add gcc abi_tag support

[jyknight]

Summary

• parse abi_tag attribute
• emit abi tags in name Itanium Mangling
• try to emit the same tags as gcc5

[stbuehler]

Just for the record: substitution doesn't work yet (it should add available tags from the substitution, but doesn't).

This example shows the difference: my patch mangles T::F() as N::Name(N::__test::Result)::T::F[abi:test](N::__test::Result), but actually shouldn't emit the abi tag.

namespace N {
  inline namespace __test
  __attribute__((abi_tag("test"))) {
    struct Result {
      const char *str;
    };
  }

  inline Result Name(Result p1) {
    struct T {
      Result F(Result p2) {
        struct S {
          Result F(Result p3) {
            static Result s3 = p3;
            return s3;
          }
        };
        static Result s2 = S().F(p2);
        return s2;
      }
    };
    static Result s1 = T().F(p1);
    return s1;
  }

  void F() {
    // instantiate Name()
    Name(Result());
  }
}

[majnemer]

Adding John, Richard and Reid as reviewers.

[sberg]

I can't figure out how to add code to someone else's Phabricator review, so sent it to the mailing list
instead: <http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20151207/144636.html> "[PATCH] Re D12834 add gcc abi_tag support."

[foutrelis]

We have received a few reports of clang crashes after applying the abi_tag support patch to our llvm/clang package in Arch Linux.

Compiling

test.cc856 BDownload

with clang++ -std=c++14 -c test.cc should trigger the segmentation fault due to (what I think is) a runaway recursion.

I'm also attaching an excerpt from GDB that shows the loop in the execution:

gdb.txt2 KBDownload

HALP! :3

[• elizafox]

In D12834#311890, @foutrelis wrote:

We have received a few reports of clang crashes after applying the abi_tag support patch to our llvm/clang package in Arch Linux.

Why would you put a patch clearly marked as "needs review" into a distribution?!?!?!?!

In any case, the recursion source seems obvious to me, but I don't know how to add patches to this reviewboard item.

[jroelofs]

In D12834#305787, @sberg wrote:

I can't figure out how to add code to someone else's Phabricator review, so sent it to the mailing list
instead: <http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20151207/144636.html> "[PATCH] Re D12834 add gcc abi_tag support."

There's a "Commandeer Revision" action, that will let you take ownership of the diff. After you do that, you should be able to upload your own diff in the review.

[foutrelis]

In D12834#314136, @elizafox wrote:

In D12834#311890, @foutrelis wrote:

We have received a few reports of clang crashes after applying the abi_tag support patch to our llvm/clang package in Arch Linux.

Why would you put a patch clearly marked as "needs review" into a distribution?!?!?!?!

We waited 6 months before switching to the new ABI in libstdc++. Ideally, we would have waited until the patch was reviewed and merged but did not want to wait much longer. I also (wrongly) considered the patch to be relative stable.

In any case, the recursion source seems obvious to me, but I don't know how to add patches to this reviewboard item.

If the correction is obvious as well and not very complex, would you mind sharing it?

[• elizafox]

In D12834#316988, @foutrelis wrote:

In D12834#314136, @elizafox wrote:

In D12834#311890, @foutrelis wrote:

We have received a few reports of clang crashes after applying the abi_tag support patch to our llvm/clang package in Arch Linux.

Why would you put a patch clearly marked as "needs review" into a distribution?!?!?!?!

We waited 6 months before switching to the new ABI in libstdc++. Ideally, we would have waited until the patch was reviewed and merged but did not want to wait much longer. I also (wrongly) considered the patch to be relative stable.

What would make you believe this was stable?! This patch has known differences in ABI anyway.

In any case, the recursion source seems obvious to me, but I don't know how to add patches to this reviewboard item.

If the correction is obvious as well and not very complex, would you mind sharing it?

I see the recursion source, but I don't know if my fix would be "correct," as I do not know enough about the new ABI nor GCC's internals (and therefore cross-checking it) to ensure it will behave correctly. I'm erring on the side of caution for now.

[tavianator]

lib/AST/ItaniumMangle.cpp

1119

This looks bogus, should be writeAbiTags(qualifier) right?

[stbuehler]

• disable dervied abi tags in some recursions to fix infinite recursions
• don't emit abi tags for NestedNameSpecifier::Identifier

[stbuehler]

integrate patch by Stephan Bergmann <[e-mail]>:

• Fix handling of abi_tag attribute on namespaces to match GCC behavior:
  • Forbid the attribute on unnamed namespaces. (GCC merely produces a warning instead of an error for unnamed or non-inline namespaces, though.)
  • When no tags are given for a (named, inline) namespace, use the namespace's name as tag.

[foutrelis]

The latest patch still applies (relatively cleanly) to Clang 3.7.x which is nice. The infinite recursion segfaults appear to be gone, thanks!

There are a few test failures which I assume will be need to be addressed later on:

clang-diff-44973-test-failures.log14 KBDownload

[aaron.ballman]

I don't have the expertise to review the semantics of the attribute, but here are some cursory thoughts on the attribute itself.

include/clang/Basic/Attr.td

355

The diagnostic doesn't match the subject -- "methods" refers to Objective-C methods, which should either be listed in the SubjectList or removed from the diagnostic.

357

No new undocumented attributes, please -- you should add documentation to AttrDocs.td and link to it from here.

include/clang/Basic/DiagnosticSemaKinds.td

4149

Should quote 'abi_tag' in the diagnostic (here and below as well). Also, why an error instead of a warning and dropping the attribute from the declaration?

Perhaps: 'abi_tag' attribute on %select{inline|anonymous}0 namespace ignored" as a warning, and remove the below diagnostic entirely?

lib/Sema/SemaDeclAttr.cpp

4456

Why a vector of std::string instead of StringRef?

4476

Indentation is incorrect; also, elide braces for single statement ifs.

4479

Comments should be complete sentences (with capitalization and punctuation); here and elsewhere.

[DmitryPolukhin]

I think Sema part should be exacted to separate patch and committed first after fixing Aaron's comments. Mangling part requires more work and much more tests. I'm still looking what actually GCC does and how it can be re-implemented in Clang without calling mangler twice.

lib/Sema/SemaDeclAttr.cpp

4450

Nit, all args fits to single line and the whole function needs clang-format.

[DmitryPolukhin]

Hi Stefan,

What are your plans about this patch? The patch has number of comments about Sema part from Aaron and me but in general there are no major issues with Sema for the attribute.

As for mangling part I think recursive approach looks reasonable because alternative implementation will require re-implementation significant part of name mangling just to calculate abi_tags for types. So your approach minimizes code duplication. I made following changes in your patch

abi_tags.patch53 KBDownload

:

• rebase on top-of-the-tree
• fixed all failing tests (now check-clang has no unexpected fails)
• fixed issue with mangling for variables in global namespace with abi_tags
• partially fixed style/clang-format issues
• added more tests on abi_tags (used the newest GCC mangling for templates with Jason's patches from Jan 31, 2016)

If you are going to keep working on this patch, please integrate my patch in your patch. If you are not going to keep working on this patch, please let me know.

Thanks,
Dmitry Polukhin

Software Engineer
Intel Compiler Team

[Pavel_Odintsov]

Hello, folks!

Sorry for bothering you because I'm just user of clang 3.8 and gcc 5.3 and haven't any experience in compiler development. But I've hit issue mentioned in this ticket.

I've C++ 11 enabled libraries (gRPC, protobuf, bson and mongodb c++ 11 client) in my project which are compiled with gcc 5.3. So I've tried to move to clang for this project for testing purposes (and awesome clang static analyzer!).

But actually it becoming nightmare because I need to rebuild each C++ 11 library for both compilers because of this issue :(

Could you look on this issue and offer some workaround? Will be fine to have support of already created by gcc 5.3 tags.

[DmitryPolukhin]

Hi Pavel,

Code review is not a good place to discuss such type of things. There is no
really good workaround for this issue if you already have libraries built
with C++ 11 and GCC configured to use abi_tag. But if you are doing it only
for testing and you are willing to build Clang yourself, you could try to
get top of the tree Clang and apply this http://reviews.llvm.org/D18035. If
you do so, please report any issues with abi_tag to the patch or to me
directly. I tested patch http://reviews.llvm.org/D18035 on tests and small
things only so testing on something bigger will be helpful.

[jyknight]

I believe this was fully superseded by
D17567: [GCC] PR23529 Sema part of attrbute abi_tag support
D18035: [GCC] PR23529 Mangler part of attrbute abi_tag support
which are now committed. So, abandoning.

docs/ItaniumMangleAbiTags.rst

+========
+Abi Tags
+========
+
+Introduction
+============
+
+This text tries to describe gcc semantic for mangling "abi_tag" attributes
+described in https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html
+
+There is no guarantee the following rules are correct, complete or make sense
+in any way as they were determined empirically by experiments with gcc5.
+
+Declaration
+===========
+
+Abi tags are declared in an abi_tag attribute and can be applied to a
+function, variable, class or inline namespace declaration. The attribute takes
+one or more strings (called tags); the order does not matter.
+
+See https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html for
+details.
+
+Tags on an inline namespace are called "implicit tags", all other tags are
+"explicit tags".
+
+Mangling
+========
+
+All tags that are "active" on a <unqualified-name> are emitted after the
+<unqualified-name>, before <template-args> or <discriminator>, and are part of
+the same <substitution> the <unqualified-name> is.
+
+They are mangled as:
+
+    <abi-tags> ::= <abi-tag>*   # sort by name
+    <abi-tag> ::= B <tag source-name>
+
+Example:
+
+    __attribute__((abi_tag("test")))
+    void Func();
+
+    gets mangled as: _Z4FuncB4testv (prettified as `Func[abi:test]()`)
+
+Active tags
+===========
+
+A namespace has never any active tags; for types (class / struct / union /
+enum) the explicit tags are the active tags.
+
+For variables and functions the active tags are the explicit tags plus any
+"required tags" which are not in the "available tags" set:
+
+    derived-tags := (required-tags - available-tags)
+    active-tags := explicit-tags + derived-tags
+
+Required tags for a function
+============================
+
+If a function is used as a local scope for another name, and is part of
+another function as local scope, it doesn't have any required tags.
+
+If a function is used as a local scope for a guard variable name, it doesn't
+have any required tags.
+
+Otherwise the function requires any implicit or explicit tag used in the name
+for the return type.
+
+Required tags for a variable
+============================
+
+A variable requires any implicit or explicit tag used in its type.
+
+Available tags
+==============
+
+All tags used in the prefix and in the template arguments for a name are
+available; for functions also all  tags from the <bare-function-type> (which
+might include the return type for template functions) are available.
+
+For <local-name>s all active tags used in the local part (<function-
+encoding>) are available, but not implicit tags which were not active!
+
+Implicit and explicit tags used in the <unqualified-name> for a function (as
+in the type of a cast operator) are NOT available.
+
+Example: a cast operator to std::string (which is
+std::__cxx11::basic_string<...>) will use 'cxx11' as active tag, as it is
+required from the return type `std::string` but not available.

include/clang/Basic/Attr.td

class IgnoredAttr : Attr {
   let SemaHandler = 0;
   let Documentation = [Undocumented];
 }
 
 //
 // Attributes begin here
 //
 
+def AbiTag : Attr {
+  let Spellings = [GCC<"abi_tag">];
+  let Args = [VariadicStringArgument<"Tags">];
+  let Subjects = SubjectList<[Struct, Var, Function, Namespace], ErrorDiag,
+                             "ExpectedStructClassVariableFunctionMethodOrInlineNamespace">;
+  let Documentation = [Undocumented];
+}
+
 def AddressSpace : TypeAttr {
   let Spellings = [GNU<"address_space">];
   let Args = [IntArgument<"AddressSpace">];
   let Documentation = [Undocumented];
 }
 
 def Alias : Attr {
   let Spellings = [GCC<"alias">];

include/clang/Basic/DiagnosticSemaKinds.td

def warn_attribute_wrong_decl_type : Warning<
   "variables, functions and labels|fields and global variables|structs|"
   "variables and typedefs|thread-local variables|"
   "variables and fields|variables, data members and tag types|"
   "types and namespaces|Objective-C interfaces|methods and properties|"
   "struct or union|struct, union or class|types|"
   "Objective-C instance methods|init methods of interface or class extension declarations|"
   "variables, functions and classes|Objective-C protocols|"
   "functions and global variables|structs, unions, and typedefs|structs and typedefs|"
-  "interface or protocol declarations|kernel functions|non-K&R-style functions}1">,
+  "interface or protocol declarations|kernel functions|non-K&R-style functions|"
+  "structs, classes, variables, functions, methods and inline namespaces}1">,
   InGroup<IgnoredAttributes>;
 def err_attribute_wrong_decl_type : Error<warn_attribute_wrong_decl_type.Text>;
 def warn_type_attribute_wrong_type : Warning<
   "'%0' only applies to %select{function|pointer|"
   "Objective-C object or block pointer}1 types; type here is %2">,
   InGroup<IgnoredAttributes>;
 def warn_incomplete_encoded_type : Warning<
   "encoding of %0 type is incomplete because %1 component has unknown encoding">,
def err_definition_of_implicitly_declared_member : Error<
   "operator|destructor|function}1">;
 def err_definition_of_explicitly_defaulted_member : Error<
   "definition of explicitly defaulted %select{default constructor|copy "
   "constructor|move constructor|copy assignment operator|move assignment "
   "operator|destructor}0">;
 def err_redefinition_extern_inline : Error<
   "redefinition of a 'extern inline' function %0 is not supported in "
   "%select{C99 mode|C++}1">;
+def err_attr_abi_tag_only_on_inline_namespace :
+  Error<"abi_tag attribute only allowed on inline namespaces">;
+def err_abi_tag_on_redeclaration :
+  Error<"cannot add abi_tag attribute in redeclaration">;
+def err_new_abi_tag_on_redeclaration :
+  Error<"abi_tag %0 missing in original declaration">;
+
 
 def note_deleted_dtor_no_operator_delete : Note<
   "virtual destructor requires an unambiguous, accessible 'operator delete'">;
 def note_deleted_special_member_class_subobject : Note<
   "%select{default constructor|copy constructor|move constructor|"
   "copy assignment operator|move assignment operator|destructor}0 of "
   "%1 is implicitly deleted because "
   "%select{base class %3|%select{||||variant }4field %3}2 has "

include/clang/Sema/AttributeList.h

enum AttributeDeclKind {
   ExpectedObjCInterfaceDeclInitMethod,
   ExpectedFunctionVariableOrClass,
   ExpectedObjectiveCProtocol,
   ExpectedFunctionGlobalVarMethodOrProperty,
   ExpectedStructOrUnionOrTypedef,
   ExpectedStructOrTypedef,
   ExpectedObjectiveCInterfaceOrProtocol,
   ExpectedKernelFunction,
-  ExpectedFunctionWithProtoType
+  ExpectedFunctionWithProtoType,
+  ExpectedStructClassVariableFunctionMethodOrInlineNamespace
 };
 
 }  // end namespace clang
 
 #endif

lib/AST/ItaniumMangle.cpp

public:
   }
   /// @}
 };
 
 /// Manage the mangling of a single name.
 class CXXNameMangler {
   ItaniumMangleContextImpl &Context;
   raw_ostream &Out;
+  bool NullOut = false;
+  bool DisableDerivedAbiTags = false;
 
   /// The "structor" is the top-level declaration being mangled, if
   /// that's not a template specialization; otherwise it's the pattern
   /// for that specialization.
   const NamedDecl *Structor;
   unsigned StructorType;
 
   /// The next substitution sequence number.
public:
 
     void pop(FunctionTypeDepthState saved) {
       assert(getDepth() == saved.getDepth() + 1);
       Bits = saved.Bits;
     }
 
   } FunctionTypeDepth;
 
+  // abi_tag is a gcc attribute, taking one or more strings called "tags".
+  //
+  // the goal is to annotage against which version of a library an object was
+  // build and to be able to provide backwards compatibility ("dual abi").
+  //
+  // for this the emitted mangled names have to be different, while you don't
+  // want the user to have to use different names in the source.
+  //
+  // the abi_tag can be present on Struct, Var and Function  declarations as
+  // "explicit" tag, and on inline Namespace as "implicit" tag. Explicit tags
+  // are always emitted after the unqualified name, and (implicit) tags on
+  // namespace are not.
+  //
+  // For functions and variables there is a set of "implicitly available"
+  // tags. These tags are: all tags from the namespace/structs the name is
+  // embedded in, all tags from any template arguments of the name, and, for
+  // functions, alls tags used anywhere in the <bare-function-type> (i.e.
+  // parameters and sometimes the return type).
+  //
+  // For functions this is basically the list of all tags from the signature
+  // without the unqualified name and usually without the return type of the
+  // function. In `operator Type()` Type is NOT part of that list, as it is
+  // part of the unqualified name!
+  //
+  // Now all tags from the function return type/variable type which are not
+  // "implicitly available" must be added to the explicit list of tags, and
+  // are emitted after the unqualified name.
+  //
+  // Example:
+  // namespace std {
+  //   inline namespace __cxx11 __attribute__((__abi_tag__("cxx11"))) { }
+  //   inline namespace __cxx11 {
+  //     struct string { };
+  //   }
+  // }
+  //
+  // std::string foo(); // needs abi tag "cxx11" on foo
+  // std::string foo(std::string); // does NOT need abi tag "cxx11" on foo
+  // __attribute__((__abi_tag__("cxx11")))
+  // std::string foo2(std::string); // emit abi tag "cxx11" on foo anyway
+  //
+  // The tags are sorted by name before emitting, and are serialized as
+  //   <abitag> ::= B <"tag" source-name>
+
+  typedef SmallVector<StringRef, 4> AbiTagList;
+
+  // state to gather all implicit and explicit tags used in a mangled name.
+  // must always have an instance of this while emitting any name to keep
+  // track.
+  //
+  // TODO(abitags): how to handle substituted names? they should add the tags used in
+  // the substitution to the list of available tags.
+  class AbiTagState final {
+  public:
+    //! all abi tags used implicitly or explicitly
+    std::set<StringRef> UsedAbiTags;
+    //! all explicit abi tags (i.e. not from namespace)
+    std::set<StringRef> EmittedAbiTags;
+
+    AbiTagState* &LinkHead;
+    AbiTagState *Parent{nullptr};
+
+    bool LinkActive{false};
+
+    explicit AbiTagState(AbiTagState* &linkHead)
+      : LinkHead(linkHead) {
+      Parent = LinkHead;
+      LinkHead = this;
+      LinkActive = true;
+    }
+
+    // no copy, no move
+    AbiTagState(AbiTagState const&) = delete;
+    AbiTagState& operator=(AbiTagState const&) = delete;
+
+    ~AbiTagState() {
+      pop();
+    }
+
+    void pop() {
+      if (!LinkActive) return;
+
+      assert(LinkHead == this && "abi tag link head must point to us on destruction");
+      LinkActive = false;
+      if (Parent) {
+        Parent->UsedAbiTags.insert(UsedAbiTags.begin(), UsedAbiTags.end());
+        Parent->EmittedAbiTags.insert(EmittedAbiTags.begin(), EmittedAbiTags.end());
+      }
+      LinkHead = Parent;
+    }
+
+    void write(raw_ostream &Out, const NamedDecl *ND, const AbiTagList *AdditionalAbiTags) {
+      ND = cast<NamedDecl>(ND->getCanonicalDecl());
+
+      if (dyn_cast<FunctionDecl>(ND) || dyn_cast<VarDecl>(ND)) {
+        // assert(AdditionalAbiTags && "function and variables need a list of additional abi tags");
+      } else {
+        assert(!AdditionalAbiTags && "only function and variables need a list of additional abi tags");
+        if (const auto* NS = dyn_cast<NamespaceDecl>(ND)) {
+          if (const auto* AbiTag = NS->getAttr<AbiTagAttr>()) {
+            for (const auto& Tag: AbiTag->tags()) {
+              UsedAbiTags.insert(Tag);
+            }
+          }
+          // don't emit abi tags for namespaces
+          return;
+        }
+      }
+
+      AbiTagList TagList;
+      if (const auto* AbiTag = ND->getAttr<AbiTagAttr>()) {
+        for (const auto& Tag: AbiTag->tags()) {
+          UsedAbiTags.insert(Tag);
+          // AbiTag->tags() is sorted and has no duplicates
+          TagList.push_back(Tag);
+        }
+      }
+
+      if (AdditionalAbiTags) {
+        for (const auto& Tag: *AdditionalAbiTags) {
+          UsedAbiTags.insert(Tag);
+          if (std::find(TagList.begin(), TagList.end(), Tag) == TagList.end()) {
+            // don't insert duplicates
+            TagList.push_back(Tag);
+          }
+        }
+        // AbiTag->tags() are already sorted; only add if we had additional tags
+        std::sort(TagList.begin(), TagList.end());
+      }
+
+      writeSortedUniqueAbiTags(Out, TagList);
+    }
+
+  protected:
+    template<typename TagList>
+    void writeSortedUniqueAbiTags(raw_ostream &Out, TagList const& AbiTags) {
+      for (const auto& Tag: AbiTags) {
+        EmittedAbiTags.insert(Tag);
+        Out << "B";
+        Out << Tag.size();
+        Out << Tag;
+      }
+    }
+  } *AbiTags = nullptr;
+  AbiTagState AbiTagsRoot{AbiTags};
+
+  struct TemporaryDisableDerivedAbiTags {
+    bool& StateRef;
+    bool OldState;
+
+    TemporaryDisableDerivedAbiTags(bool& State, bool Disable = true)
+    : StateRef(State) {
+      OldState = StateRef;
+      StateRef = Disable;
+    }
+    TemporaryDisableDerivedAbiTags(TemporaryDisableDerivedAbiTags const&) = delete;
+    ~TemporaryDisableDerivedAbiTags() {
+      StateRef = OldState;
+    }
+  };
+
   llvm::DenseMap<uintptr_t, unsigned> Substitutions;
 
   ASTContext &getASTContext() const { return Context.getASTContext(); }
 
 public:
   CXXNameMangler(ItaniumMangleContextImpl &C, raw_ostream &Out_,
                  const NamedDecl *D = nullptr)
     : Context(C), Out(Out_), Structor(getStructor(D)), StructorType(0),
       SeqID(0) {
     // These can't be mangled without a ctor type or dtor type.
     assert(!D || (!isa<CXXDestructorDecl>(D) &&
                   !isa<CXXConstructorDecl>(D)));
   }
   CXXNameMangler(ItaniumMangleContextImpl &C, raw_ostream &Out_,
                  const CXXConstructorDecl *D, CXXCtorType Type)
     : Context(C), Out(Out_), Structor(getStructor(D)), StructorType(Type),
       SeqID(0) { }
   CXXNameMangler(ItaniumMangleContextImpl &C, raw_ostream &Out_,
                  const CXXDestructorDecl *D, CXXDtorType Type)
     : Context(C), Out(Out_), Structor(getStructor(D)), StructorType(Type),
       SeqID(0) { }
 
+  CXXNameMangler(CXXNameMangler &Outer, llvm::raw_null_ostream &Out_)
+    : Context(Outer.Context), Out(Out_), NullOut(true), Structor(Outer.Structor), StructorType(Outer.StructorType),
+      SeqID(Outer.SeqID) { }
+
 #if MANGLE_CHECKER
   ~CXXNameMangler() {
     if (Out.str()[0] == '\01')
       return;
 
     int status = 0;
     char *result = abi::__cxa_demangle(Out.str().str().c_str(), 0, 0, &status);
     assert(status == 0 && "Could not demangle mangled name!");
     free(result);
   }
 #endif
   raw_ostream &getStream() { return Out; }
 
+  void disableDerivedAbiTags() { DisableDerivedAbiTags = true; }
+
   void mangle(const NamedDecl *D);
   void mangleCallOffset(int64_t NonVirtual, int64_t Virtual);
   void mangleNumber(const llvm::APSInt &I);
   void mangleNumber(int64_t Number);
   void mangleFloat(const llvm::APFloat &F);
-  void mangleFunctionEncoding(const FunctionDecl *FD);
+  void mangleFunctionEncoding(const FunctionDecl *FD, bool ExcludeUnqualifiedName = false);
   void mangleSeqID(unsigned SeqID);
-  void mangleName(const NamedDecl *ND);
+  void mangleName(const NamedDecl *ND, bool ExcludeUnqualifiedName = false);
   void mangleType(QualType T);
   void mangleNameOrStandardSubstitution(const NamedDecl *ND);
   
 private:
+  void writeAbiTags(const NamedDecl *ND, const AbiTagList *AdditionalAbiTags = nullptr);
 
   bool mangleSubstitution(const NamedDecl *ND);
   bool mangleSubstitution(QualType T);
   bool mangleSubstitution(TemplateName Template);
   bool mangleSubstitution(uintptr_t Ptr);
 
   void mangleExistingSubstitution(QualType type);
   void mangleExistingSubstitution(TemplateName name);
private:
   void addSubstitution(uintptr_t Ptr);
 
   void mangleUnresolvedPrefix(NestedNameSpecifier *qualifier,
                               bool recursive = false);
   void mangleUnresolvedName(NestedNameSpecifier *qualifier,
                             DeclarationName name,
                             unsigned KnownArity = UnknownArity);
 
-  void mangleName(const TemplateDecl *TD,
+  void mangleFunctionEncodingBareType(const FunctionDecl *FD);
+
+  void mangleNameWithAbiTags(const NamedDecl *ND,
+                             const AbiTagList *AdditionalAbiTags,
+                             bool ExcludeUnqualifiedName);
+  void mangleTemplateName(const TemplateDecl *TD,
+                  const AbiTagList *AdditionalAbiTags,
+                  bool ExcludeUnqualifiedName,
                   const TemplateArgument *TemplateArgs,
                   unsigned NumTemplateArgs);
-  void mangleUnqualifiedName(const NamedDecl *ND) {
-    mangleUnqualifiedName(ND, ND->getDeclName(), UnknownArity);
+  void mangleUnqualifiedName(const NamedDecl *ND, const AbiTagList *AdditionalAbiTags) {
+    mangleUnqualifiedName(ND, ND->getDeclName(), UnknownArity, AdditionalAbiTags);
   }
   void mangleUnqualifiedName(const NamedDecl *ND, DeclarationName Name,
-                             unsigned KnownArity);
-  void mangleUnscopedName(const NamedDecl *ND);
-  void mangleUnscopedTemplateName(const TemplateDecl *ND);
-  void mangleUnscopedTemplateName(TemplateName);
+                             unsigned KnownArity, const AbiTagList *AdditionalAbiTags);
+  void mangleUnscopedName(const NamedDecl *ND, const AbiTagList *AdditionalAbiTags);
+  void mangleUnscopedTemplateName(const TemplateDecl *ND,
+                                  const AbiTagList *AdditionalAbiTags);
+  void mangleUnscopedTemplateName(TemplateName,
+                                  const AbiTagList *AdditionalAbiTags);
   void mangleSourceName(const IdentifierInfo *II);
-  void mangleLocalName(const Decl *D);
+  void mangleLocalName(const Decl *D,
+                       const AbiTagList *AdditionalAbiTags,
+                       bool ExcludeUnqualifiedName);
   void mangleBlockForPrefix(const BlockDecl *Block);
   void mangleUnqualifiedBlock(const BlockDecl *Block);
   void mangleLambda(const CXXRecordDecl *Lambda);
   void mangleNestedName(const NamedDecl *ND, const DeclContext *DC,
-                        bool NoFunction=false);
+                        const AbiTagList *AdditionalAbiTags,
+                        bool NoFunction,
+                        bool ExcludeUnqualifiedName);
   void mangleNestedName(const TemplateDecl *TD,
+                        const AbiTagList *AdditionalAbiTags,
+                        bool ExcludeUnqualifiedName,
                         const TemplateArgument *TemplateArgs,
                         unsigned NumTemplateArgs);
   void manglePrefix(NestedNameSpecifier *qualifier);
   void manglePrefix(const DeclContext *DC, bool NoFunction=false);
   void manglePrefix(QualType type);
-  void mangleTemplatePrefix(const TemplateDecl *ND, bool NoFunction=false);
+  void mangleTemplatePrefix(const TemplateDecl *ND,
+                            const AbiTagList *AdditionalAbiTags,
+                            bool NoFunction = false,
+                            bool ExcludeUnqualifiedName = false);
   void mangleTemplatePrefix(TemplateName Template);
   bool mangleUnresolvedTypeOrSimpleId(QualType DestroyedType,
                                       StringRef Prefix = "");
   void mangleOperatorName(DeclarationName Name, unsigned Arity);
   void mangleOperatorName(OverloadedOperatorKind OO, unsigned Arity);
   void mangleQualifiers(Qualifiers Quals);
   void mangleRefQualifier(RefQualifierKind RefQualifier);
 
#include "clang/AST/TypeNodes.def"
   void mangleTemplateArgs(const TemplateArgument *TemplateArgs,
                           unsigned NumTemplateArgs);
   void mangleTemplateArgs(const TemplateArgumentList &AL);
   void mangleTemplateArg(TemplateArgument A);
 
   void mangleTemplateParameter(unsigned Index);
 
   void mangleFunctionParam(const ParmVarDecl *parm);
+
+  std::set<StringRef> getTagsFromPrefixAndTemplateArguments(const NamedDecl *ND);
+  AbiTagList makeAdditionalTagsForFunction(const FunctionDecl *FD);
+  AbiTagList makeAdditionalTagsForVariable(const VarDecl *VD);
 };
 
 }
 
 bool ItaniumMangleContextImpl::shouldMangleCXXName(const NamedDecl *D) {
   const FunctionDecl *FD = dyn_cast<FunctionDecl>(D);
   if (FD) {
     LanguageLinkage L = FD->getLanguageLinkage();
if (VD) {
     if (DC->isTranslationUnit() && D->getFormalLinkage() != InternalLinkage &&
         !isa<VarTemplateSpecializationDecl>(D))
       return false;
   }
 
   return true;
 }
 
+void CXXNameMangler::writeAbiTags(const NamedDecl *ND, const AbiTagList *AdditionalAbiTags) {
+  assert(AbiTags && "require AbiTagState");
+  if (AbiTags) AbiTags->write(Out, ND, DisableDerivedAbiTags ? nullptr : AdditionalAbiTags);
+}
+
 void CXXNameMangler::mangle(const NamedDecl *D) {
   // <mangled-name> ::= _Z <encoding>
   //            ::= <data name>
   //            ::= <special-name>
   Out << "_Z";
   if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
     mangleFunctionEncoding(FD);
   else if (const VarDecl *VD = dyn_cast<VarDecl>(D))
     mangleName(VD);
   else if (const IndirectFieldDecl *IFD = dyn_cast<IndirectFieldDecl>(D))
     mangleName(IFD->getAnonField());
   else
     mangleName(cast<FieldDecl>(D));
 }
 
-void CXXNameMangler::mangleFunctionEncoding(const FunctionDecl *FD) {
-  // <encoding> ::= <function name> <bare-function-type>
-  mangleName(FD);
-
+void CXXNameMangler::mangleFunctionEncoding(const FunctionDecl *FD, bool ExcludeUnqualifiedName) {
   // Don't mangle in the type if this isn't a decl we should typically mangle.
-  if (!Context.shouldMangleDeclName(FD))
+  if (!Context.shouldMangleDeclName(FD)) {
+    mangleNameWithAbiTags(FD, /* AdditionalAbiTags */ nullptr, ExcludeUnqualifiedName);
     return;
+  }
 
+  // <encoding> ::= <function name> <bare-function-type>
+
+  if (ExcludeUnqualifiedName)
+  {
+    // running makeAdditionalTagsForFunction would loop, don't need it here anyway
+    mangleNameWithAbiTags(FD, /* AdditionalAbiTags */ nullptr, ExcludeUnqualifiedName);
+  } else {
+    AbiTagList AdditionalAbiTags = makeAdditionalTagsForFunction(FD);
+    mangleNameWithAbiTags(FD, &AdditionalAbiTags, ExcludeUnqualifiedName);
+  }
+
+  mangleFunctionEncodingBareType(FD);
+}
+
+void CXXNameMangler::mangleFunctionEncodingBareType(const FunctionDecl *FD) {
   if (FD->hasAttr<EnableIfAttr>()) {
     FunctionTypeDepthState Saved = FunctionTypeDepth.push();
     Out << "Ua9enable_ifI";
     // FIXME: specific_attr_iterator iterates in reverse order. Fix that and use
     // it here.
     for (AttrVec::const_reverse_iterator I = FD->getAttrs().rbegin(),
                                          E = FD->getAttrs().rend();
          I != E; ++I) {
if (const VarTemplateSpecializationDecl *Spec =
           dyn_cast<VarTemplateSpecializationDecl>(ND)) {
     TemplateArgs = &Spec->getTemplateArgs();
     return Spec->getSpecializedTemplate();
   }
 
   return nullptr;
 }
 
-void CXXNameMangler::mangleName(const NamedDecl *ND) {
+// must not be run from mangleLocalName for the <entity name> as it would loop otherwise.
+void CXXNameMangler::mangleName(const NamedDecl *ND, bool ExcludeUnqualifiedName) {
+  if (!ExcludeUnqualifiedName) {
+    if (const VarDecl *VD = dyn_cast<VarDecl>(ND)) {
+      AbiTagList VariableAdditionalAbiTags = makeAdditionalTagsForVariable(VD);
+      mangleNameWithAbiTags(VD, &VariableAdditionalAbiTags, ExcludeUnqualifiedName);
+      return;
+    }
+  }
+  mangleNameWithAbiTags(ND, nullptr, ExcludeUnqualifiedName);
+}
+
+void CXXNameMangler::mangleNameWithAbiTags(const NamedDecl *ND,
+                                           const AbiTagList *AdditionalAbiTags,
+                                           bool ExcludeUnqualifiedName) {
   //  <name> ::= <nested-name>
   //         ::= <unscoped-name>
   //         ::= <unscoped-template-name> <template-args>
   //         ::= <local-name>
   //
   const DeclContext *DC = getEffectiveDeclContext(ND);
 
   // If this is an extern variable declared locally, the relevant DeclContext
   // is that of the containing namespace, or the translation unit.
   // FIXME: This is a hack; extern variables declared locally should have
   // a proper semantic declaration context!
   if (isLocalContainerContext(DC) && ND->hasLinkage() && !isLambda(ND))
     while (!DC->isNamespace() && !DC->isTranslationUnit())
       DC = getEffectiveParentContext(DC);
   else if (GetLocalClassDecl(ND)) {
-    mangleLocalName(ND);
+    mangleLocalName(ND, AdditionalAbiTags, ExcludeUnqualifiedName);
     return;
   }
 
   DC = IgnoreLinkageSpecDecls(DC);
 
   if (DC->isTranslationUnit() || isStdNamespace(DC)) {
     // Check if we have a template.
     const TemplateArgumentList *TemplateArgs = nullptr;
     if (const TemplateDecl *TD = isTemplate(ND, TemplateArgs)) {
-      mangleUnscopedTemplateName(TD);
+      if (!ExcludeUnqualifiedName)
+        mangleUnscopedTemplateName(TD, AdditionalAbiTags);
       mangleTemplateArgs(*TemplateArgs);
       return;
     }
 
-    mangleUnscopedName(ND);
+    if (!ExcludeUnqualifiedName)
+      mangleUnscopedName(ND, AdditionalAbiTags);
     return;
   }
 
   if (isLocalContainerContext(DC)) {
-    mangleLocalName(ND);
+    mangleLocalName(ND, AdditionalAbiTags, ExcludeUnqualifiedName);
     return;
   }
 
-  mangleNestedName(ND, DC);
+  mangleNestedName(ND, DC, AdditionalAbiTags, /* NoFunction */ false, ExcludeUnqualifiedName);
 }
-void CXXNameMangler::mangleName(const TemplateDecl *TD,
+
+void CXXNameMangler::mangleTemplateName(const TemplateDecl *TD,
+                                        const AbiTagList *AdditionalAbiTags,
+                                        bool ExcludeUnqualifiedName,
                                         const TemplateArgument *TemplateArgs,
                                         unsigned NumTemplateArgs) {
   const DeclContext *DC = IgnoreLinkageSpecDecls(getEffectiveDeclContext(TD));
 
   if (DC->isTranslationUnit() || isStdNamespace(DC)) {
-    mangleUnscopedTemplateName(TD);
+    if (!ExcludeUnqualifiedName)
+      mangleUnscopedTemplateName(TD, AdditionalAbiTags);
     mangleTemplateArgs(TemplateArgs, NumTemplateArgs);
   } else {
-    mangleNestedName(TD, TemplateArgs, NumTemplateArgs);
+    mangleNestedName(TD, AdditionalAbiTags, ExcludeUnqualifiedName, TemplateArgs, NumTemplateArgs);
   }
 }
 
-void CXXNameMangler::mangleUnscopedName(const NamedDecl *ND) {
+void CXXNameMangler::mangleUnscopedName(const NamedDecl *ND, const AbiTagList *AdditionalAbiTags) {
   //  <unscoped-name> ::= <unqualified-name>
   //                  ::= St <unqualified-name>   # ::std::
 
   if (isStdNamespace(IgnoreLinkageSpecDecls(getEffectiveDeclContext(ND))))
     Out << "St";
 
-  mangleUnqualifiedName(ND);
+  mangleUnqualifiedName(ND, AdditionalAbiTags);
 }
 
-void CXXNameMangler::mangleUnscopedTemplateName(const TemplateDecl *ND) {
+void CXXNameMangler::mangleUnscopedTemplateName(const TemplateDecl *ND,
+                                                const AbiTagList *AdditionalAbiTags) {
   //     <unscoped-template-name> ::= <unscoped-name>
   //                              ::= <substitution>
   if (mangleSubstitution(ND))
     return;
 
   // <template-template-param> ::= <template-param>
-  if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(ND))
+  if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(ND)) {
+    assert(!AdditionalAbiTags && "template template param cannot have abi tags"); // TODO(abitags)
     mangleTemplateParameter(TTP->getIndex());
-  else
-    mangleUnscopedName(ND->getTemplatedDecl());
+  } else {
+    mangleUnscopedName(ND->getTemplatedDecl(), AdditionalAbiTags);
+  }
 
   addSubstitution(ND);
 }
 
-void CXXNameMangler::mangleUnscopedTemplateName(TemplateName Template) {
+void CXXNameMangler::mangleUnscopedTemplateName(TemplateName Template,
+                                                const AbiTagList *AdditionalAbiTags) {
   //     <unscoped-template-name> ::= <unscoped-name>
   //                              ::= <substitution>
   if (TemplateDecl *TD = Template.getAsTemplateDecl())
-    return mangleUnscopedTemplateName(TD);
+    return mangleUnscopedTemplateName(TD, AdditionalAbiTags);
   
   if (mangleSubstitution(Template))
     return;
 
+  assert(!AdditionalAbiTags && "dependent template name cannot have abi tags"); // TODO(abitags)
+
   DependentTemplateName *Dependent = Template.getAsDependentTemplateName();
   assert(Dependent && "Not a dependent template name?");
   if (const IdentifierInfo *Id = Dependent->getIdentifier())
     mangleSourceName(Id);
   else
     mangleOperatorName(Dependent->getOperator(), UnknownArity);
-  
   addSubstitution(Template);
 }
 
 void CXXNameMangler::mangleFloat(const llvm::APFloat &f) {
   // ABI:
   //   Floating-point literals are encoded using a fixed-length
   //   lowercase hexadecimal string corresponding to the internal
   //   representation (IEEE on Itanium), high-order bytes first,
void CXXNameMangler::mangleUnresolvedPrefix(NestedNameSpecifier *qualifier,
 
   case NestedNameSpecifier::Namespace:
     if (qualifier->getPrefix())
       mangleUnresolvedPrefix(qualifier->getPrefix(),
                              /*recursive*/ true);
     else
       Out << "sr";
     mangleSourceName(qualifier->getAsNamespace()->getIdentifier());
+    writeAbiTags(qualifier->getAsNamespace());
     break;
   case NestedNameSpecifier::NamespaceAlias:
     if (qualifier->getPrefix())
       mangleUnresolvedPrefix(qualifier->getPrefix(),
                              /*recursive*/ true);
     else
       Out << "sr";
     mangleSourceName(qualifier->getAsNamespaceAlias()->getIdentifier());
+    writeAbiTags(qualifier->getAsNamespaceAlias());
     break;
 
   case NestedNameSpecifier::TypeSpec:
   case NestedNameSpecifier::TypeSpecWithTemplate: {
     const Type *type = qualifier->getAsType();
 
     // We only want to use an unresolved-type encoding if this is one of:
     //   - a decltype
case NestedNameSpecifier::Identifier:
     // Member expressions can have these without prefixes.
     if (qualifier->getPrefix())
       mangleUnresolvedPrefix(qualifier->getPrefix(),
                              /*recursive*/ true);
     else
       Out << "sr";
 
     mangleSourceName(qualifier->getAsIdentifier());
+    // an Identifier has no type information, so we can't emit abi tags for it
     break;
   }
 
   // If this was the innermost part of the NNS, and we fell out to
   // here, append an 'E'.
   if (!recursive)
     Out << 'E';
 }
switch (name.getNameKind()) {
     case DeclarationName::ObjCOneArgSelector:
     case DeclarationName::ObjCZeroArgSelector:
       llvm_unreachable("Can't mangle Objective-C selector names here!");
   }
 }
 
 void CXXNameMangler::mangleUnqualifiedName(const NamedDecl *ND,
                                            DeclarationName Name,
-                                           unsigned KnownArity) {
+                                           unsigned KnownArity,
+                                           const AbiTagList *AdditionalAbiTags) {
   unsigned Arity = KnownArity;
   //  <unqualified-name> ::= <operator-name>
   //                     ::= <ctor-dtor-name>
   //                     ::= <source-name>
   switch (Name.getNameKind()) {
   case DeclarationName::Identifier: {
     if (const IdentifierInfo *II = Name.getAsIdentifierInfo()) {
       // We must avoid conflicts between internally- and externally-
       // linked variable and function declaration names in the same TU:
       //   void test() { extern void foo(); }
       //   static void foo();
       // This naming convention is the same as that followed by GCC,
       // though it shouldn't actually matter.
       if (ND && ND->getFormalLinkage() == InternalLinkage &&
           getEffectiveDeclContext(ND)->isFileContext())
         Out << 'L';
 
       mangleSourceName(II);
+      writeAbiTags(ND, AdditionalAbiTags);
       break;
     }
 
     // Otherwise, an anonymous entity.  We must have a declaration.
     assert(ND && "mangling empty name without declaration");
 
     if (const NamespaceDecl *NS = dyn_cast<NamespaceDecl>(ND)) {
       if (NS->isAnonymousNamespace()) {
if (const VarDecl *VD = dyn_cast<VarDecl>(ND)) {
 
       // It's actually possible for various reasons for us to get here
       // with an empty anonymous struct / union.  Fortunately, it
       // doesn't really matter what name we generate.
       if (!FD) break;
       assert(FD->getIdentifier() && "Data member name isn't an identifier!");
 
       mangleSourceName(FD->getIdentifier());
+      // TODO(abitags): not emitting abi tags: internal name anyway
       break;
     }
 
     // Class extensions have no name as a category, and it's possible
     // for them to be the semantic parent of certain declarations
     // (primarily, tag decls defined within declarations).  Such
     // declarations will always have internal linkage, so the name
     // doesn't really matter, but we shouldn't crash on them.  For
     // safety, just handle all ObjC containers here.
     if (isa<ObjCContainerDecl>(ND))
       break;
     
     // We must have an anonymous struct.
     const TagDecl *TD = cast<TagDecl>(ND);
     if (const TypedefNameDecl *D = TD->getTypedefNameForAnonDecl()) {
       assert(TD->getDeclContext() == D->getDeclContext() &&
              "Typedef should not be in another decl context!");
       assert(D->getDeclName().getAsIdentifierInfo() &&
              "Typedef was not named!");
       mangleSourceName(D->getDeclName().getAsIdentifierInfo());
+      assert(!AdditionalAbiTags && "Type cannot have additional abi tags");
+      // explicit abi tags are still possible; take from underlying type, not from typedef.
+      writeAbiTags(TD, nullptr);
       break;
     }
 
     // <unnamed-type-name> ::= <closure-type-name>
     // 
     // <closure-type-name> ::= Ul <lambda-sig> E [ <nonnegative number> ] _
     // <lambda-sig> ::= <parameter-type>+   # Parameter types or 'v' for 'void'.
     if (const CXXRecordDecl *Record = dyn_cast<CXXRecordDecl>(TD)) {
       if (Record->isLambda() && Record->getLambdaManglingNumber()) {
+        assert(!AdditionalAbiTags && "Lambda type cannot have additional abi tags");
         mangleLambda(Record);
         break;
       }
     }
 
     if (TD->isExternallyVisible()) {
       unsigned UnnamedMangle = getASTContext().getManglingNumber(TD);
       Out << "Ut";
       if (UnnamedMangle > 1)
         Out << UnnamedMangle - 2;
       Out << '_';
+      writeAbiTags(TD, AdditionalAbiTags);
       break;
     }
 
     // Get a unique id for the anonymous struct.
     unsigned AnonStructId = Context.getAnonymousStructId(TD);
 
     // Mangle it as a source name in the form
     // [n] $_<id>
case DeclarationName::CXXConstructorName:
     if (ND == Structor)
       // If the named decl is the C++ constructor we're mangling, use the type
       // we were given.
       mangleCXXCtorType(static_cast<CXXCtorType>(StructorType));
     else
       // Otherwise, use the complete constructor name. This is relevant if a
       // class with a constructor is declared within a constructor.
       mangleCXXCtorType(Ctor_Complete);
+    writeAbiTags(ND, AdditionalAbiTags);
     break;
 
   case DeclarationName::CXXDestructorName:
     if (ND == Structor)
       // If the named decl is the C++ destructor we're mangling, use the type we
       // were given.
       mangleCXXDtorType(static_cast<CXXDtorType>(StructorType));
     else
       // Otherwise, use the complete destructor name. This is relevant if a
       // class with a destructor is declared within a destructor.
       mangleCXXDtorType(Dtor_Complete);
+    writeAbiTags(ND, AdditionalAbiTags);
     break;
 
   case DeclarationName::CXXOperatorName:
     if (ND && Arity == UnknownArity) {
       Arity = cast<FunctionDecl>(ND)->getNumParams();
 
       // If we have a member function, we need to include the 'this' pointer.
       if (const auto *MD = dyn_cast<CXXMethodDecl>(ND))
         if (!MD->isStatic())
           Arity++;
     }
   // FALLTHROUGH
   case DeclarationName::CXXConversionFunctionName:
   case DeclarationName::CXXLiteralOperatorName:
     mangleOperatorName(Name, Arity);
+    writeAbiTags(ND, AdditionalAbiTags);
     break;
 
   case DeclarationName::CXXUsingDirective:
     llvm_unreachable("Can't mangle a using directive name!");
   }
 }
 
 void CXXNameMangler::mangleSourceName(const IdentifierInfo *II) {
   // <source-name> ::= <positive length number> <identifier>
   // <number> ::= [n] <non-negative decimal integer>
   // <identifier> ::= <unqualified source code identifier>
   Out << II->getLength() << II->getName();
 }
 
 void CXXNameMangler::mangleNestedName(const NamedDecl *ND,
                                       const DeclContext *DC,
-                                      bool NoFunction) {
+                                      const AbiTagList *AdditionalAbiTags,
+                                      bool NoFunction,
+                                      bool ExcludeUnqualifiedName) {
   // <nested-name> 
   //   ::= N [<CV-qualifiers>] [<ref-qualifier>] <prefix> <unqualified-name> E
   //   ::= N [<CV-qualifiers>] [<ref-qualifier>] <template-prefix> 
   //       <template-args> E
 
   Out << 'N';
   if (const CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(ND)) {
     Qualifiers MethodQuals =
         Qualifiers::fromCVRMask(Method->getTypeQualifiers());
     // We do not consider restrict a distinguishing attribute for overloading
     // purposes so we must not mangle it.
     MethodQuals.removeRestrict();
     mangleQualifiers(MethodQuals);
     mangleRefQualifier(Method->getRefQualifier());
   }
   
   // Check if we have a template.
   const TemplateArgumentList *TemplateArgs = nullptr;
   if (const TemplateDecl *TD = isTemplate(ND, TemplateArgs)) {
-    mangleTemplatePrefix(TD, NoFunction);
+    mangleTemplatePrefix(TD, AdditionalAbiTags, NoFunction, ExcludeUnqualifiedName);
     mangleTemplateArgs(*TemplateArgs);
   }
   else {
     manglePrefix(DC, NoFunction);
-    mangleUnqualifiedName(ND);
+    if (!ExcludeUnqualifiedName)
+      mangleUnqualifiedName(ND, AdditionalAbiTags);
   }
 
   Out << 'E';
 }
 void CXXNameMangler::mangleNestedName(const TemplateDecl *TD,
+                                      const AbiTagList *AdditionalAbiTags,
+                                      bool ExcludeUnqualifiedName,
                                       const TemplateArgument *TemplateArgs,
                                       unsigned NumTemplateArgs) {
   // <nested-name> ::= N [<CV-qualifiers>] <template-prefix> <template-args> E
 
   Out << 'N';
 
-  mangleTemplatePrefix(TD);
+  mangleTemplatePrefix(TD, AdditionalAbiTags, ExcludeUnqualifiedName);
   mangleTemplateArgs(TemplateArgs, NumTemplateArgs);
 
   Out << 'E';
 }
 
-void CXXNameMangler::mangleLocalName(const Decl *D) {
+void CXXNameMangler::mangleLocalName(const Decl *D,
+                                     const AbiTagList *AdditionalAbiTags,
+                                     bool ExcludeUnqualifiedName) {
   // <local-name> := Z <function encoding> E <entity name> [<discriminator>]
   //              := Z <function encoding> E s [<discriminator>]
   // <local-name> := Z <function encoding> E d [ <parameter number> ] 
   //                 _ <entity name>
   // <discriminator> := _ <non-negative number>
   assert(isa<NamedDecl>(D) || isa<BlockDecl>(D));
   const RecordDecl *RD = GetLocalClassDecl(D);
   const DeclContext *DC = getEffectiveDeclContext(RD ? RD : D);
 
   Out << 'Z';
 
+  {
+    AbiTagState localAbiTags(AbiTags);
+
     if (const ObjCMethodDecl *MD = dyn_cast<ObjCMethodDecl>(DC))
       mangleObjCMethodName(MD);
     else if (const BlockDecl *BD = dyn_cast<BlockDecl>(DC))
       mangleBlockForPrefix(BD);
     else
       mangleFunctionEncoding(cast<FunctionDecl>(DC));
 
+    // implicit abi tags (from namespace) are not available in the following
+    // entity; reset to actually emitted tags, which are available.
+    localAbiTags.UsedAbiTags = localAbiTags.EmittedAbiTags;
+  }
+
   Out << 'E';
 
+  TemporaryDisableDerivedAbiTags TemporyDisable(DisableDerivedAbiTags, getStructor(dyn_cast<NamedDecl>(D)) != Structor);
+
   if (RD) {
     // The parameter number is omitted for the last parameter, 0 for the 
     // second-to-last parameter, 1 for the third-to-last parameter, etc. The 
     // <entity name> will of course contain a <closure-type-name>: Its 
     // numbering will be local to the particular argument in which it appears
     // -- other default arguments do not affect its encoding.
     const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD);
     if (CXXRD->isLambda()) {
       if (const ParmVarDecl *Parm
               = dyn_cast_or_null<ParmVarDecl>(CXXRD->getLambdaContextDecl())) {
         if (const FunctionDecl *Func
               = dyn_cast<FunctionDecl>(Parm->getDeclContext())) {
           Out << 'd';
           unsigned Num = Func->getNumParams() - Parm->getFunctionScopeIndex();
           if (Num > 1)
             mangleNumber(Num - 2);
           Out << '_';
         }
       }
     }
     
     // Mangle the name relative to the closest enclosing function.
     // equality ok because RD derived from ND above
     if (D == RD)  {
-      mangleUnqualifiedName(RD);
+      if (!ExcludeUnqualifiedName)
+        mangleUnqualifiedName(RD, AdditionalAbiTags);
     } else if (const BlockDecl *BD = dyn_cast<BlockDecl>(D)) {
       manglePrefix(getEffectiveDeclContext(BD), true /*NoFunction*/);
+      assert(!AdditionalAbiTags && "Block cannot have additional abi tags");
+      if (!ExcludeUnqualifiedName)
         mangleUnqualifiedBlock(BD);
     } else {
       const NamedDecl *ND = cast<NamedDecl>(D);
-      mangleNestedName(ND, getEffectiveDeclContext(ND), true /*NoFunction*/);
+      mangleNestedName(ND, getEffectiveDeclContext(ND),
+                       AdditionalAbiTags, true /*NoFunction*/, ExcludeUnqualifiedName);
     }
   } else if (const BlockDecl *BD = dyn_cast<BlockDecl>(D)) {
     // Mangle a block in a default parameter; see above explanation for
     // lambdas.
     if (const ParmVarDecl *Parm
             = dyn_cast_or_null<ParmVarDecl>(BD->getBlockManglingContextDecl())) {
       if (const FunctionDecl *Func
             = dyn_cast<FunctionDecl>(Parm->getDeclContext())) {
         Out << 'd';
         unsigned Num = Func->getNumParams() - Parm->getFunctionScopeIndex();
         if (Num > 1)
           mangleNumber(Num - 2);
         Out << '_';
       }
     }
 
+    assert(!AdditionalAbiTags && "Block cannot have additional abi tags");
+    if (!ExcludeUnqualifiedName)
       mangleUnqualifiedBlock(BD);
   } else {
-    mangleUnqualifiedName(cast<NamedDecl>(D));
+    if (!ExcludeUnqualifiedName)
+      mangleUnqualifiedName(cast<NamedDecl>(D), AdditionalAbiTags);
   }
 
+  if (!ExcludeUnqualifiedName) {
     if (const NamedDecl *ND = dyn_cast<NamedDecl>(RD ? RD : D)) {
       unsigned disc;
       if (Context.getNextDiscriminator(ND, disc)) {
         if (disc < 10)
           Out << '_' << disc;
         else
           Out << "__" << disc << '_';
       }
     }
   }
+}
 
 void CXXNameMangler::mangleBlockForPrefix(const BlockDecl *Block) {
   if (GetLocalClassDecl(Block)) {
-    mangleLocalName(Block);
+    mangleLocalName(Block, /* AdditionalAbiTags */ nullptr, /* ExcludeUnqualifiedName */ false);
     return;
   }
   const DeclContext *DC = getEffectiveDeclContext(Block);
   if (isLocalContainerContext(DC)) {
-    mangleLocalName(Block);
+    mangleLocalName(Block, /* AdditionalAbiTags */ nullptr, /* ExcludeUnqualifiedName */ false);
     return;
   }
   manglePrefix(getEffectiveDeclContext(Block));
   mangleUnqualifiedBlock(Block);
 }
 
 void CXXNameMangler::mangleUnqualifiedBlock(const BlockDecl *Block) {
   if (Decl *Context = Block->getBlockManglingContextDecl()) {
     if ((isa<VarDecl>(Context) || isa<FieldDecl>(Context)) &&
         Context->getDeclContext()->isRecord()) {
-      if (const IdentifierInfo *Name
-            = cast<NamedDecl>(Context)->getIdentifier()) {
+      const auto *ND = cast<NamedDecl>(Context);
+      if (const IdentifierInfo *Name = ND->getIdentifier()) {
         mangleSourceName(Name);
-        Out << 'M';            
+        writeAbiTags(ND, /* AdditionalAbiTags */ nullptr);
+        Out << 'M';
       }
     }
   }
 
   // If we have a block mangling number, use it.
   unsigned Number = Block->getBlockManglingNumber();
   // Otherwise, just make up a number. It doesn't matter what it is because
   // the symbol in question isn't externally visible.
void CXXNameMangler::mangleLambda(const CXXRecordDecl *Lambda) {
   // since a closure type will always be mangled with a prefix, it's easier
   // to emit that last part of the prefix here.
   if (Decl *Context = Lambda->getLambdaContextDecl()) {
     if ((isa<VarDecl>(Context) || isa<FieldDecl>(Context)) &&
         Context->getDeclContext()->isRecord()) {
       if (const IdentifierInfo *Name
             = cast<NamedDecl>(Context)->getIdentifier()) {
         mangleSourceName(Name);
-        Out << 'M';            
+        Out << 'M';
       }
     }
   }
 
   Out << "Ul";
   const FunctionProtoType *Proto = Lambda->getLambdaTypeInfo()->getType()->
                                    getAs<FunctionProtoType>();
   mangleBareFunctionType(Proto, /*MangleReturnType=*/false,
void CXXNameMangler::manglePrefix(const DeclContext *DC, bool NoFunction) {
 
   const NamedDecl *ND = cast<NamedDecl>(DC);  
   if (mangleSubstitution(ND))
     return;
   
   // Check if we have a template.
   const TemplateArgumentList *TemplateArgs = nullptr;
   if (const TemplateDecl *TD = isTemplate(ND, TemplateArgs)) {
-    mangleTemplatePrefix(TD);
+    mangleTemplatePrefix(TD, /* AdditionalAbiTags */ nullptr);
     mangleTemplateArgs(*TemplateArgs);
   } else {
     manglePrefix(getEffectiveDeclContext(ND), NoFunction);
-    mangleUnqualifiedName(ND);
+    mangleUnqualifiedName(ND, /* AdditionalAbiTags */ nullptr);
   }
 
   addSubstitution(ND);
 }
 
 void CXXNameMangler::mangleTemplatePrefix(TemplateName Template) {
   // <template-prefix> ::= <prefix> <template unqualified-name>
   //                   ::= <template-param>
   //                   ::= <substitution>
   if (TemplateDecl *TD = Template.getAsTemplateDecl())
-    return mangleTemplatePrefix(TD);
+    return mangleTemplatePrefix(TD, /* AdditionalAbiTags */ nullptr);
 
   if (QualifiedTemplateName *Qualified = Template.getAsQualifiedTemplateName())
     manglePrefix(Qualified->getQualifier());
-  
   if (OverloadedTemplateStorage *Overloaded
                                       = Template.getAsOverloadedTemplate()) {
     mangleUnqualifiedName(nullptr, (*Overloaded->begin())->getDeclName(),
-                          UnknownArity);
+                          UnknownArity,
+                          /* AdditionalAbiTags */ nullptr);
     return;
   }
-   
   DependentTemplateName *Dependent = Template.getAsDependentTemplateName();
   assert(Dependent && "Unknown template name kind?");
   if (NestedNameSpecifier *Qualifier = Dependent->getQualifier())
     manglePrefix(Qualifier);
-  mangleUnscopedTemplateName(Template);
+  mangleUnscopedTemplateName(Template, /* AdditionalAbiTags */ nullptr);
 }
 
 void CXXNameMangler::mangleTemplatePrefix(const TemplateDecl *ND,
-                                          bool NoFunction) {
+                                          const AbiTagList *AdditionalAbiTags,
+                                          bool NoFunction,
+                                          bool ExcludeUnqualifiedName) {
   // <template-prefix> ::= <prefix> <template unqualified-name>
   //                   ::= <template-param>
   //                   ::= <substitution>
   // <template-template-param> ::= <template-param>
   //                               <substitution>
 
   if (mangleSubstitution(ND))
     return;
 
   // <template-template-param> ::= <template-param>
   if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(ND)) {
+    // TODO(abitags): ???
     mangleTemplateParameter(TTP->getIndex());
   } else {
     manglePrefix(getEffectiveDeclContext(ND), NoFunction);
-    mangleUnqualifiedName(ND->getTemplatedDecl());
+    if (!ExcludeUnqualifiedName)
+      mangleUnqualifiedName(ND->getTemplatedDecl(), AdditionalAbiTags);
   }
 
   addSubstitution(ND);
 }
 
 /// Mangles a template name under the production <type>.  Required for
 /// template template arguments.
 ///   <type> ::= <class-enum-type>
void CXXNameMangler::mangleType(TemplateName TN) {
   case TemplateName::DependentTemplate: {
     const DependentTemplateName *Dependent = TN.getAsDependentTemplateName();
     assert(Dependent->isIdentifier());
 
     // <class-enum-type> ::= <name>
     // <name> ::= <nested-name>
     mangleUnresolvedPrefix(Dependent->getQualifier());
     mangleSourceName(Dependent->getIdentifier());
+     // writeAbiTags(Dependent);
     break;
   }
 
   case TemplateName::SubstTemplateTemplateParm: {
     // Substituted template parameters are mangled as the substituted
     // template.  This will check for the substitution twice, which is
     // fine, but we have to return early so that we don't try to *add*
     // the substitution twice.
unresolvedType:
     mangleType(Ty);
 
     // We never want to print 'E' directly after an unresolved-type,
     // so we return directly.
     return true;
 
   case Type::Typedef:
     mangleSourceName(cast<TypedefType>(Ty)->getDecl()->getIdentifier());
+    writeAbiTags(cast<TypedefType>(Ty)->getDecl());
     break;
 
   case Type::UnresolvedUsing:
     mangleSourceName(
         cast<UnresolvedUsingType>(Ty)->getDecl()->getIdentifier());
+    writeAbiTags(cast<UnresolvedUsingType>(Ty)->getDecl());
     break;
 
   case Type::Enum:
   case Type::Record:
     mangleSourceName(cast<TagType>(Ty)->getDecl()->getIdentifier());
+    writeAbiTags(cast<TagType>(Ty)->getDecl());
     break;
 
   case Type::TemplateSpecialization: {
     const TemplateSpecializationType *TST =
         cast<TemplateSpecializationType>(Ty);
     TemplateName TN = TST->getTemplateName();
     switch (TN.getKind()) {
     case TemplateName::Template:
     case TemplateName::QualifiedTemplate: {
       TemplateDecl *TD = TN.getAsTemplateDecl();
 
       // If the base is a template template parameter, this is an
       // unresolved type.
       assert(TD && "no template for template specialization type");
       if (isa<TemplateTemplateParmDecl>(TD))
         goto unresolvedType;
 
       mangleSourceName(TD->getIdentifier());
+      writeAbiTags(TD);
       break;
     }
 
     case TemplateName::OverloadedTemplate:
     case TemplateName::DependentTemplate:
       llvm_unreachable("invalid base for a template specialization type");
 
     case TemplateName::SubstTemplateTemplateParm: {
case Type::TemplateSpecialization: {
 
     mangleTemplateArgs(TST->getArgs(), TST->getNumArgs());
     break;
   }
 
   case Type::InjectedClassName:
     mangleSourceName(
         cast<InjectedClassNameType>(Ty)->getDecl()->getIdentifier());
+    writeAbiTags(cast<InjectedClassNameType>(Ty)->getDecl());
     break;
 
   case Type::DependentName:
     mangleSourceName(cast<DependentNameType>(Ty)->getIdentifier());
+    // writeAbiTags(cast<DependentNameType>(Ty));
     break;
 
   case Type::DependentTemplateSpecialization: {
     const DependentTemplateSpecializationType *DTST =
         cast<DependentTemplateSpecializationType>(Ty);
     mangleSourceName(DTST->getIdentifier());
+    // writeAbiTags(DTST);
     mangleTemplateArgs(DTST->getArgs(), DTST->getNumArgs());
     break;
   }
 
   case Type::Elaborated:
     return mangleUnresolvedTypeOrSimpleId(
         cast<ElaboratedType>(Ty)->getNamedType(), Prefix);
   }
void CXXNameMangler::mangleType(const InjectedClassNameType *T) {
   // Mangle injected class name types as if the user had written the
   // specialization out fully.  It may not actually be possible to see
   // this mangling, though.
   mangleType(T->getInjectedSpecializationType());
 }
 
 void CXXNameMangler::mangleType(const TemplateSpecializationType *T) {
   if (TemplateDecl *TD = T->getTemplateName().getAsTemplateDecl()) {
-    mangleName(TD, T->getArgs(), T->getNumArgs());
+    // types only have explicit abi tags, no addition tags
+    mangleTemplateName(TD,
+                       /* AdditionalAbiTags */ nullptr,
+                       /* ExcludeUnqualifiedName */ false,
+                       T->getArgs(), T->getNumArgs());
   } else {
     if (mangleSubstitution(QualType(T, 0)))
       return;
     
     mangleTemplatePrefix(T->getTemplateName());
     
     // FIXME: GCC does not appear to mangle the template arguments when
     // the template in question is a dependent template name. Should we
case ETK_Enum:
       break;
     default:
       llvm_unreachable("unexpected keyword for dependent type name");
   }
   // Typename types are always nested
   Out << 'N';
   manglePrefix(T->getQualifier());
   mangleSourceName(T->getIdentifier());
+  // writeAbiTags(T); // TODO(abitags)
   Out << 'E';
 }
 
 void CXXNameMangler::mangleType(const DependentTemplateSpecializationType *T) {
   // Dependently-scoped template types are nested if they have a prefix.
   Out << 'N';
 
   // TODO: avoid making this TemplateName.
void CXXNameMangler::addSubstitution(TemplateName Template) {
   addSubstitution(reinterpret_cast<uintptr_t>(Template.getAsVoidPointer()));
 }
 
 void CXXNameMangler::addSubstitution(uintptr_t Ptr) {
   assert(!Substitutions.count(Ptr) && "Substitution already exists!");
   Substitutions[Ptr] = SeqID++;
 }
 
+std::set<StringRef> CXXNameMangler::getTagsFromPrefixAndTemplateArguments(const NamedDecl *ND) {
+  llvm::raw_null_ostream NullOutStream;
+  CXXNameMangler TrackPrefixAndTemplateArguments(*this, NullOutStream);
+
+  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(ND)) {
+    TrackPrefixAndTemplateArguments.mangleFunctionEncoding(FD, /* ExcludeUnqualifiedName */ true);
+  } else {
+    TrackPrefixAndTemplateArguments.mangleName(ND, /* ExcludeUnqualifiedName */ true);
+  }
+
+  return std::move(TrackPrefixAndTemplateArguments.AbiTagsRoot.UsedAbiTags);
+}
+
+CXXNameMangler::AbiTagList CXXNameMangler::makeAdditionalTagsForFunction(const FunctionDecl *FD) {
+  // when derived abi tags are disabled there is no need to make any list
+  if (DisableDerivedAbiTags) return AbiTagList();
+
+  std::set<StringRef> ImplicitlyAvailableTags = getTagsFromPrefixAndTemplateArguments(FD);
+  std::set<StringRef> ReturnTypeTags;
+
+  {
+    llvm::raw_null_ostream NullOutStream;
+    CXXNameMangler TrackReturnTypeTags(*this, NullOutStream);
+    TrackReturnTypeTags.disableDerivedAbiTags();
+
+    const FunctionProtoType *Proto = cast<FunctionProtoType>(FD->getType()->getAs<FunctionType>());
+    TrackReturnTypeTags.FunctionTypeDepth.enterResultType();
+    TrackReturnTypeTags.mangleType(Proto->getReturnType());
+    TrackReturnTypeTags.FunctionTypeDepth.leaveResultType();
+
+    ReturnTypeTags = std::move(TrackReturnTypeTags.AbiTagsRoot.UsedAbiTags);
+  }
+
+  AbiTagList AdditionalAbiTags;
+
+  for (const auto& Tag: ReturnTypeTags) {
+    if (ImplicitlyAvailableTags.count(Tag) == 0)
+      AdditionalAbiTags.push_back(Tag);
+  }
+
+  return AdditionalAbiTags;
+}
+
+CXXNameMangler::AbiTagList CXXNameMangler::makeAdditionalTagsForVariable(const VarDecl *VD) {
+  // when derived abi tags are disabled there is no need to make any list
+  if (DisableDerivedAbiTags) return AbiTagList();
+
+  std::set<StringRef> ImplicitlyAvailableTags = getTagsFromPrefixAndTemplateArguments(VD);
+  std::set<StringRef> VariableTypeTags;
+
+  {
+    llvm::raw_null_ostream NullOutStream;
+    CXXNameMangler TrackVariableType(*this, NullOutStream);
+    TrackVariableType.disableDerivedAbiTags();
+
+    TrackVariableType.mangleType(VD->getType());
+
+    VariableTypeTags = std::move(TrackVariableType.AbiTagsRoot.UsedAbiTags);
+  }
+
+  AbiTagList AdditionalAbiTags;
+
+  for (const auto& Tag: VariableTypeTags) {
+    if (ImplicitlyAvailableTags.count(Tag) == 0)
+      AdditionalAbiTags.push_back(Tag);
+  }
+
+  return AdditionalAbiTags;
+}
+
 //
 
 /// Mangles the name of the declaration D and emits that name to the given
 /// output stream.
 ///
 /// If the declaration D requires a mangled name, this routine will emit that
 /// mangled name to \p os and return true. Otherwise, \p os will be unchanged
 /// and this routine will return false. In this case, the caller should just

 }
 
 /// Returns the mangled name for a guard variable for the passed in VarDecl.
 void ItaniumMangleContextImpl::mangleStaticGuardVariable(const VarDecl *D,
                                                          raw_ostream &Out) {
   //  <special-name> ::= GV <object name>       # Guard variable for one-time
   //                                            # initialization
   CXXNameMangler Mangler(*this, Out);
+  Mangler.disableDerivedAbiTags(); // GCC: doesn't emit derived abi tags for guard variables
   Mangler.getStream() << "_ZGV";
   Mangler.mangleName(D);
 }
 
 void ItaniumMangleContextImpl::mangleDynamicInitializer(const VarDecl *MD,
                                                         raw_ostream &Out) {
   // These symbols are internal in the Itanium ABI, so the names don't matter.
   // Clang has traditionally used this symbol and allowed LLVM to adjust it to

lib/Sema/SemaDeclAttr.cpp

static void handleDeclspecThreadAttr(Sema &S, Decl *D,
   if (VD->hasLocalStorage()) {
     S.Diag(Attr.getLoc(), diag::err_thread_non_global) << "__declspec(thread)";
     return;
   }
   VD->addAttr(::new (S.Context) ThreadAttr(
       Attr.getRange(), S.Context, Attr.getAttributeSpellingListIndex()));
 }
 
+static void handleAbiTagAttr(Sema &S, Decl *D,
+                             const AttributeList &Attr) {
+  if (!checkAttributeAtLeastNumArgs(S, Attr, 1))
+    return;
+
+  SmallVector<std::string, 4> Tags;
+
+  for (unsigned I = 0, E = Attr.getNumArgs(); I != E; ++I) {
+    StringRef Tag;
+
+    if (!S.checkStringLiteralArgumentAttr(Attr, I, Tag))
+      return;
+
+    Tags.push_back(Tag);
+  }
+  // store tags sorted and without duplicates
+  std::sort(Tags.begin(), Tags.end());
+  Tags.erase(std::unique(Tags.begin(), Tags.end()), Tags.end());
+
+  if (const auto *NS = dyn_cast<NamespaceDecl>(D)) {
+    if (!NS->isInline()) {
+      S.Diag(Attr.getLoc(), diag::err_attr_abi_tag_only_on_inline_namespace);
+      return;
+    }
+  }
+
+  const auto *CD = D->getCanonicalDecl();
+  if (CD != D) {
+    // redeclarations must not add new abi tags, or abi tags in the first place
+    const auto *OldAbiTagAttr = D->getAttr<AbiTagAttr>();
+    if (nullptr == OldAbiTagAttr) {
+      S.Diag(Attr.getLoc(), diag::err_abi_tag_on_redeclaration);
+      S.Diag(CD->getLocation(), diag::note_previous_definition);
+      return;
+    }
+    for (const auto& NewTag: Tags) {
+      if (std::find(OldAbiTagAttr->tags_begin(),
+                    OldAbiTagAttr->tags_end(),
+                    NewTag) == OldAbiTagAttr->tags_end()) {
+        S.Diag(Attr.getLoc(), diag::err_new_abi_tag_on_redeclaration) << NewTag;
+        S.Diag(OldAbiTagAttr->getLocation(), diag::note_previous_definition);
+        return;
+      }
+    }
+    return;
+  }
+
+  D->addAttr(::new (S.Context) AbiTagAttr(Attr.getRange(), S.Context,
+                                          Tags.data(), Tags.size(),
+                                          Attr.getAttributeSpellingListIndex()));
+}
+
 static void handleARMInterruptAttr(Sema &S, Decl *D,
                                    const AttributeList &Attr) {
   // Check the attribute arguments.
   if (Attr.getNumArgs() > 1) {
     S.Diag(Attr.getLoc(), diag::err_attribute_too_many_arguments)
       << Attr.getName() << 1;
     return;
   }
case AttributeList::AT_MSInheritance:
     handleMSInheritanceAttr(S, D, Attr);
     break;
   case AttributeList::AT_SelectAny:
     handleSimpleAttribute<SelectAnyAttr>(S, D, Attr);
     break;
   case AttributeList::AT_Thread:
     handleDeclspecThreadAttr(S, D, Attr);
     break;
+  case AttributeList::AT_AbiTag:
+    handleAbiTagAttr(S, D, Attr);
+    break;
 
   // Thread safety attributes:
   case AttributeList::AT_AssertExclusiveLock:
     handleAssertExclusiveLockAttr(S, D, Attr);
     break;
   case AttributeList::AT_AssertSharedLock:
     handleAssertSharedLockAttr(S, D, Attr);
     break;