diff --git a/clang/include/clang/AST/Mangle.h b/clang/include/clang/AST/Mangle.h
--- a/clang/include/clang/AST/Mangle.h
+++ b/clang/include/clang/AST/Mangle.h
@@ -194,6 +194,8 @@
 
   virtual void mangleDynamicStermFinalizer(const VarDecl *D, raw_ostream &) = 0;
 
+  virtual void mangleModuleInitializer(const Module *Module, raw_ostream &) = 0;
+
   // This has to live here, otherwise the CXXNameMangler won't have access to
   // it.
   virtual DiscriminatorOverrideTy getDiscriminatorOverride() const = 0;
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -1540,6 +1540,11 @@
 }
 
 Module *Decl::getOwningModuleForLinkage(bool IgnoreLinkage) const {
+  if (isa<NamespaceDecl>(this))
+    // Namespaces never have module linkage.  It is the entities within them
+    // that [may] do.
+    return nullptr;
+
   Module *M = getOwningModule();
   if (!M)
     return nullptr;
diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -130,6 +130,8 @@
 
   void mangleLambdaSig(const CXXRecordDecl *Lambda, raw_ostream &) override;
 
+  void mangleModuleInitializer(const Module *Module, raw_ostream &) override;
+
   bool getNextDiscriminator(const NamedDecl *ND, unsigned &disc) {
     // Lambda closure types are already numbered.
     if (isLambda(ND))
@@ -438,6 +440,7 @@
   void mangleType(QualType T);
   void mangleNameOrStandardSubstitution(const NamedDecl *ND);
   void mangleLambdaSig(const CXXRecordDecl *Lambda);
+  void mangleModuleNamePrefix(StringRef Name, bool IsPartition = false);
 
 private:
 
@@ -473,22 +476,21 @@
 
   void mangleNameWithAbiTags(GlobalDecl GD,
                              const AbiTagList *AdditionalAbiTags);
-  void mangleModuleName(const Module *M);
-  void mangleModuleNamePrefix(StringRef Name);
+  void mangleModuleName(const NamedDecl *ND);
   void mangleTemplateName(const TemplateDecl *TD,
                           const TemplateArgument *TemplateArgs,
                           unsigned NumTemplateArgs);
-  void mangleUnqualifiedName(GlobalDecl GD,
+  void mangleUnqualifiedName(GlobalDecl GD, const DeclContext *DC,
                              const AbiTagList *AdditionalAbiTags) {
-    mangleUnqualifiedName(GD, cast<NamedDecl>(GD.getDecl())->getDeclName(), UnknownArity,
-                          AdditionalAbiTags);
+    mangleUnqualifiedName(GD, cast<NamedDecl>(GD.getDecl())->getDeclName(), DC,
+                          UnknownArity, AdditionalAbiTags);
   }
   void mangleUnqualifiedName(GlobalDecl GD, DeclarationName Name,
-                             unsigned KnownArity,
+                             const DeclContext *DC, unsigned KnownArity,
                              const AbiTagList *AdditionalAbiTags);
-  void mangleUnscopedName(GlobalDecl GD,
+  void mangleUnscopedName(GlobalDecl GD, const DeclContext *DC,
                           const AbiTagList *AdditionalAbiTags);
-  void mangleUnscopedTemplateName(GlobalDecl GD,
+  void mangleUnscopedTemplateName(GlobalDecl GD, const DeclContext *DC,
                                   const AbiTagList *AdditionalAbiTags);
   void mangleSourceName(const IdentifierInfo *II);
   void mangleRegCallName(const IdentifierInfo *II);
@@ -733,7 +735,8 @@
     if (VD->isExternC())
       return false;
 
-    // Variables at global scope with non-internal linkage are not mangled.
+    // Variables at global scope are not mangled unless they have internal
+    // linkage or are specializations or are attached to a named module.
     const DeclContext *DC = getEffectiveDeclContext(D);
     // Check for extern variable declared locally.
     if (DC->isFunctionOrMethod() && D->hasLinkage())
@@ -741,7 +744,8 @@
         DC = getEffectiveParentContext(DC);
     if (DC->isTranslationUnit() && D->getFormalLinkage() != InternalLinkage &&
         !CXXNameMangler::shouldHaveAbiTags(*this, VD) &&
-        !isa<VarTemplateSpecializationDecl>(VD))
+        !isa<VarTemplateSpecializationDecl>(VD) &&
+        !VD->getOwningModuleForLinkage())
       return false;
   }
 
@@ -1016,14 +1020,6 @@
     return;
   }
 
-  // Do not mangle the owning module for an external linkage declaration.
-  // This enables backwards-compatibility with non-modular code, and is
-  // a valid choice since conflicts are not permitted by C++ Modules TS
-  // [basic.def.odr]/6.2.
-  if (!ND->hasExternalFormalLinkage())
-    if (Module *M = ND->getOwningModuleForLinkage())
-      mangleModuleName(M);
-
   // Closures can require a nested-name mangling even if they're semantically
   // in the global namespace.
   if (const NamedDecl *PrefixND = getClosurePrefix(ND)) {
@@ -1035,38 +1031,35 @@
     // Check if we have a template.
     const TemplateArgumentList *TemplateArgs = nullptr;
     if (GlobalDecl TD = isTemplate(GD, TemplateArgs)) {
-      mangleUnscopedTemplateName(TD, AdditionalAbiTags);
+      mangleUnscopedTemplateName(TD, DC, AdditionalAbiTags);
       mangleTemplateArgs(asTemplateName(TD), *TemplateArgs);
       return;
     }
 
-    mangleUnscopedName(GD, AdditionalAbiTags);
+    mangleUnscopedName(GD, DC, AdditionalAbiTags);
     return;
   }
 
   mangleNestedName(GD, DC, AdditionalAbiTags);
 }
 
-void CXXNameMangler::mangleModuleName(const Module *M) {
-  // Implement the C++ Modules TS name mangling proposal; see
-  //     https://gcc.gnu.org/wiki/cxx-modules?action=AttachFile
-  //
-  //   <module-name> ::= W <unscoped-name>+ E
-  //                 ::= W <module-subst> <unscoped-name>* E
-  Out << 'W';
-  mangleModuleNamePrefix(M->Name);
-  Out << 'E';
+void CXXNameMangler::mangleModuleName(const NamedDecl *ND) {
+  if (ND->isExternallyVisible())
+    if (Module *M = ND->getOwningModuleForLinkage())
+      mangleModuleNamePrefix(M->getPrimaryModuleInterfaceName());
 }
 
-void CXXNameMangler::mangleModuleNamePrefix(StringRef Name) {
-  //  <module-subst> ::= _ <seq-id>          # 0 < seq-id < 10
-  //                 ::= W <seq-id - 10> _   # otherwise
+// <module-name> ::= <module-subname>
+//		 ::= <module-name> <module-subname>
+//	 	 ::= <substitution>
+// <module-subname> ::= W <source-name>
+//		    ::= W P <source-name>
+void CXXNameMangler::mangleModuleNamePrefix(StringRef Name, bool IsPartition) {
+  //  <substitution> ::= S <seq-id> _
   auto It = ModuleSubstitutions.find(Name);
   if (It != ModuleSubstitutions.end()) {
-    if (It->second < 10)
-      Out << '_' << static_cast<char>('0' + It->second);
-    else
-      Out << 'W' << (It->second - 10) << '_';
+    Out << 'S';
+    mangleSeqID(It->second);
     return;
   }
 
@@ -1075,11 +1068,16 @@
   auto Parts = Name.rsplit('.');
   if (Parts.second.empty())
     Parts.second = Parts.first;
-  else
-    mangleModuleNamePrefix(Parts.first);
+  else {
+    mangleModuleNamePrefix(Parts.first, IsPartition);
+    IsPartition = false;
+  }
 
+  Out << 'W';
+  if (IsPartition)
+    Out << 'P';
   Out << Parts.second.size() << Parts.second;
-  ModuleSubstitutions.insert({Name, ModuleSubstitutions.size()});
+  ModuleSubstitutions.insert({Name, SeqID++});
 }
 
 void CXXNameMangler::mangleTemplateName(const TemplateDecl *TD,
@@ -1088,27 +1086,27 @@
   const DeclContext *DC = Context.getEffectiveDeclContext(TD);
 
   if (DC->isTranslationUnit() || isStdNamespace(DC)) {
-    mangleUnscopedTemplateName(TD, nullptr);
+    mangleUnscopedTemplateName(TD, DC, nullptr);
     mangleTemplateArgs(asTemplateName(TD), TemplateArgs, NumTemplateArgs);
   } else {
     mangleNestedName(TD, TemplateArgs, NumTemplateArgs);
   }
 }
 
-void CXXNameMangler::mangleUnscopedName(GlobalDecl GD,
+void CXXNameMangler::mangleUnscopedName(GlobalDecl GD, const DeclContext *DC,
                                         const AbiTagList *AdditionalAbiTags) {
-  const NamedDecl *ND = cast<NamedDecl>(GD.getDecl());
   //  <unscoped-name> ::= <unqualified-name>
   //                  ::= St <unqualified-name>   # ::std::
 
-  if (isStdNamespace(Context.getEffectiveDeclContext(ND)))
+  assert(!isa<LinkageSpecDecl>(DC) && "unskipped LinkageSpecDecl");
+  if (isStdNamespace(DC))
     Out << "St";
 
-  mangleUnqualifiedName(GD, AdditionalAbiTags);
+  mangleUnqualifiedName(GD, DC, AdditionalAbiTags);
 }
 
 void CXXNameMangler::mangleUnscopedTemplateName(
-    GlobalDecl GD, const AbiTagList *AdditionalAbiTags) {
+    GlobalDecl GD, const DeclContext *DC, const AbiTagList *AdditionalAbiTags) {
   const TemplateDecl *ND = cast<TemplateDecl>(GD.getDecl());
   //     <unscoped-template-name> ::= <unscoped-name>
   //                              ::= <substitution>
@@ -1121,9 +1119,10 @@
            "template template param cannot have abi tags");
     mangleTemplateParameter(TTP->getDepth(), TTP->getIndex());
   } else if (isa<BuiltinTemplateDecl>(ND) || isa<ConceptDecl>(ND)) {
-    mangleUnscopedName(GD, AdditionalAbiTags);
+    mangleUnscopedName(GD, DC, AdditionalAbiTags);
   } else {
-    mangleUnscopedName(GD.getWithDecl(ND->getTemplatedDecl()), AdditionalAbiTags);
+    mangleUnscopedName(GD.getWithDecl(ND->getTemplatedDecl()), DC,
+                       AdditionalAbiTags);
   }
 
   addSubstitution(ND);
@@ -1399,15 +1398,19 @@
     mangleTemplateArgs(TemplateName(), TemplateArgs, NumTemplateArgs);
 }
 
-void CXXNameMangler::mangleUnqualifiedName(GlobalDecl GD,
-                                           DeclarationName Name,
-                                           unsigned KnownArity,
-                                           const AbiTagList *AdditionalAbiTags) {
+void CXXNameMangler::mangleUnqualifiedName(
+    GlobalDecl GD, DeclarationName Name, const DeclContext *DC,
+    unsigned KnownArity, const AbiTagList *AdditionalAbiTags) {
   const NamedDecl *ND = cast_or_null<NamedDecl>(GD.getDecl());
-  unsigned Arity = KnownArity;
-  //  <unqualified-name> ::= <operator-name>
+  //  <unqualified-name> ::= [<module-name>] <operator-name>
   //                     ::= <ctor-dtor-name>
-  //                     ::= <source-name>
+  //                     ::= [<module-name>] <source-name>
+  //                     ::= [<module-name>] DC <source-name>* E
+
+  if (ND && DC && DC->isFileContext())
+    mangleModuleName(ND);
+
+  unsigned Arity = KnownArity;
   switch (Name.getNameKind()) {
   case DeclarationName::Identifier: {
     const IdentifierInfo *II = Name.getAsIdentifierInfo();
@@ -1418,8 +1421,6 @@
       //
       //  <unqualified-name> ::= DC <source-name>* E
       //
-      // These can never be referenced across translation units, so we do
-      // not need a cross-vendor mangling for anything other than demanglers.
       // Proposed on cxx-abi-dev on 2016-08-12
       Out << "DC";
       for (auto *BD : DD->bindings())
@@ -1716,7 +1717,7 @@
     mangleTemplateArgs(asTemplateName(TD), *TemplateArgs);
   } else {
     manglePrefix(DC, NoFunction);
-    mangleUnqualifiedName(GD, AdditionalAbiTags);
+    mangleUnqualifiedName(GD, DC, AdditionalAbiTags);
   }
 
   Out << 'E';
@@ -1746,7 +1747,7 @@
   Out << 'N';
 
   mangleClosurePrefix(PrefixND);
-  mangleUnqualifiedName(GD, AdditionalAbiTags);
+  mangleUnqualifiedName(GD, nullptr, AdditionalAbiTags);
 
   Out << 'E';
 }
@@ -1824,7 +1825,7 @@
     // Mangle the name relative to the closest enclosing function.
     // equality ok because RD derived from ND above
     if (D == RD)  {
-      mangleUnqualifiedName(RD, AdditionalAbiTags);
+      mangleUnqualifiedName(RD, DC, AdditionalAbiTags);
     } else if (const BlockDecl *BD = dyn_cast<BlockDecl>(D)) {
       if (const NamedDecl *PrefixND = getClosurePrefix(BD))
         mangleClosurePrefix(PrefixND, true /*NoFunction*/);
@@ -1855,7 +1856,7 @@
     assert(!AdditionalAbiTags && "Block cannot have additional abi tags");
     mangleUnqualifiedBlock(BD);
   } else {
-    mangleUnqualifiedName(GD, AdditionalAbiTags);
+    mangleUnqualifiedName(GD, DC, AdditionalAbiTags);
   }
 
   if (const NamedDecl *ND = dyn_cast<NamedDecl>(RD ? RD : D)) {
@@ -2082,10 +2083,11 @@
     mangleTemplateArgs(asTemplateName(TD), *TemplateArgs);
   } else if (const NamedDecl *PrefixND = getClosurePrefix(ND)) {
     mangleClosurePrefix(PrefixND, NoFunction);
-    mangleUnqualifiedName(ND, nullptr);
+    mangleUnqualifiedName(ND, nullptr, nullptr);
   } else {
-    manglePrefix(Context.getEffectiveDeclContext(ND), NoFunction);
-    mangleUnqualifiedName(ND, nullptr);
+    const DeclContext *DC = Context.getEffectiveDeclContext(ND);
+    manglePrefix(DC, NoFunction);
+    mangleUnqualifiedName(ND, DC, nullptr);
   }
 
   addSubstitution(ND);
@@ -2138,11 +2140,13 @@
   if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(ND)) {
     mangleTemplateParameter(TTP->getDepth(), TTP->getIndex());
   } else {
-    manglePrefix(Context.getEffectiveDeclContext(ND), NoFunction);
+    const DeclContext *DC = Context.getEffectiveDeclContext(ND);
+    manglePrefix(DC, NoFunction);
     if (isa<BuiltinTemplateDecl>(ND) || isa<ConceptDecl>(ND))
-      mangleUnqualifiedName(GD, nullptr);
+      mangleUnqualifiedName(GD, DC, nullptr);
     else
-      mangleUnqualifiedName(GD.getWithDecl(ND->getTemplatedDecl()), nullptr);
+      mangleUnqualifiedName(GD.getWithDecl(ND->getTemplatedDecl()), DC,
+                            nullptr);
   }
 
   addSubstitution(ND);
@@ -2183,8 +2187,9 @@
     mangleTemplatePrefix(TD, NoFunction);
     mangleTemplateArgs(asTemplateName(TD), *TemplateArgs);
   } else {
-    manglePrefix(Context.getEffectiveDeclContext(ND), NoFunction);
-    mangleUnqualifiedName(ND, nullptr);
+    const auto *DC = Context.getEffectiveDeclContext(ND);
+    manglePrefix(DC, NoFunction);
+    mangleUnqualifiedName(ND, DC, nullptr);
   }
 
   Out << 'M';
@@ -6026,6 +6031,9 @@
   if (TemplateArgs[0].getAsType() != A)
     return false;
 
+  if (SD->getSpecializedTemplate()->getOwningModuleForLinkage())
+    return false;
+
   return true;
 }
 
@@ -6057,6 +6065,9 @@
       !isSpecializedAs(TemplateArgs[2].getAsType(), "allocator", A))
     return false;
 
+  if (SD->getSpecializedTemplate()->getOwningModuleForLinkage())
+    return false;
+
   return true;
 }
 
@@ -6074,6 +6085,9 @@
     if (!isStdNamespace(Context.getEffectiveDeclContext(TD)))
       return false;
 
+    if (TD->getOwningModuleForLinkage())
+      return false;
+
     // <substitution> ::= Sa # ::std::allocator
     if (TD->getIdentifier()->isStr("allocator")) {
       Out << "Sa";
@@ -6093,6 +6107,9 @@
     if (!isStdNamespace(Context.getEffectiveDeclContext(SD)))
       return false;
 
+    if (SD->getSpecializedTemplate()->getOwningModuleForLinkage())
+      return false;
+
     //    <substitution> ::= Ss # ::std::basic_string<char,
     //                            ::std::char_traits<char>,
     //                            ::std::allocator<char> >
@@ -6447,6 +6464,19 @@
   Mangler.mangleLambdaSig(Lambda);
 }
 
+void ItaniumMangleContextImpl::mangleModuleInitializer(const Module *M,
+                                                       raw_ostream &Out) {
+  CXXNameMangler Mangler(*this, Out);
+  Mangler.getStream() << "_ZGI";
+  Mangler.mangleModuleNamePrefix(M->getPrimaryModuleInterfaceName());
+  if (M->isModulePartition()) {
+    auto Partition = M->Name.find(':');
+    Mangler.mangleModuleNamePrefix(
+        StringRef(&M->Name[Partition + 1], M->Name.size() - Partition - 1),
+        /*IsPartition*/ true);
+  }
+}
+
 ItaniumMangleContext *ItaniumMangleContext::create(ASTContext &Context,
                                                    DiagnosticsEngine &Diags) {
   return new ItaniumMangleContextImpl(
diff --git a/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/module.cpp b/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/module.cpp
--- a/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/module.cpp
+++ b/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/module.cpp
@@ -1,21 +1,21 @@
 // RUN: %clang_cc1 -fmodules-ts %S/module.cppm -triple %itanium_abi_triple -emit-module-interface -o %t
 // RUN: %clang_cc1 -fmodules-ts %s -triple %itanium_abi_triple -fmodule-file=%t -emit-llvm -o - | FileCheck %s --implicit-check-not=unused --implicit-check-not=global_module
 
-// CHECK-DAG: @extern_var_exported = external {{(dso_local )?}}global
-// CHECK-DAG: @inline_var_exported = linkonce_odr {{(dso_local )?}}global
-// CHECK-DAG: @const_var_exported = available_externally {{(dso_local )?}}constant i32 3,
+// CHECK-DAG: @_ZW6Module19extern_var_exported = external {{(dso_local )?}}global
+// CHECK-DAG: @_ZW6Module19inline_var_exported = linkonce_odr {{(dso_local )?}}global
+// CHECK-DAG: @_ZW6Module18const_var_exported = available_externally {{(dso_local )?}}constant i32 3,
 //
-// CHECK-DAG: @_ZW6ModuleE25extern_var_module_linkage = external {{(dso_local )?}}global
-// CHECK-DAG: @_ZW6ModuleE25inline_var_module_linkage = linkonce_odr {{(dso_local )?}}global
-// CHECK-DAG: @_ZW6ModuleE25static_var_module_linkage = available_externally {{(dso_local )?}}global i32 0,
-// CHECK-DAG: @_ZW6ModuleE24const_var_module_linkage = available_externally {{(dso_local )?}}constant i32 3,
+// CHECK-DAG: @_ZW6Module25extern_var_module_linkage = external {{(dso_local )?}}global
+// CHECK-DAG: @_ZW6Module25inline_var_module_linkage = linkonce_odr {{(dso_local )?}}global
+// CHECK-DAG: @_ZW6Module25static_var_module_linkage = available_externally {{(dso_local )?}}global i32 0,
+// CHECK-DAG: @_ZW6Module24const_var_module_linkage = available_externally {{(dso_local )?}}constant i32 3,
 
 module Module;
 
 void use() {
-  // CHECK: define linkonce_odr {{.*}}@_Z20used_inline_exportedv
+  // CHECK: define linkonce_odr {{.*}}@_ZW6Module20used_inline_exportedv
   used_inline_exported();
-  // CHECK: declare {{.*}}@_Z18noninline_exportedv
+  // CHECK: declare {{.*}}@_ZW6Module18noninline_exportedv
   noninline_exported();
 
   (void)&extern_var_exported;
@@ -23,13 +23,13 @@
   (void)&const_var_exported;
 
   // FIXME: This symbol should not be visible here.
-  // CHECK: declare {{.*}}@_ZW6ModuleE26used_static_module_linkagev
+  // CHECK: declare {{.*}}@_ZW6Module26used_static_module_linkagev
   used_static_module_linkage();
 
-  // CHECK: define linkonce_odr {{.*}}@_ZW6ModuleE26used_inline_module_linkagev
+  // CHECK: define linkonce_odr {{.*}}@_ZW6Module26used_inline_module_linkagev
   used_inline_module_linkage();
 
-  // CHECK: declare {{.*}}@_ZW6ModuleE24noninline_module_linkagev
+  // CHECK: declare {{.*}}@_ZW6Module24noninline_module_linkagev
   noninline_module_linkage();
 
   (void)&extern_var_module_linkage;
diff --git a/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/module.cppm b/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/module.cppm
--- a/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/module.cppm
+++ b/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/module.cppm
@@ -6,24 +6,24 @@
 // CHECK-DAG: @_ZL23const_var_global_module = internal constant
 //
 // For ABI compatibility, these symbols do not include the module name.
-// CHECK-DAG: @extern_var_exported = external {{(dso_local )?}}global
+// CHECK-DAG: @_ZW6Module19extern_var_exported = external {{(dso_local )?}}global
 // FIXME: Should this be 'weak_odr global'? Presumably it must be, since we
 // can discard this global and its initializer (if any), and other TUs are not
 // permitted to run the initializer for this variable.
-// CHECK-DAG: @inline_var_exported = linkonce_odr {{(dso_local )?}}global
-// CHECK-DAG: @const_var_exported = {{(dso_local )?}}constant
+// CHECK-DAG: @_ZW6Module19inline_var_exported = linkonce_odr {{(dso_local )?}}global
+// CHECK-DAG: @_ZW6Module18const_var_exported = {{(dso_local )?}}constant
 //
-// CHECK-DAG: @_ZW6ModuleE25extern_var_module_linkage = external {{(dso_local )?}}global
+// CHECK-DAG: @_ZW6Module25extern_var_module_linkage = external {{(dso_local )?}}global
 // FIXME: Should this be 'weak_odr global'? Presumably it must be, since we
 // can discard this global and its initializer (if any), and other TUs are not
 // permitted to run the initializer for this variable.
-// CHECK-DAG: @_ZW6ModuleE25inline_var_module_linkage = linkonce_odr {{(dso_local )?}}global
-// CHECK-DAG: @_ZW6ModuleE25static_var_module_linkage = {{(dso_local )?}}global
-// CHECK-DAG: @_ZW6ModuleE24const_var_module_linkage = {{(dso_local )?}}constant
+// CHECK-DAG: @_ZW6Module25inline_var_module_linkage = linkonce_odr {{(dso_local )?}}global
+// CHECK-DAG: @_ZW6Module25static_var_module_linkage = {{(dso_local )?}}global
+// CHECK-DAG: @_ZW6Module24const_var_module_linkage = {{(dso_local )?}}constant
 //
-// CHECK-DAG: @_ZW6ModuleE25unused_var_module_linkage = {{(dso_local )?}}global i32 4
-// CHECK-DAG: @_ZW6ModuleE32unused_static_var_module_linkage = {{(dso_local )?}}global i32 5
-// CHECK-DAG: @_ZW6ModuleE31unused_const_var_module_linkage = {{(dso_local )?}}constant i32 7
+// CHECK-DAG: @_ZW6Module25unused_var_module_linkage = {{(dso_local )?}}global i32 4
+// CHECK-DAG: @_ZW6Module32unused_static_var_module_linkage = {{(dso_local )?}}global i32 5
+// CHECK-DAG: @_ZW6Module31unused_const_var_module_linkage = {{(dso_local )?}}constant i32 7
 
 static void unused_static_global_module() {}
 static void used_static_global_module() {}
@@ -64,7 +64,7 @@
   inline int inline_var_exported;
   const int const_var_exported = 3;
 
-  // CHECK: define {{(dso_local )?}}void {{.*}}@_Z18noninline_exportedv
+  // CHECK: define {{(dso_local )?}}void {{.*}}@_ZW6Module18noninline_exportedv
   void noninline_exported() {
     (void)&extern_var_exported;
     (void)&inline_var_exported;
@@ -75,9 +75,9 @@
 // FIXME: Ideally we wouldn't emit this as its name is not visible outside this
 // TU, but this module interface might contain a template that can use this
 // function so we conservatively emit it for now.
-// CHECK: define {{(dso_local )?}}void {{.*}}@_ZW6ModuleE28unused_static_module_linkagev
+// CHECK: define {{(dso_local )?}}void {{.*}}@_ZW6Module28unused_static_module_linkagev
 static void unused_static_module_linkage() {}
-// CHECK: define {{(dso_local )?}}void {{.*}}@_ZW6ModuleE26used_static_module_linkagev
+// CHECK: define {{(dso_local )?}}void {{.*}}@_ZW6Module26used_static_module_linkagev
 static void used_static_module_linkage() {}
 
 inline void unused_inline_module_linkage() {}
@@ -88,10 +88,10 @@
 static int static_var_module_linkage;
 const int const_var_module_linkage = 3;
 
-// CHECK: define {{(dso_local )?}}void {{.*}}@_ZW6ModuleE24noninline_module_linkagev
+// CHECK: define {{(dso_local )?}}void {{.*}}@_ZW6Module24noninline_module_linkagev
 void noninline_module_linkage() {
   used_static_module_linkage();
-  // CHECK: define linkonce_odr {{.*}}@_ZW6ModuleE26used_inline_module_linkagev
+  // CHECK: define linkonce_odr {{.*}}@_ZW6Module26used_inline_module_linkagev
   used_inline_module_linkage();
 
   (void)&extern_var_module_linkage;
@@ -109,5 +109,5 @@
   struct b {};
   struct c {};
 };
-// CHECK: define {{(dso_local )?}}void @_ZW6ModuleE1fW_0EN1a1bEW_0ENS_1cE(
+// CHECK: define {{(dso_local )?}}void @_ZW6Module1fNS_1a1bENS0_1cE(
 void f(a::b, a::c) {}
diff --git a/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/user.cpp b/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/user.cpp
--- a/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/user.cpp
+++ b/clang/test/CXX/modules-ts/basic/basic.def.odr/p4/user.cpp
@@ -1,16 +1,16 @@
 // RUN: %clang_cc1 -fmodules-ts %S/module.cppm -triple %itanium_abi_triple -emit-module-interface -o %t
 // RUN: %clang_cc1 -fmodules-ts %s -triple %itanium_abi_triple -fmodule-file=%t -emit-llvm -o - | FileCheck %s --implicit-check-not=unused --implicit-check-not=global_module
 
-// CHECK-DAG: @extern_var_exported = external {{(dso_local )?}}global
-// CHECK-DAG: @inline_var_exported = linkonce_odr {{(dso_local )?}}global
-// CHECK-DAG: @const_var_exported = available_externally {{(dso_local )?}}constant i32 3
+// CHECK-DAG: @_ZW6Module19extern_var_exported = external {{(dso_local )?}}global
+// CHECK-DAG: @_ZW6Module19inline_var_exported = linkonce_odr {{(dso_local )?}}global
+// CHECK-DAG: @_ZW6Module18const_var_exported = available_externally {{(dso_local )?}}constant i32 3
 
 import Module;
 
 void use() {
-  // CHECK: define linkonce_odr {{.*}}@_Z20used_inline_exportedv
+  // CHECK: define linkonce_odr {{.*}}@_ZW6Module20used_inline_exportedv
   used_inline_exported();
-  // CHECK: declare {{.*}}@_Z18noninline_exportedv
+  // CHECK: declare {{.*}}@_ZW6Module18noninline_exportedv
   noninline_exported();
 
   (void)&extern_var_exported;
diff --git a/clang/test/CXX/modules-ts/basic/basic.link/p3.cppm b/clang/test/CXX/modules-ts/basic/basic.link/p3.cppm
--- a/clang/test/CXX/modules-ts/basic/basic.link/p3.cppm
+++ b/clang/test/CXX/modules-ts/basic/basic.link/p3.cppm
@@ -3,9 +3,9 @@
 
 export module M;
 
-// CHECK-DAG: @_ZW1ME1a ={{.*}} constant i32 1
+// CHECK-DAG: @_ZW1M1a ={{.*}} constant i32 1
 const int a = 1;
-// CHECK-DAG: @b ={{.*}} constant i32 2
+// CHECK-DAG: @_ZW1M1b ={{.*}} constant i32 2
 export const int b = 2;
 
 export int f() { return a + b; }
diff --git a/clang/test/CXX/modules-ts/codegen-basics.cppm b/clang/test/CXX/modules-ts/codegen-basics.cppm
--- a/clang/test/CXX/modules-ts/codegen-basics.cppm
+++ b/clang/test/CXX/modules-ts/codegen-basics.cppm
@@ -4,14 +4,14 @@
 export module FooBar;
 
 export {
-  // CHECK-DAG: define{{.*}} i32 @_Z1fv(
+  // CHECK-DAG: define{{.*}} i32 @_ZW6FooBar1fv(
   int f() { return 0; }
 }
 
-// CHECK-DAG: define weak_odr void @_ZW6FooBarE2f2v(
+// CHECK-DAG: define weak_odr void @_ZW6FooBar2f2v(
 inline void f2() {}
 
-// CHECK-DAG: define{{.*}} void @_ZW6FooBarE2f3v(
+// CHECK-DAG: define{{.*}} void @_ZW6FooBar2f3v(
 static void f3() {}
 export void use_f3() { f3(); }
 
diff --git a/clang/test/CodeGenCXX/Inputs/cxx20-module-impl-1a.cpp b/clang/test/CodeGenCXX/Inputs/cxx20-module-impl-1a.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/Inputs/cxx20-module-impl-1a.cpp
@@ -0,0 +1,3 @@
+export module Foo;
+export void Exported();
+void Module();
diff --git a/clang/test/CodeGenCXX/Inputs/cxx20-module-std-subst-2a.cpp b/clang/test/CodeGenCXX/Inputs/cxx20-module-std-subst-2a.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/Inputs/cxx20-module-std-subst-2a.cpp
@@ -0,0 +1,11 @@
+module;
+# 5 __FILE__ 1
+namespace std {
+template <typename A> struct allocator {};
+template <typename C, typename T, typename A>
+class basic_string;
+} // namespace std
+# 12 "" 2
+export module RenameString;
+export template <typename C, typename T>
+using str = std::basic_string<C, T, std::allocator<C>>;
diff --git a/clang/test/CodeGenCXX/cxx20-module-decomp-1.cpp b/clang/test/CodeGenCXX/cxx20-module-decomp-1.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-decomp-1.cpp
@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -emit-llvm -o - | FileCheck %s
+
+export module Foo;
+
+// CHECK-DAG: @_ZW3Foo3ary =
+int ary[2];
+
+// CHECK-DAG: @_ZW3FooDC1a1bE =
+export auto &[a, b] = ary;
+
+namespace N {
+// CHECK-DAG: @_ZN1NW3FooDC1a1bEE =
+export auto &[a, b] = ary;
+// CHECK-DAG: @_ZN1NW3FooDC1c1dEE =
+auto &[c, d] = ary;
+// FIXME: We mangle the module name here, as we give this ModuleInternalLinkage
+// That's no longer needed.
+// CHECK DAG: @_ZN1MDC1e1fEE =
+static auto &__attribute__((used))[e, f] = ary;
+} // namespace N
diff --git a/clang/test/CodeGenCXX/cxx20-module-extern-1.cppm b/clang/test/CodeGenCXX/cxx20-module-extern-1.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-extern-1.cppm
@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -emit-llvm -o - | FileCheck %s
+
+// module-purview extern "C++" semantics not implemented
+// XFAIL: *
+
+export module FOO;
+extern "C++" export class A;
+export class B;
+
+// CHECK-DAG: void @_ZW3FOO3FooP1APNS_1B(
+export void Foo (A *, B*) {
+}
+
+extern "C++" {
+// CHECK-DAG: void @_Z3BarP1APW3FOO1B(
+export void Bar (A *, B*) {
+}
+}
diff --git a/clang/test/CodeGenCXX/cxx20-module-impl-1a.cpp b/clang/test/CodeGenCXX/cxx20-module-impl-1a.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-impl-1a.cpp
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -std=c++20 %S/Inputs/cxx20-module-impl-1a.cpp -triple %itanium_abi_triple -emit-module-interface -o %t
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -fmodule-file=%t -emit-llvm -o - | FileCheck %s
+
+module Foo;
+
+// CHECK-DAG: @_ZW3Foo8Exportedv(
+void Exported() {
+}
+
+// CHECK-DAG: @_ZW3Foo6Modulev(
+void Module() {
+}
+
+// CHECK-DAG: @_ZW3Foo7Module2v(
+void Module2() {
+}
+
diff --git a/clang/test/CodeGenCXX/cxx20-module-nested-1.cppm b/clang/test/CodeGenCXX/cxx20-module-nested-1.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-nested-1.cppm
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -std=c++20 -fmodules-ts %s -triple %itanium_abi_triple -emit-llvm -o - | FileCheck %s
+module;
+# 4 __FILE__ 1
+namespace Outer::Inner {
+class X;
+// CHECK-DAG: define dso_local void @_ZN5Outer5Inner3BarERNS0_1XE(
+void Bar (X &) {}
+} // namespace Outer::Inner
+# 10 "" 2
+export module FOO;	
+namespace Outer {
+class Y;
+namespace Inner {
+// CHECK-DAG: define dso_local void @_ZN5Outer5InnerW3FOO2FnERNS0_1XERNS_S1_1YE(
+void Fn (X &, Y &){}  // #1
+} // namespace Inner
+} // namespace Outer
diff --git a/clang/test/CodeGenCXX/cxx20-module-nested-2.cppm b/clang/test/CodeGenCXX/cxx20-module-nested-2.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-nested-2.cppm
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fmodules-ts %s -triple %itanium_abi_triple -emit-llvm -o - | FileCheck %s
+export module FOO;
+namespace Outer {
+class Y;
+class Inner {
+  class X;
+  void Fn (X &, Y &); // #2
+};
+// CHECK-DAG: define dso_local void @_ZN5OuterW3FOO5Inner2FnERNS1_1XERNS_S0_1YE(
+void Inner::Fn (X &, Y &) {}
+}
+
diff --git a/clang/test/CodeGenCXX/cxx20-module-part-1a.cpp b/clang/test/CodeGenCXX/cxx20-module-part-1a.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-part-1a.cpp
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -emit-llvm -o - | FileCheck %s
+export module Foo:inter;
+
+// CHECK-DAG: @_ZW3Foo4Frobv(
+export void Frob() {
+}
diff --git a/clang/test/CodeGenCXX/cxx20-module-part-1b.cpp b/clang/test/CodeGenCXX/cxx20-module-part-1b.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-part-1b.cpp
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -emit-llvm -o - | FileCheck %s
+export module Foo:impl;
+
+// CHECK-DAG: @_ZW3Foo4Quuxv(
+export void Quux() {
+}
diff --git a/clang/test/CodeGenCXX/cxx20-module-part-1c.cpp b/clang/test/CodeGenCXX/cxx20-module-part-1c.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-part-1c.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -std=c++20 %S/cxx20-module-part-1a.cpp -triple %itanium_abi_triple -emit-module-interface -o %t-inter
+// RUN: %clang_cc1 -std=c++20 %S/cxx20-module-part-1b.cpp -triple %itanium_abi_triple -emit-module-interface -o %t-impl
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -fmodule-file=Foo:inter=%t-inter -fmodule-file=Foo:impl=%t-impl -emit-llvm -o - | FileCheck %s
+export module Foo;
+export import :inter;
+import :impl;
+
+void Wrap() {
+  // CHECK: call void @_ZW3Foo4Frobv()
+  Frob();
+  // CHECK: call void @_ZW3Foo4Quuxv()
+  Quux();
+}
diff --git a/clang/test/CodeGenCXX/cxx20-module-std-subst-1.cppm b/clang/test/CodeGenCXX/cxx20-module-std-subst-1.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-std-subst-1.cppm
@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -emit-llvm -o - | FileCheck %s
+
+module;
+# 5 __FILE__ 1
+class Pooh;
+class Piglet;
+# 8 "" 2
+
+export module std; // might happen, you can't say it won't!
+
+namespace std {
+export template<typename T> class allocator {
+// just for testing, not real!
+void M (T *);
+template <typename U> U *N (T *);
+};
+
+template<typename T> void allocator<T>::M (T *) {}
+template<typename T> template<typename U> U *allocator<T>::N (T *) {
+return nullptr;
+}
+
+// CHECK-DAG: void @_ZNStW3std9allocatorIiE1MEPi(
+template void allocator<int>::M (int *);
+// CHECK-DAG: @_ZNStW3std9allocatorIiE1NIfEEPT_Pi(
+template float *allocator<int>::N<float> (int *);
+}
+
+// CHECK-DAG: @_ZNStW3std9allocatorI4PoohE1MEPS1_(
+template void std::allocator<Pooh>::M (Pooh *);
+// CHECK-DAG: @_ZNStW3std9allocatorI4PoohE1NI6PigletEEPT_PS1_(
+template Piglet *std::allocator<Pooh>::N<Piglet> (Pooh *);
diff --git a/clang/test/CodeGenCXX/cxx20-module-std-subst-2b.cpp b/clang/test/CodeGenCXX/cxx20-module-std-subst-2b.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-std-subst-2b.cpp
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -std=c++20 %S/Inputs/cxx20-module-std-subst-2a.cpp -triple %itanium_abi_triple -emit-module-interface -o %t
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -fmodule-file=%t -emit-llvm -o - | FileCheck %s
+
+export module Foo;
+import RenameString;
+
+namespace std {
+template <typename T> struct char_traits {};
+} // namespace std
+
+// use Sb mangling, not Ss as this is not global-module std::char_traits
+// std::char_traits.
+// CHECK-DAG: void @_ZW3Foo1fRSbIcStS_11char_traitsIcESaIcEE(
+void f(str<char, std::char_traits<char>> &s) {
+}
diff --git a/clang/test/CodeGenCXX/cxx20-module-std-subst-2c.cpp b/clang/test/CodeGenCXX/cxx20-module-std-subst-2c.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-std-subst-2c.cpp
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -std=c++20 %S/Inputs/cxx20-module-std-subst-2a.cpp -triple %itanium_abi_triple -emit-module-interface -o %t
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -fmodule-file=%t -emit-llvm -o - | FileCheck %s
+module;
+# 5 __FILE__ 1
+namespace std {
+template <typename A> struct char_traits {};
+} // namespace std
+# 9 "" 2
+export module Bar;
+import RenameString;
+
+// Use Ss as this is global-module std::char_traits
+// CHECK-DAG: void @_ZW3Bar1gRSs(
+void g(str<char, std::char_traits<char>> &s) {
+}
diff --git a/clang/test/CodeGenCXX/cxx20-module-sub-1a.cppm b/clang/test/CodeGenCXX/cxx20-module-sub-1a.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-sub-1a.cppm
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -emit-llvm -o - | FileCheck %s
+
+export module FOO.BAR;
+export class A;
+namespace Bob {
+export class B;
+
+// CHECK-DAG: void @_ZN3BobW3FOOW3BAR3BarEPS1_1APNS_S1_1BE(
+export void Bar (A *, B*) {
+}
+}
+
+// CHECK-DAG: void @_ZW3FOOW3BAR3FooPS0_1APN3BobS0_1BE(
+export void Foo (A *, Bob::B*) {
+}
diff --git a/clang/test/CodeGenCXX/cxx20-module-sub-1b.cppm b/clang/test/CodeGenCXX/cxx20-module-sub-1b.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-sub-1b.cppm
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -std=c++20 %S/cxx20-module-sub-1a.cppm -triple %itanium_abi_triple -emit-module-interface -o %t
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -fmodule-file=%t -emit-llvm -o - | FileCheck %s
+
+export module FOO.BAZ;
+import FOO.BAR;
+
+namespace Bob {
+
+// CHECK-DAG: void @_ZN3BobW3FOOW3BAZ3FooEPS0_W3BAR1APNS_S2_1BE(
+void Foo (A *, B*) {
+}
+}
+
+// CHECK-DAG: void @_ZW3FOOW3BAZ3BarPS_W3BAR1APN3BobS1_1BE(
+void Bar (A *, Bob::B*) {
+}
diff --git a/clang/test/CodeGenCXX/cxx20-module-tmpl-1.cppm b/clang/test/CodeGenCXX/cxx20-module-tmpl-1.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx20-module-tmpl-1.cppm
@@ -0,0 +1,43 @@
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -emit-llvm -o - | FileCheck %s
+
+export module FOO;
+
+class One;
+class Two;
+
+export template<typename T> struct TPL
+{
+void M (T *);
+template<typename U> void N (T *, U*);
+};
+
+template<typename T>
+void TPL<T>::M (T *) {}
+
+template<typename T> template<typename U> void TPL<T>::N (T *, U*) {}
+
+// CHECK-DAG: void @_ZNW3FOO3TPLIS_3OneE1MEPS1_(
+template void TPL<One>::M (One *);
+// CHECK-DAG: void @_ZNW3FOO3TPLIS_3OneE1NIS_3TwoEEvPS1_PT_(
+template void TPL<One>::N<Two> (One *, Two *);
+
+namespace NMS {
+class One;
+class Two;
+
+export template<typename T> struct TPL
+{
+void M (T *);
+template<typename U> void N (T *, U*);
+};
+
+template<typename T>
+void TPL<T>::M (T *) {}
+
+template<typename T> template<typename U> void TPL<T>::N (T *, U*) {}
+
+// CHECK-DAG: void @_ZN3NMSW3FOO3TPLINS_S0_3OneEE1MEPS2_(
+template void TPL<One>::M (One *);
+// CHECK-DAG: void @_ZN3NMSW3FOO3TPLINS_S0_3OneEE1NINS_S0_3TwoEEEvPS2_PT_
+template void TPL<One>::N<Two> (One *, Two *);
+}