Index: llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -493,6 +493,8 @@
 
     AMemSet = Builder.CreateMemSet(StartPtr, ByteVal, Range.End - Range.Start,
                                    MaybeAlign(Range.Alignment));
+    AMemSet->mergeDIAssignID(Range.TheStores);
+
     LLVM_DEBUG(dbgs() << "Replace stores:\n"; for (Instruction *SI
                                                    : Range.TheStores) dbgs()
                                               << *SI << '\n';
@@ -727,6 +729,7 @@
             M = Builder.CreateMemCpy(
                 SI->getPointerOperand(), SI->getAlign(),
                 LI->getPointerOperand(), LI->getAlign(), Size);
+          M->copyMetadata(*SI, LLVMContext::MD_DIAssignID);
 
           LLVM_DEBUG(dbgs() << "Promoting " << *LI << " to " << *SI << " => "
                             << *M << "\n");
@@ -813,6 +816,7 @@
       IRBuilder<> Builder(SI);
       auto *M = Builder.CreateMemSet(SI->getPointerOperand(), ByteVal, Size,
                                      SI->getAlign());
+      M->copyMetadata(*SI, LLVMContext::MD_DIAssignID);
 
       LLVM_DEBUG(dbgs() << "Promoting " << *SI << " to " << *M << "\n");
 
@@ -1154,6 +1158,7 @@
     NewM = Builder.CreateMemCpy(M->getRawDest(), M->getDestAlign(),
                                 MDep->getRawSource(), MDep->getSourceAlign(),
                                 M->getLength(), M->isVolatile());
+  NewM->copyMetadata(*M, LLVMContext::MD_DIAssignID);
 
   assert(isa<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(M)));
   auto *LastDef = cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(M));
Index: llvm/test/DebugInfo/Generic/assignment-tracking/memcpyopt/merge-stores.ll
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/Generic/assignment-tracking/memcpyopt/merge-stores.ll
@@ -0,0 +1,160 @@
+; RUN: opt %s -S -memcpyopt -o - -experimental-assignment-tracking | FileCheck %s
+
+;; $ cat test.cpp
+;; struct v {
+;;   float c[4];
+;;
+;;   v(float d) {
+;;     c[0] = c[1] = c[2] = d;
+;;     c[3] = 0.f;
+;;   }
+;; };
+;;
+;; void esc(v*);
+;;
+;; v f() {
+;;   v g(0);
+;;   esc(&g);
+;; }
+;; IR grabbed before memcpyopt in:
+;; clang++ -Xclang -fexperimental-assignment-tracking -g -c -O2 test.cpp
+
+;; Check that the memset that memcpyopt creates to merge 4 stores merges the
+;; DIASsignIDs from the stores.
+
+; CHECK: call void @llvm.dbg.assign(metadata float 0.000000e+00, metadata ![[VAR:[0-9]+]], metadata !DIExpression(DW_OP_LLVM_fragment, 64, 32), metadata ![[ID:[0-9]+]], metadata float* %arrayidx.i, metadata !DIExpression())
+; CHECK: call void @llvm.dbg.assign(metadata float 0.000000e+00, metadata ![[VAR]], metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32), metadata ![[ID]], metadata float* %arrayidx3.i, metadata !DIExpression())
+; CHECK: call void @llvm.dbg.assign(metadata float 0.000000e+00, metadata ![[VAR]], metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32), metadata ![[ID]], metadata float* %arrayidx5.i, metadata !DIExpression())
+; CHECK: call void @llvm.dbg.assign(metadata float 0.000000e+00, metadata ![[VAR]], metadata !DIExpression(DW_OP_LLVM_fragment, 96, 32), metadata ![[ID]], metadata float* %arrayidx7.i, metadata !DIExpression())
+; CHECK: call void @llvm.memset.p0i8.i64(i8* align 4 %1, i8 0, i64 16, i1 false),{{.*}} !DIAssignID ![[ID]]
+
+%struct.v = type { [4 x float] }
+
+$_ZN1vC2Ef = comdat any
+
+; Function Attrs: uwtable
+define dso_local void @_Z1fv() local_unnamed_addr #0 !dbg !7 {
+entry:
+  %g = alloca %struct.v, align 4, !DIAssignID !23
+  call void @llvm.dbg.assign(metadata i1 undef, metadata !11, metadata !DIExpression(), metadata !23, metadata %struct.v* %g, metadata !DIExpression()), !dbg !24
+  %0 = bitcast %struct.v* %g to i8*, !dbg !25
+  call void @llvm.lifetime.start.p0i8(i64 16, i8* nonnull %0) #5, !dbg !25
+  call void @llvm.dbg.assign(metadata i1 undef, metadata !26, metadata !DIExpression(), metadata !31, metadata %struct.v** undef, metadata !DIExpression()), !dbg !32
+  call void @llvm.dbg.assign(metadata i1 undef, metadata !29, metadata !DIExpression(), metadata !34, metadata float* undef, metadata !DIExpression()), !dbg !32
+  call void @llvm.dbg.assign(metadata %struct.v* %g, metadata !26, metadata !DIExpression(), metadata !35, metadata %struct.v** undef, metadata !DIExpression()), !dbg !32
+  call void @llvm.dbg.assign(metadata float 0.000000e+00, metadata !29, metadata !DIExpression(), metadata !36, metadata float* undef, metadata !DIExpression()), !dbg !32
+  %arrayidx.i = getelementptr inbounds %struct.v, %struct.v* %g, i64 0, i32 0, i64 2, !dbg !37
+  store float 0.000000e+00, float* %arrayidx.i, align 4, !dbg !39, !DIAssignID !44
+  call void @llvm.dbg.assign(metadata float 0.000000e+00, metadata !11, metadata !DIExpression(DW_OP_LLVM_fragment, 64, 32), metadata !44, metadata float* %arrayidx.i, metadata !DIExpression()), !dbg !24
+  %arrayidx3.i = getelementptr inbounds %struct.v, %struct.v* %g, i64 0, i32 0, i64 1, !dbg !45
+  store float 0.000000e+00, float* %arrayidx3.i, align 4, !dbg !46, !DIAssignID !47
+  call void @llvm.dbg.assign(metadata float 0.000000e+00, metadata !11, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32), metadata !47, metadata float* %arrayidx3.i, metadata !DIExpression()), !dbg !24
+  %arrayidx5.i = getelementptr inbounds %struct.v, %struct.v* %g, i64 0, i32 0, i64 0, !dbg !48
+  store float 0.000000e+00, float* %arrayidx5.i, align 4, !dbg !49, !DIAssignID !50
+  call void @llvm.dbg.assign(metadata float 0.000000e+00, metadata !11, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32), metadata !50, metadata float* %arrayidx5.i, metadata !DIExpression()), !dbg !24
+  %arrayidx7.i = getelementptr inbounds %struct.v, %struct.v* %g, i64 0, i32 0, i64 3, !dbg !51
+  store float 0.000000e+00, float* %arrayidx7.i, align 4, !dbg !52, !DIAssignID !53
+  call void @llvm.dbg.assign(metadata float 0.000000e+00, metadata !11, metadata !DIExpression(DW_OP_LLVM_fragment, 96, 32), metadata !53, metadata float* %arrayidx7.i, metadata !DIExpression()), !dbg !24
+  call void @_Z3escP1v(%struct.v* nonnull %g), !dbg !54
+  call void @llvm.lifetime.end.p0i8(i64 16, i8* nonnull %0) #5, !dbg !55
+  ret void, !dbg !55
+}
+
+; Function Attrs: argmemonly nofree nosync nounwind willreturn
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #1
+
+; Function Attrs: nounwind uwtable
+define linkonce_odr dso_local void @_ZN1vC2Ef(%struct.v* %this, float %d) unnamed_addr #2 comdat align 2 !dbg !27 {
+entry:
+  call void @llvm.dbg.assign(metadata i1 undef, metadata !26, metadata !DIExpression(), metadata !56, metadata %struct.v** undef, metadata !DIExpression()), !dbg !57
+  call void @llvm.dbg.assign(metadata i1 undef, metadata !29, metadata !DIExpression(), metadata !58, metadata float* undef, metadata !DIExpression()), !dbg !57
+  call void @llvm.dbg.assign(metadata %struct.v* %this, metadata !26, metadata !DIExpression(), metadata !59, metadata %struct.v** undef, metadata !DIExpression()), !dbg !57
+  call void @llvm.dbg.assign(metadata float %d, metadata !29, metadata !DIExpression(), metadata !60, metadata float* undef, metadata !DIExpression()), !dbg !57
+  %arrayidx = getelementptr inbounds %struct.v, %struct.v* %this, i64 0, i32 0, i64 2, !dbg !61
+  store float %d, float* %arrayidx, align 4, !dbg !62
+  %arrayidx3 = getelementptr inbounds %struct.v, %struct.v* %this, i64 0, i32 0, i64 1, !dbg !63
+  store float %d, float* %arrayidx3, align 4, !dbg !64
+  %arrayidx5 = getelementptr inbounds %struct.v, %struct.v* %this, i64 0, i32 0, i64 0, !dbg !65
+  store float %d, float* %arrayidx5, align 4, !dbg !66
+  %arrayidx7 = getelementptr inbounds %struct.v, %struct.v* %this, i64 0, i32 0, i64 3, !dbg !67
+  store float 0.000000e+00, float* %arrayidx7, align 4, !dbg !68
+  ret void, !dbg !69
+}
+
+declare !dbg !70 dso_local void @_Z3escP1v(%struct.v*) local_unnamed_addr
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture)
+declare void @llvm.dbg.assign(metadata, metadata, metadata, 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 12.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "reduce.cpp", directory: "/")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 12.0.0"}
+!7 = distinct !DISubprogram(name: "f", linkageName: "_Z1fv", scope: !1, file: !1, line: 12, type: !8, scopeLine: 12, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !10)
+!8 = !DISubroutineType(types: !9)
+!9 = !{null}
+!10 = !{!11}
+!11 = !DILocalVariable(name: "g", scope: !7, file: !1, line: 13, type: !12)
+!12 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "v", file: !1, line: 1, size: 128, flags: DIFlagTypePassByValue | DIFlagNonTrivial, elements: !13, identifier: "_ZTS1v")
+!13 = !{!14, !19}
+!14 = !DIDerivedType(tag: DW_TAG_member, name: "c", scope: !12, file: !1, line: 2, baseType: !15, size: 128)
+!15 = !DICompositeType(tag: DW_TAG_array_type, baseType: !16, size: 128, elements: !17)
+!16 = !DIBasicType(name: "float", size: 32, encoding: DW_ATE_float)
+!17 = !{!18}
+!18 = !DISubrange(count: 4)
+!19 = !DISubprogram(name: "v", scope: !12, file: !1, line: 4, type: !20, scopeLine: 4, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized)
+!20 = !DISubroutineType(types: !21)
+!21 = !{null, !22, !16}
+!22 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 64, flags: DIFlagArtificial | DIFlagObjectPointer)
+!23 = distinct !DIAssignID()
+!24 = !DILocation(line: 0, scope: !7)
+!25 = !DILocation(line: 13, column: 3, scope: !7)
+!26 = !DILocalVariable(name: "this", arg: 1, scope: !27, type: !30, flags: DIFlagArtificial | DIFlagObjectPointer)
+!27 = distinct !DISubprogram(name: "v", linkageName: "_ZN1vC2Ef", scope: !12, file: !1, line: 4, type: !20, scopeLine: 4, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, declaration: !19, retainedNodes: !28)
+!28 = !{!26, !29}
+!29 = !DILocalVariable(name: "d", arg: 2, scope: !27, file: !1, line: 4, type: !16)
+!30 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 64)
+!31 = distinct !DIAssignID()
+!32 = !DILocation(line: 0, scope: !27, inlinedAt: !33)
+!33 = distinct !DILocation(line: 13, column: 5, scope: !7)
+!34 = distinct !DIAssignID()
+!35 = distinct !DIAssignID()
+!36 = distinct !DIAssignID()
+!37 = !DILocation(line: 5, column: 19, scope: !38, inlinedAt: !33)
+!38 = distinct !DILexicalBlock(scope: !27, file: !1, line: 4, column: 14)
+!39 = !DILocation(line: 5, column: 24, scope: !38, inlinedAt: !33)
+!44 = distinct !DIAssignID()
+!45 = !DILocation(line: 5, column: 12, scope: !38, inlinedAt: !33)
+!46 = !DILocation(line: 5, column: 17, scope: !38, inlinedAt: !33)
+!47 = distinct !DIAssignID()
+!48 = !DILocation(line: 5, column: 5, scope: !38, inlinedAt: !33)
+!49 = !DILocation(line: 5, column: 10, scope: !38, inlinedAt: !33)
+!50 = distinct !DIAssignID()
+!51 = !DILocation(line: 6, column: 5, scope: !38, inlinedAt: !33)
+!52 = !DILocation(line: 6, column: 10, scope: !38, inlinedAt: !33)
+!53 = distinct !DIAssignID()
+!54 = !DILocation(line: 14, column: 3, scope: !7)
+!55 = !DILocation(line: 15, column: 1, scope: !7)
+!56 = distinct !DIAssignID()
+!57 = !DILocation(line: 0, scope: !27)
+!58 = distinct !DIAssignID()
+!59 = distinct !DIAssignID()
+!60 = distinct !DIAssignID()
+!61 = !DILocation(line: 5, column: 19, scope: !38)
+!62 = !DILocation(line: 5, column: 24, scope: !38)
+!63 = !DILocation(line: 5, column: 12, scope: !38)
+!64 = !DILocation(line: 5, column: 17, scope: !38)
+!65 = !DILocation(line: 5, column: 5, scope: !38)
+!66 = !DILocation(line: 5, column: 10, scope: !38)
+!67 = !DILocation(line: 6, column: 5, scope: !38)
+!68 = !DILocation(line: 6, column: 10, scope: !38)
+!69 = !DILocation(line: 7, column: 3, scope: !27)
+!70 = !DISubprogram(name: "esc", linkageName: "_Z3escP1v", scope: !1, file: !1, line: 10, type: !71, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !2)
+!71 = !DISubroutineType(types: !72)
+!72 = !{null, !30}