else if (const auto *CI = dyn_cast<ConstantInt>(V)) {
@@ -1442,8 +1442,7 @@
       BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, II, IsIndirect, Reg,
               DI->getVariable(), DI->getExpression());
     } else {
-      // We can't yet handle anything else here because it would require
-      // generating code, thus altering codegen because of debug info.
+      // We don't know how to handle other cases, so we drop.
       LLVM_DEBUG(dbgs() << "Dropping debug info for " << *DI << "\n");
     }
     return true;
diff --git a/llvm/test/CodeGen/AArch64/fastisel-debugvalue-undef.ll b/llvm/test/CodeGen/AArch64/fastisel-debugvalue-undef.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/fastisel-debugvalue-undef.ll
@@ -0,0 +1,66 @@
+; RUN: llc -O0 -fast-isel=1 -o - -print-after="finalize-isel" %s 2>&1 | FileCheck %s
+
+; Check that we emit a DBG_VALUE for the `@llvm.dbg.value` which has `undef` has first arg.
+; CHECK: DBG_VALUE $noreg, $noreg, !"MyStruct", !DIExpression(), debug-location
+
+target triple = "arm64-apple-ios13.4.0"
+%T1a8PatatinoC = type <{ %swift.refcounted, %TSi }>
+%swift.refcounted = type { %swift.type*, i64 }
+%TSi = type <{ i64 }>
+%swift.type = type { i64 }
+%swift.metadata_response = type { %swift.type*, i64 }
+%T1a5TinkyV = type opaque
+declare hidden swiftcc %T1a8PatatinoC* @"$s1a8PatatinoCyACSicfC"(i64, %swift.type* swiftself) #0
+declare hidden swiftcc i64 @"$s1a6lambdaySiAA8PatatinoCF"(%T1a8PatatinoC*) #0
+define hidden swiftcc i64 @"$s1a4mainSiyF"() #0 !dbg !37 {
+entry:
+  %LiveObj = alloca %T1a8PatatinoC*, align 8
+  %0 = bitcast %T1a8PatatinoC** %LiveObj to i8*
+  %Ret = alloca %TSi, align 8
+  %1 = bitcast %TSi* %Ret to i8*
+  %2 = bitcast %T1a8PatatinoC** %LiveObj to i8*, !dbg !48
+  %3 = call swiftcc %swift.metadata_response @"$s1a8PatatinoCMa"(i64 0) #5, !dbg !51
+  %4 = extractvalue %swift.metadata_response %3, 0, !dbg !51
+  %5 = call swiftcc %T1a8PatatinoC* @"$s1a8PatatinoCyACSicfC"(i64 37, %swift.type* swiftself %4), !dbg !51
+  %6 = bitcast %T1a8PatatinoC* %5 to %swift.refcounted*, !dbg !51
+  %7 = call %swift.refcounted* @swift_retain(%swift.refcounted* returned %6) #4, !dbg !51
+  %8 = bitcast %TSi* %Ret to i8*, !dbg !48
+  %9 = call swiftcc i64 @"$s1a6lambdaySiAA8PatatinoCF"(%T1a8PatatinoC* %5), !dbg !52
+  call void @llvm.dbg.value(metadata %T1a5TinkyV* undef, metadata !53, metadata !DIExpression()), !dbg !55
+  ret i64 %9, !dbg !57
+}
+declare void @llvm.dbg.value(metadata, metadata, metadata) #2
+declare hidden swiftcc %swift.metadata_response @"$s1a8PatatinoCMa"(i64) #3
+declare %swift.refcounted* @swift_retain(%swift.refcounted* returned) #4
+!llvm.module.flags = !{!0, !1, !2, !3, !4, !5, !6, !7, !8, !9, !10}
+!llvm.dbg.cu = !{!11}
+!0 = !{i32 2, !"SDK Version", [2 x i32] [i32 13, i32 4]}
+!1 = !{i32 1, !"Objective-C Version", i32 2}
+!2 = !{i32 1, !"Objective-C Image Info Version", i32 0}
+!3 = !{i32 1, !"Objective-C Image Info Section", !"__DATA,__objc_imageinfo,regular,no_dead_strip"}
+!4 = !{i32 4, !"Objective-C Garbage Collection", i32 84018944}
+!5 = !{i32 1, !"Objective-C Class Properties", i32 64}
+!6 = !{i32 2, !"Dwarf Version", i32 4}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{i32 1, !"wchar_size", i32 4}
+!9 = !{i32 7, !"PIC Level", i32 2}
+!10 = !{i32 1, !"Swift Version", i32 7}
+!11 = distinct !DICompileUnit(language: DW_LANG_Swift, file: !12, producer: "Swift version 5.2-dev (Swift f5c0fc7bbf)", isOptimized: false, runtimeVersion: 5, emissionKind: FullDebug, enums: !13, imports: !14)
+!12 = !DIFile(filename: "/Users/local/lldb-swift-device/llvm-project/lldb/packages/Python/lldbsuite/test/lang/swift/swift_reference_counting/main.swift", directory: "/Users/local/lldb-swift-device/build/Ninja-ReleaseAssert+stdlib-Release/lldb-macosx-x86_64/bin")
+!13 = !{}
+!14 = !{}
+!37 = distinct !DISubprogram(name: "main", linkageName: "$s1a4mainSiyF", file: !12, line: 26, type: !38, scopeLine: 26, spFlags: DISPFlagDefinition, unit: !11, retainedNodes: !13)
+!38 = !DISubroutineType(types: !39)
+!39 = !{!40}
+!40 = !DICompositeType(tag: DW_TAG_structure_type, name: "Int", file: !41, size: 64, elements: !13, runtimeLang: DW_LANG_Swift, identifier: "$sSiD")
+!41 = !DIFile(filename: "tino", directory: "/Users/local/lldb-swift-device/build")
+!43 = distinct !DILexicalBlock(scope: !37, file: !12, line: 26, column: 20)
+!48 = !DILocation(line: 0, scope: !49)
+!49 = !DILexicalBlockFile(scope: !43, file: !50, discriminator: 0)
+!50 = !DIFile(filename: "<compiler-generated>", directory: "")
+!51 = !DILocation(line: 27, column: 17, scope: !43)
+!52 = !DILocation(line: 28, column: 19, scope: !43)
+!53 = !DILocalVariable(name: "MyStruct", scope: !43, file: !12, line: 29, type: !54)
+!54 = !DICompositeType(tag: DW_TAG_structure_type, name: "Tinky", file: !12, line: 20, elements: !13, runtimeLang: DW_LANG_Swift, identifier: "$s1a5TinkyVD")
+!55 = !DILocation(line: 29, column: 7, scope: !43)
+!57 = !DILocation(line: 30, column: 3, scope: !43)