[clang] Implement objc_non_runtime_protocol to remove protocol metadata

Summary:
Motivated by the new objc_direct attribute, this change adds a new
attribute that remotes metadata from Protocols that the programmer knows
isn't going to be used at runtime. We simply have the frontend skip
generating any protocol metadata entries (e.g. OBJC_CLASS_NAME,
_OBJC_$_PROTOCOL_INSTANCE_METHDOS, _OBJC_PROTOCOL, etc) for a protocol
marked with `__attribute__((objc_non_runtime_protocol))`.

There are a few APIs used to retrieve a protocol at runtime.
`@protocol(SomeProtocol)` will now error out of the requested protocol
is marked with attribute. `objc_getProtocol` will return `NULL` which
is consistent with the behavior of a non-existing protocol.

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75574

clang/include/clang/AST/DeclObjC.h

@@ -2178,6 +2178,14 @@ class ObjCProtocolDecl : public ObjCContainerDecl,
     data().ReferencedProtocols.set(List, Num, Locs, C);
   }
 
+  /// This is true iff the protocol is tagged with the `objc_static_protocol`
+  /// attribute.
+  bool isNonRuntimeProtocol() const;
+
+  /// Get the set of all protocols implied by this protocols inheritance
+  /// hierarchy.
+  void getImpliedProtocols(llvm::DenseSet<const ObjCProtocolDecl *> &IPs) const;
+
   ObjCProtocolDecl *lookupProtocolNamed(IdentifierInfo *PName);
 
   // Lookup a method. First, we search locally. If a method isn't

clang/include/clang/Basic/Attr.td

@@ -2024,6 +2024,13 @@ def ObjCDirectMembers : Attr {
   let Documentation = [ObjCDirectMembersDocs];
 }
 
+def ObjCNonRuntimeProtocol : Attr {
+  let Spellings = [Clang<"objc_non_runtime_protocol">];
+  let Subjects = SubjectList<[ObjCProtocol], ErrorDiag>;
+  let LangOpts = [ObjC];
+  let Documentation = [ObjCNonRuntimeProtocolDocs];
+}
+
 def ObjCRuntimeName : Attr {
   let Spellings = [Clang<"objc_runtime_name">];
   let Subjects = SubjectList<[ObjCInterface, ObjCProtocol], ErrorDiag>;

clang/include/clang/Basic/AttrDocs.td

@@ -4620,6 +4620,22 @@ properties, including auto-synthesized properties.
   }];
 }
 
+def ObjCNonRuntimeProtocolDocs : Documentation {
+  let Category = DocCatDecl;
+  let Content = [{
+The ``objc_non_runtime_protocol`` attribute can be used to mark that an
+Objective-C protocol is only used during static type-checking and doesn't need
+to be represented dynamically. This avoids several small code-size and run-time
+overheads associated with handling the protocol's metadata. A non-runtime
+protocol cannot be used as the operand of a ``@protocol`` expression, and
+dynamic attempts to find it with ``objc_getProtocol`` will fail.
+
+If a non-runtime protocol inherits from any ordinary protocols, classes and
+derived protocols that declare conformance to the non-runtime protocol will
+dynamically list their conformance to those bare protocols.
+  }];
+}
+
 def SelectAnyDocs : Documentation {
   let Category = DocCatDecl;
   let Content = [{

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1034,6 +1034,8 @@ def warn_objc_boxing_invalid_utf8_string : Warning<
   "string is ill-formed as UTF-8 and will become a null %0 when boxed">,
   InGroup<ObjCBoxing>;
 
+def err_objc_non_runtime_protocol_in_protocol_expr : Error<
+  "cannot use a protocol declared 'objc_non_runtime_protocol' in a @protocol expression">;
 def err_objc_direct_on_protocol : Error<
   "'objc_direct' attribute cannot be applied to %select{methods|properties}0 "
   "declared in an Objective-C protocol">;

clang/lib/AST/DeclObjC.cpp

@@ -33,6 +33,7 @@
 #include <cassert>
 #include <cstdint>
 #include <cstring>
+#include <queue>
 #include <utility>
 
 using namespace clang;
@@ -1905,6 +1906,27 @@ ObjCProtocolDecl *ObjCProtocolDecl::CreateDeserialized(ASTContext &C,
   return Result;
 }
 
+bool ObjCProtocolDecl::isNonRuntimeProtocol() const {
+  return hasAttr<ObjCNonRuntimeProtocolAttr>();
+}
+
+void ObjCProtocolDecl::getImpliedProtocols(
+    llvm::DenseSet<const ObjCProtocolDecl *> &IPs) const {
+  std::queue<const ObjCProtocolDecl *> WorkQueue;
+  WorkQueue.push(this);
+
+  while (!WorkQueue.empty()) {
+    const auto *PD = WorkQueue.front();
+    WorkQueue.pop();
+    for (const auto *Parent : PD->protocols()) {
+      const auto *Can = Parent->getCanonicalDecl();
+      auto Result = IPs.insert(Can);
+      if (Result.second)
+        WorkQueue.push(Parent);
+    }
+  }
+}
+
 ObjCProtocolDecl *ObjCProtocolDecl::lookupProtocolNamed(IdentifierInfo *Name) {
   ObjCProtocolDecl *PDecl = this;
 

clang/lib/CodeGen/CGObjC.cpp

@@ -445,6 +445,75 @@ CodeGen::RValue CGObjCRuntime::GeneratePossiblySpecializedMessageSend(
                              Method);
 }
 
+static void AppendFirstImpliedRuntimeProtocols(
+    const ObjCProtocolDecl *PD,
+    llvm::UniqueVector<const ObjCProtocolDecl *> &PDs) {
+  if (!PD->isNonRuntimeProtocol()) {
+    const auto *Can = PD->getCanonicalDecl();
+    PDs.insert(Can);
+    return;
+  }
+
+  for (const auto *ParentPD : PD->protocols())
+    AppendFirstImpliedRuntimeProtocols(ParentPD, PDs);
+}
+
+std::vector<const ObjCProtocolDecl *>
+CGObjCRuntime::GetRuntimeProtocolList(ObjCProtocolDecl::protocol_iterator begin,
+                                      ObjCProtocolDecl::protocol_iterator end) {
+  std::vector<const ObjCProtocolDecl *> RuntimePds;
+  llvm::DenseSet<const ObjCProtocolDecl *> NonRuntimePDs;
+
+  for (; begin != end; ++begin) {
+    const auto *It = *begin;
+    const auto *Can = It->getCanonicalDecl();
+    if (Can->isNonRuntimeProtocol())
+      NonRuntimePDs.insert(Can);
+    else
+      RuntimePds.push_back(Can);
+  }
+
+  // If there are no non-runtime protocols then we can just stop now.
+  if (NonRuntimePDs.empty())
+    return RuntimePds;
+
+  // Else we have to search through the non-runtime protocol's inheritancy
+  // hierarchy DAG stopping whenever a branch either finds a runtime protocol or
+  // a non-runtime protocol without any parents. These are the "first-implied"
+  // protocols from a non-runtime protocol.
+  llvm::UniqueVector<const ObjCProtocolDecl *> FirstImpliedProtos;
+  for (const auto *PD : NonRuntimePDs)
+    AppendFirstImpliedRuntimeProtocols(PD, FirstImpliedProtos);
+
+  // Walk the Runtime list to get all protocols implied via the inclusion of
+  // this protocol, e.g. all protocols it inherits from including itself.
+  llvm::DenseSet<const ObjCProtocolDecl *> AllImpliedProtocols;
+  for (const auto *PD : RuntimePds) {
+    const auto *Can = PD->getCanonicalDecl();
+    AllImpliedProtocols.insert(Can);
+    Can->getImpliedProtocols(AllImpliedProtocols);
+  }
+
+  // Similar to above, walk the list of first-implied protocols to find the set
+  // all the protocols implied excluding the listed protocols themselves since
+  // they are not yet a part of the `RuntimePds` list.
+  for (const auto *PD : FirstImpliedProtos) {
+    PD->getImpliedProtocols(AllImpliedProtocols);
+  }
+
+  // From the first-implied list we have to finish building the final protocol
+  // list. If a protocol in the first-implied list was already implied via some
+  // inheritance path through some other protocols then it would be redundant to
+  // add it here and so we skip over it.
+  for (const auto *PD : FirstImpliedProtos) {
+    if (!AllImpliedProtocols.contains(PD)) {
+      RuntimePds.push_back(PD);
+    }
+  }
+
+  return RuntimePds;
+}
+
 /// Instead of '[[MyClass alloc] init]', try to generate
 /// 'objc_alloc_init(MyClass)'. This provides a code size improvement on the
 /// caller side, as well as the optimized objc_alloc.

clang/lib/CodeGen/CGObjCGNU.cpp

@@ -1187,8 +1187,11 @@ class CGObjCGNUstep2 : public CGObjCGNUstep {
   }
   llvm::Constant *GenerateCategoryProtocolList(const ObjCCategoryDecl *OCD)
     override {
-    SmallVector<llvm::Constant*, 16> Protocols;
-    for (const auto *PI : OCD->getReferencedProtocols())
+    const auto &ReferencedProtocols = OCD->getReferencedProtocols();
+    auto RuntimeProtocols = GetRuntimeProtocolList(ReferencedProtocols.begin(),
+                                                   ReferencedProtocols.end());
+    SmallVector<llvm::Constant *, 16> Protocols;
+    for (const auto *PI : RuntimeProtocols)
       Protocols.push_back(
           llvm::ConstantExpr::getBitCast(GenerateProtocolRef(PI),
             ProtocolPtrTy));
@@ -1371,7 +1374,9 @@ class CGObjCGNUstep2 : public CGObjCGNUstep {
     }
 
     SmallVector<llvm::Constant*, 16> Protocols;
-    for (const auto *PI : PD->protocols())
+    auto RuntimeProtocols =
+        GetRuntimeProtocolList(PD->protocol_begin(), PD->protocol_end());
+    for (const auto *PI : RuntimeProtocols)
       Protocols.push_back(
           llvm::ConstantExpr::getBitCast(GenerateProtocolRef(PI),
             ProtocolPtrTy));
@@ -1910,8 +1915,10 @@ class CGObjCGNUstep2 : public CGObjCGNUstep {
     // struct objc_class *sibling_class
     classFields.addNullPointer(PtrTy);
     // struct objc_protocol_list *protocols;
-    SmallVector<llvm::Constant*, 16> Protocols;
-    for (const auto *I : classDecl->protocols())
+    auto RuntimeProtocols = GetRuntimeProtocolList(classDecl->protocol_begin(),
+                                                   classDecl->protocol_end());
+    SmallVector<llvm::Constant *, 16> Protocols;
+    for (const auto *I : RuntimeProtocols)
       Protocols.push_back(
           llvm::ConstantExpr::getBitCast(GenerateProtocolRef(I),
             ProtocolPtrTy));
@@ -3076,6 +3083,9 @@ CGObjCGNU::GenerateEmptyProtocol(StringRef ProtocolName) {
 }
 
 void CGObjCGNU::GenerateProtocol(const ObjCProtocolDecl *PD) {
+  if (PD->isNonRuntimeProtocol())
+    return;
+
   std::string ProtocolName = PD->getNameAsString();
 
   // Use the protocol definition, if there is one.
@@ -3228,8 +3238,11 @@ llvm::Constant *CGObjCGNU::MakeBitField(ArrayRef<bool> bits) {
 
 llvm::Constant *CGObjCGNU::GenerateCategoryProtocolList(const
     ObjCCategoryDecl *OCD) {
+  const auto &RefPro = OCD->getReferencedProtocols();
+  const auto RuntimeProtos =
+      GetRuntimeProtocolList(RefPro.begin(), RefPro.end());
   SmallVector<std::string, 16> Protocols;
-  for (const auto *PD : OCD->getReferencedProtocols())
+  for (const auto *PD : RuntimeProtos)
     Protocols.push_back(PD->getNameAsString());
   return GenerateProtocolList(Protocols);
 }
@@ -3515,8 +3528,11 @@ void CGObjCGNU::GenerateClass(const ObjCImplementationDecl *OID) {
   llvm::Constant *Properties = GeneratePropertyList(OID, ClassDecl);
 
   // Collect the names of referenced protocols
+  auto RefProtocols = ClassDecl->protocols();
+  auto RuntimeProtocols =
+      GetRuntimeProtocolList(RefProtocols.begin(), RefProtocols.end());
   SmallVector<std::string, 16> Protocols;
-  for (const auto *I : ClassDecl->protocols())
+  for (const auto *I : RuntimeProtocols)
     Protocols.push_back(I->getNameAsString());
 
   // Get the superclass pointer.

clang/lib/CodeGen/CGObjCMac.cpp

@@ -32,6 +32,7 @@
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/UniqueVector.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/InlineAsm.h"
 #include "llvm/IR/IntrinsicInst.h"
@@ -3196,7 +3197,8 @@ CGObjCMac::EmitProtocolList(Twine name,
                             ObjCProtocolDecl::protocol_iterator begin,
                             ObjCProtocolDecl::protocol_iterator end) {
   // Just return null for empty protocol lists
-  if (begin == end)
+  auto PDs = GetRuntimeProtocolList(begin, end);
+  if (PDs.empty())
     return llvm::Constant::getNullValue(ObjCTypes.ProtocolListPtrTy);
 
   ConstantInitBuilder builder(CGM);
@@ -3209,9 +3211,9 @@ CGObjCMac::EmitProtocolList(Twine name,
   auto countSlot = values.addPlaceholder();
 
   auto refsArray = values.beginArray(ObjCTypes.ProtocolPtrTy);
-  for (; begin != end; ++begin) {
-    refsArray.add(GetProtocolRef(*begin));
-  }
+  for (const auto *Proto : PDs)
+    refsArray.add(GetProtocolRef(Proto));
+
   auto count = refsArray.size();
 
   // This list is null terminated.
@@ -6648,7 +6650,8 @@ llvm::Value *CGObjCNonFragileABIMac::GenerateProtocolRef(CodeGenFunction &CGF,
 
   // This routine is called for @protocol only. So, we must build definition
   // of protocol's meta-data (not a reference to it!)
-  //
+  assert(!PD->isNonRuntimeProtocol() &&
+         "attempting to get a protocol ref to a static protocol.");
   llvm::Constant *Init =
     llvm::ConstantExpr::getBitCast(GetOrEmitProtocol(PD),
                                    ObjCTypes.getExternalProtocolPtrTy());
@@ -7005,6 +7008,8 @@ llvm::Constant *CGObjCNonFragileABIMac::GetOrEmitProtocolRef(
   const ObjCProtocolDecl *PD) {
   llvm::GlobalVariable *&Entry = Protocols[PD->getIdentifier()];
 
+  assert(!PD->isNonRuntimeProtocol() &&
+         "attempting to GetOrEmit a non-runtime protocol");
   if (!Entry) {
     // We use the initializer as a marker of whether this is a forward
     // reference or not. At module finalization we add the empty
@@ -7148,10 +7153,20 @@ llvm::Constant *
 CGObjCNonFragileABIMac::EmitProtocolList(Twine Name,
                                       ObjCProtocolDecl::protocol_iterator begin,
                                       ObjCProtocolDecl::protocol_iterator end) {
+  // Just return null for empty protocol lists
+  auto Protocols = GetRuntimeProtocolList(begin, end);
+  if (Protocols.empty())
+    return llvm::Constant::getNullValue(ObjCTypes.ProtocolListnfABIPtrTy);
+
   SmallVector<llvm::Constant *, 16> ProtocolRefs;
+  ProtocolRefs.reserve(Protocols.size());
 
-  // Just return null for empty protocol lists
-  if (begin == end)
+  for (const auto *PD : Protocols)
+    ProtocolRefs.push_back(GetProtocolRef(PD));
+
+  // If all of the protocols in the protocol list are objc_non_runtime_protocol
+  // just return null
+  if (ProtocolRefs.size() == 0)
     return llvm::Constant::getNullValue(ObjCTypes.ProtocolListnfABIPtrTy);
 
   // FIXME: We shouldn't need to do this lookup here, should we?
@@ -7168,8 +7183,8 @@ CGObjCNonFragileABIMac::EmitProtocolList(Twine Name,
 
   // A null-terminated array of protocols.
   auto array = values.beginArray(ObjCTypes.ProtocolnfABIPtrTy);
-  for (; begin != end; ++begin)
-    array.add(GetProtocolRef(*begin));  // Implemented???
+  for (auto const &proto : ProtocolRefs)
+    array.add(proto);
   auto count = array.size();
   array.addNullPointer(ObjCTypes.ProtocolnfABIPtrTy);
 

clang/lib/CodeGen/CGObjCRuntime.h

@@ -20,6 +20,7 @@
 #include "CGValue.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/Basic/IdentifierTable.h" // Selector
+#include "llvm/ADT/UniqueVector.h"
 
 namespace llvm {
   class Constant;
@@ -205,6 +206,16 @@ class CGObjCRuntime {
                            const CallArgList &CallArgs,
                            const ObjCMethodDecl *Method = nullptr) = 0;
 
+  /// Walk the list of protocol references from a class, category or
+  /// protocol to traverse the DAG formed from it's inheritance hierarchy. Find
+  /// the list of protocols that ends each walk at either a runtime
+  /// protocol or a non-runtime protocol with no parents. For the common case of
+  /// just a list of standard runtime protocols this just returns the same list
+  /// that was passed in.
+  std::vector<const ObjCProtocolDecl *>
+  GetRuntimeProtocolList(ObjCProtocolDecl::protocol_iterator begin,
+                         ObjCProtocolDecl::protocol_iterator end);
+
   /// Emit the code to return the named protocol as an object, as in a
   /// \@protocol expression.
   virtual llvm::Value *GenerateProtocolRef(CodeGenFunction &CGF,

clang/lib/Sema/SemaDeclAttr.cpp

@@ -2617,6 +2617,11 @@ static void handleVisibilityAttr(Sema &S, Decl *D, const ParsedAttr &AL,
     D->addAttr(newAttr);
 }
 
+static void handleObjCNonRuntimeProtocolAttr(Sema &S, Decl *D,
+                                             const ParsedAttr &AL) {
+  handleSimpleAttribute<ObjCNonRuntimeProtocolAttr>(S, D, AL);
+}
+
 static void handleObjCDirectAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   // objc_direct cannot be set on methods declared in the context of a protocol
   if (isa<ObjCProtocolDecl>(D->getDeclContext())) {
@@ -7665,6 +7670,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case ParsedAttr::AT_ObjCDirect:
     handleObjCDirectAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_ObjCNonRuntimeProtocol:
+    handleObjCNonRuntimeProtocolAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_ObjCDirectMembers:
     handleObjCDirectMembersAttr(S, D, AL);
     handleSimpleAttribute<ObjCDirectMembersAttr>(S, D, AL);

clang/lib/Sema/SemaExprObjC.cpp

@@ -1394,6 +1394,9 @@ ExprResult Sema::ParseObjCProtocolExpression(IdentifierInfo *ProtocolId,
     Diag(ProtoLoc, diag::err_undeclared_protocol) << ProtocolId;
     return true;
   }
+  if (PDecl->isNonRuntimeProtocol())
+    Diag(ProtoLoc, diag::err_objc_non_runtime_protocol_in_protocol_expr)
+        << PDecl;
   if (!PDecl->hasDefinition()) {
     Diag(ProtoLoc, diag::err_atprotocol_protocol) << PDecl;
     Diag(PDecl->getLocation(), diag::note_entity_declared_at) << PDecl;