Index: llvm/include/llvm/CodeGen/GlobalISel/LegalizerHelper.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/LegalizerHelper.h
+++ llvm/include/llvm/CodeGen/GlobalISel/LegalizerHelper.h
@@ -32,6 +32,7 @@
 class Legalizer;
 class MachineRegisterInfo;
 class GISelChangeObserver;
+class LostDebugLocObserver;
 class TargetLowering;
 
 class LegalizerHelper {
@@ -78,10 +79,11 @@
   ///
   /// Considered as an opaque blob, the legal code will use and define the same
   /// registers as \p MI.
-  LegalizeResult legalizeInstrStep(MachineInstr &MI);
+  LegalizeResult legalizeInstrStep(MachineInstr &MI,
+                                   LostDebugLocObserver &LocObserver);
 
   /// Legalize an instruction by emiting a runtime library call instead.
-  LegalizeResult libcall(MachineInstr &MI);
+  LegalizeResult libcall(MachineInstr &MI, LostDebugLocObserver &LocObserver);
 
   /// Legalize an instruction by reducing the width of the underlying scalar
   /// type.
@@ -408,9 +410,9 @@
               ArrayRef<CallLowering::ArgInfo> Args);
 
 /// Create a libcall to memcpy et al.
-LegalizerHelper::LegalizeResult createMemLibcall(MachineIRBuilder &MIRBuilder,
-                                                 MachineRegisterInfo &MRI,
-                                                 MachineInstr &MI);
+LegalizerHelper::LegalizeResult
+createMemLibcall(MachineIRBuilder &MIRBuilder, MachineRegisterInfo &MRI,
+                 MachineInstr &MI, LostDebugLocObserver &LocObserver);
 
 } // End namespace llvm.
 
Index: llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
+++ llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
@@ -230,7 +230,7 @@
       }
 
       // Do the legalization for this instruction.
-      auto Res = Helper.legalizeInstrStep(MI);
+      auto Res = Helper.legalizeInstrStep(MI, LocObserver);
       // Error out if we couldn't legalize this instruction. We may want to
       // fall back to DAG ISel instead in the future.
       if (Res == LegalizerHelper::UnableToLegalize) {
Index: llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -16,6 +16,7 @@
 #include "llvm/CodeGen/GlobalISel/CallLowering.h"
 #include "llvm/CodeGen/GlobalISel/GISelChangeObserver.h"
 #include "llvm/CodeGen/GlobalISel/LegalizerInfo.h"
+#include "llvm/CodeGen/GlobalISel/LostDebugLocObserver.h"
 #include "llvm/CodeGen/GlobalISel/MIPatternMatch.h"
 #include "llvm/CodeGen/GlobalISel/Utils.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
@@ -101,7 +102,8 @@
     TLI(*MF.getSubtarget().getTargetLowering()) { }
 
 LegalizerHelper::LegalizeResult
-LegalizerHelper::legalizeInstrStep(MachineInstr &MI) {
+LegalizerHelper::legalizeInstrStep(MachineInstr &MI,
+                                   LostDebugLocObserver &LocObserver) {
   LLVM_DEBUG(dbgs() << "Legalizing: " << MI);
 
   MIRBuilder.setInstrAndDebugLoc(MI);
@@ -116,7 +118,7 @@
     return AlreadyLegal;
   case Libcall:
     LLVM_DEBUG(dbgs() << ".. Convert to libcall\n");
-    return libcall(MI);
+    return libcall(MI, LocObserver);
   case NarrowScalar:
     LLVM_DEBUG(dbgs() << ".. Narrow scalar\n");
     return narrowScalar(MI, Step.TypeIdx, Step.NewType);
@@ -562,7 +564,7 @@
 
 LegalizerHelper::LegalizeResult
 llvm::createMemLibcall(MachineIRBuilder &MIRBuilder, MachineRegisterInfo &MRI,
-                       MachineInstr &MI) {
+                       MachineInstr &MI, LostDebugLocObserver &LocObserver) {
   auto &Ctx = MIRBuilder.getMF().getFunction().getContext();
 
   SmallVector<CallLowering::ArgInfo, 3> Args;
@@ -620,8 +622,13 @@
   if (!CLI.lowerCall(MIRBuilder, Info))
     return LegalizerHelper::UnableToLegalize;
 
+
   if (Info.LoweredTailCall) {
     assert(Info.IsTailCall && "Lowered tail call when it wasn't a tail call?");
+
+    // Check debug locations before removing the return.
+    LocObserver.checkpoint(true);
+
     // We must have a return following the call (or debug insts) to get past
     // isLibCallInTailPosition.
     do {
@@ -632,6 +639,9 @@
       // Delete the old return.
       Next->eraseFromParent();
     } while (MI.getNextNode());
+
+    // We expect to lose the debug location from the return.
+    LocObserver.checkpoint(false);
   }
 
   return LegalizerHelper::Legalized;
@@ -668,7 +678,7 @@
 }
 
 LegalizerHelper::LegalizeResult
-LegalizerHelper::libcall(MachineInstr &MI) {
+LegalizerHelper::libcall(MachineInstr &MI, LostDebugLocObserver &LocObserver) {
   LLT LLTy = MRI.getType(MI.getOperand(0).getReg());
   unsigned Size = LLTy.getSizeInBits();
   auto &Ctx = MIRBuilder.getMF().getFunction().getContext();
@@ -765,7 +775,7 @@
   case TargetOpcode::G_MEMMOVE:
   case TargetOpcode::G_MEMSET: {
     LegalizeResult Result =
-        createMemLibcall(MIRBuilder, *MIRBuilder.getMRI(), MI);
+        createMemLibcall(MIRBuilder, *MIRBuilder.getMRI(), MI, LocObserver);
     if (Result != Legalized)
       return Result;
     MI.eraseFromParent();
Index: llvm/test/CodeGen/AArch64/GlobalISel/debug-loc-legalize-tail-call.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/GlobalISel/debug-loc-legalize-tail-call.mir
@@ -0,0 +1,43 @@
+# RUN: llc %s -mtriple=aarch64-unknown-unknown -run-pass=legalizer -verify-machineinstrs -pass-remarks-missed=gisel* -o - 2>&1 | FileCheck %s
+
+# When we create a tail call, we expect to drop the return's debug location.
+# Ensure that we don't get a missed remark for debug locations in this case.
+
+# CHECK-NOT: remark: file.ll:[[#]]:[[#]]: lost [[#]] debug locations during pass
+
+--- |  
+  define void @snork() !dbg !6 { unreachable }
+  
+  !llvm.module.flags = !{!0}
+  !llvm.dbg.cu = !{!1}
+  !llvm.debugify = !{!4, !5}
+  
+  !0 = !{i32 2, !"Debug Info Version", i32 3}
+  !1 = distinct !DICompileUnit(language: DW_LANG_C, file: !2, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !3)
+  !2 = !DIFile(filename: "file.ll", directory: "/")
+  !3 = !{}
+  !4 = !{i32 2}
+  !5 = !{i32 1}
+  !6 = distinct !DISubprogram(name: "snork", linkageName: "snork", scope: null, file: !2, line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !1, retainedNodes: !8)
+  !7 = !DISubroutineType(types: !3)
+  !8 = !{!9}
+  !9 = !DILocalVariable(name: "1", scope: !6, file: !2, line: 2, type: !10)
+  !10 = !DIBasicType(name: "ty32", size: 32, encoding: DW_ATE_unsigned)
+  !11 = !DILocation(line: 1, column: 1, scope: !6)
+  !12 = !DILocation(line: 2, column: 1, scope: !6)
+
+...
+---
+name:            snork
+alignment:       4
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    %0:_(p0) = G_IMPLICIT_DEF debug-location !DILocation(line: 0, scope: !6)
+    %1:_(s8) = G_CONSTANT i8 0
+    %2:_(s64) = G_IMPLICIT_DEF debug-location !DILocation(line: 0, scope: !6)
+    G_MEMSET %0(p0), %1(s8), %2(s64), 1, debug-location !11 :: (store 1)
+    DBG_VALUE 0, 0, !9, !DIExpression(), debug-location !12
+    RET_ReallyLR debug-location !12
+
+...