Add ObjC category merging to ObjCPass

[pete]

Summary

This ports the start of the ObjC category merging pass from ld64 to lld.

This is used in code such as the following:

@interface A @end
@implementation A
+(void) foo {
}
@end

@interface A (Cat) @end
@implementation A (Cat)
+(void) bar {
}
@end

In this example, we would have a category list with the element 'A (Cat)'. This pass takes the methods from the category and adds them to the method list on the class 'A'. That is, class 'A' had a method list { foo } but after this pass will have { foo, bar }.

This is only the first of several optimizations in this area. This handles instance methods, but future commits will handle class methods, protocols, and properties. The code here is meant to be extensible in that i'll later need to add support for all the different offsets and atom types for those optimizations too.

lib/ReaderWriter/MachO/ObjCPass.cpp

 
 #include "ArchHandler.h"
 #include "File.h"
+#include "MachONormalizedFileBinaryUtils.h"
 #include "MachOPasses.h"
 #include "lld/Core/DefinedAtom.h"
 #include "lld/Core/File.h"

   } Data;
 };
 
+///
+/// ObjC Method List Atom created by the ObjC pass.
+///
+/// struct _method_list_t {
+///   uint32_t SizeOfMethod = sizeof(method_t);
+///   uint32_t NumMethods;
+///   method_t Methods[NumMethods];
+/// }
+///
+class ObjCMethodListAtom : public MachODefinedCustomSectionAtom {
+public:
+  ObjCMethodListAtom(const File &file,
+                     StringRef classAtomName,
+                     unsigned methodCount,
+                     const MachOLinkingContext &ctx,
+                     llvm::BumpPtrAllocator &allocator)
+    : MachODefinedCustomSectionAtom(file, getAtomName(classAtomName, allocator),
+                                    Scope::scopeGlobal,
+                                    DefinedAtom::typeObjCMethodList,
+                                    Merge::mergeNo,
+                                    false /* thumb */, true /* noDeadStrip */,
+                                    getBuffer(methodCount, ctx, allocator),
+                                    "__DATA/__objc_const", 8) {
+
+    bool isBig = MachOLinkingContext::isBigEndian(ctx.arch());
+    uint8_t *BufferData = const_cast<uint8_t *>(rawContent().data());
+    normalized::write32(BufferData, getMethodSize(ctx), isBig);
+    normalized::write32(BufferData + 4, methodCount, isBig);
+    bzero(BufferData + 8, size() - 8);
+  }
+
+  ContentPermissions permissions() const override {
+    return DefinedAtom::permR__;
+  }
+
+private:
+  /// Constructs a buffer for a method list with the given number of methods.
+  static ArrayRef<uint8_t> getBuffer(unsigned methodCount,
+                                     const MachOLinkingContext &ctx,
+                                     llvm::BumpPtrAllocator &allocator) {
+    unsigned BufferSize = 8 + (getMethodSize(ctx) * methodCount);
+    uint8_t *BufferData = (uint8_t*)allocator.Allocate(BufferSize, 8);
+    return llvm::makeArrayRef(BufferData, BufferSize);
+  }
+
+  static StringRef getAtomName(StringRef classAtomName,
+                               llvm::BumpPtrAllocator &allocator) {
+    SmallString<128> name;
+    name += "l_";
+    name += classAtomName;
+    name += "_objc_merged_method_list";
+    return name.str().copy(allocator);
+  }
+
+  static unsigned getMethodSize(const MachOLinkingContext &ctx) {
+    // A single method is exactly 3 pointers in size
+    return 3 * (ctx.is64Bit() ? 8 : 4);
+  }
+};
+
 class ObjCPass : public Pass {
 public:
   ObjCPass(const MachOLinkingContext &context)

     // Add the image info.
     mergedFile.addAtom(*getImageInfo());
 
+    if (_ctx.mergeObjCCategories())
+      if (auto EC = optimizeCategories(mergedFile))
+        return EC;
+
     return std::error_code();
   }
 

                                                      _ctx.swiftVersion());
   }
 
+  std::error_code optimizeCategories(SimpleFile &mergedFile);
+
   const MachOLinkingContext   &_ctx;
   MachOFile                   _file;
 };
 
+/// Provides accessors to an in memory category atom.
+///
+/// struct category_t {
+///   ptr_t name;
+///   ptr_t class;
+///   ptr_t instance methods;
+///   ptr_t class methods;
+///   ptr_t protocols;
+///   ptr_t instance properties;
+/// }
+class Category {
+public:
+  Category(const DefinedAtom *atom, MachOLinkingContext::Arch arch)
+    : AtomAndIs64Bit(atom, MachOLinkingContext::is64Bit(arch)) {
+  }
+
+  /// Verifies that the category is correct.  Otherwise returns an error.
+  std::error_code verify() const {
+    // Categories are 6 pointers.  Make sure the size reflects that.
+    const DefinedAtom *atom = AtomAndIs64Bit.getPointer();
+    if (atom->size() != (6 * (is64Bit() ? 8 : 4)))
+      return make_dynamic_error_code("ObjC category size must be 6 pointers");
+    return std::error_code();
+  }
+
+  ErrorOr<const DefinedAtom *> getClass() const {
+    auto ErrorOrAtom = getReferenceAtOffset(1 * (is64Bit() ? 8 : 4));
+    if (!ErrorOrAtom)
+      return ErrorOrAtom.getError();
+    if (auto *atom = ErrorOrAtom.get()) {
+      if (auto *definedAtom = dyn_cast<DefinedAtom>(atom))
+        return definedAtom;
+      return make_dynamic_error_code("ObjC atom must be defined");
+    }
+    return nullptr;
+  }
+
+  ErrorOr<const DefinedAtom *> getInstanceMethods() const {
+    auto ErrorOrAtom = getReferenceAtOffset(2 * (is64Bit() ? 8 : 4));
+    if (!ErrorOrAtom)
+      return ErrorOrAtom.getError();
+    if (auto *atom = ErrorOrAtom.get()) {
+      if (auto *definedAtom = dyn_cast<DefinedAtom>(atom))
+        return definedAtom;
+      return make_dynamic_error_code("ObjC atom must be defined");
+    }
+    return nullptr;
+  }
+
+  bool hasInstanceMethods() const {
+    return hasReferenceAtOffset(2 * (is64Bit() ? 8 : 4));
+  }
+
+private:
+
+  ErrorOr<const Atom *> getReferenceAtOffset(unsigned Offset) const {
+    const DefinedAtom *atom = AtomAndIs64Bit.getPointer();
+    for (const Reference *ref : *atom) {
+      if (ref->offsetInAtom() != Offset)
+        continue;
+      if (ref->addend())
+        return make_dynamic_error_code("ObjC category field"
+                                       " addend is not supported");
+      return ref->target();
+    }
+    return nullptr;
+  }
+
+  bool hasReferenceAtOffset(unsigned Offset) const {
+    const DefinedAtom *atom = AtomAndIs64Bit.getPointer();
+    for (const Reference *ref : *atom)
+      if (ref->offsetInAtom() == Offset)
+        return true;
+    return false;
+  }
+
+  bool is64Bit() const {
+    return AtomAndIs64Bit.getInt();
+  }
+
+  llvm::PointerIntPair<const DefinedAtom *, 1> AtomAndIs64Bit;
+};
+
+/// Provides accessors to an in memory class atom.
+///
+/// struct _class_t {
+///   struct _class_t *isa;
+///   struct _class_t * const superclass;
+///   void *cache;
+///   IMP *vtable;
+///   struct class_ro_t *ro;
+/// }
+class Class {
+public:
+  Class(const DefinedAtom *atom, MachOLinkingContext::Arch arch)
+    : AtomAndIs64Bit(atom, MachOLinkingContext::is64Bit(arch)) {
+  }
 
+  /// Verifies that the class is correct.  Otherwise returns an error.
+  std::error_code verify() const {
+    // classes are 5 pointers.  Make sure the size reflects that.
+    const DefinedAtom *atom = AtomAndIs64Bit.getPointer();
+    if (atom->size() != (5 * (is64Bit() ? 8 : 4)))
+      return make_dynamic_error_code("ObjC class size must be 6 pointers");
+    return std::error_code();
+  }
+
+  ErrorOr<const DefinedAtom *> getData() const {
+    auto ErrorOrAtom = getReferenceAtOffset(4 * (is64Bit() ? 8 : 4));
+    if (!ErrorOrAtom)
+      return ErrorOrAtom.getError();
+    if (auto *atom = ErrorOrAtom.get()) {
+      if (auto *definedAtom = dyn_cast<DefinedAtom>(atom))
+        return definedAtom;
+      return make_dynamic_error_code("ObjC atom must be defined");
+    }
+    return nullptr;
+  }
+
+  ErrorOr<const Atom *> getReferenceAtOffset(unsigned Offset) const {
+    const DefinedAtom *atom = AtomAndIs64Bit.getPointer();
+    for (const Reference *ref : *atom) {
+      if (ref->offsetInAtom() != Offset)
+        continue;
+      if (ref->addend())
+        return make_dynamic_error_code("ObjC class field"
+                                       " addend is not supported");
+      return ref->target();
+    }
+    return make_dynamic_error_code("Could not find Objc class field");
+  }
+
+private:
+
+  bool is64Bit() const {
+    return AtomAndIs64Bit.getInt();
+  }
+
+  llvm::PointerIntPair<const DefinedAtom *, 1> AtomAndIs64Bit;
+};
+
+/// Provides accessors to an in memory class RO data atom.
+///
+/// struct _class_ro_t {
+///   uint32_t const flags;
+///   uint32_t const instanceStart;
+///   uint32_t const instanceSize;
+///   uint32_t const reserved;  // only when building for 64bit targets
+///   const uint8_t * const ivarLayout;
+///   const char *const name;
+///   const struct _method_list_t * const baseMethods;
+///   const struct _protocol_list_t *const baseProtocols;
+///   const struct _ivar_list_t *const ivars;
+///   const uint8_t * const weakIvarLayout;
+///   const struct _prop_list_t * const properties;
+/// }
+///
+class ClassROData {
+public:
+  ClassROData(const DefinedAtom *atom, MachOLinkingContext::Arch arch)
+    : AtomAndIs64Bit(atom, MachOLinkingContext::is64Bit(arch)) {
+  }
+
+  /// Verifies that the RO data is correct.  Otherwise returns an error.
+  std::error_code verify() const {
+    // There are 7 pointers of data after the header.  Verify that this is the
+    // case.
+    const DefinedAtom *atom = AtomAndIs64Bit.getPointer();
+    if (atom->size() != (getHeaderSizeInBytes() + (7 * (is64Bit() ? 8 : 4))))
+      return make_dynamic_error_code("ObjC RO data size in incorrect");
+    return std::error_code();
+  }
+
+  unsigned offsetOfBaseMethods() const {
+    return getHeaderSizeInBytes() + 2 * (is64Bit() ? 8 : 4);
+  }
+
+  ErrorOr<const DefinedAtom *> getBaseMethods() const {
+    auto ErrorOrAtom = getReferenceAtOffset(offsetOfBaseMethods());
+    if (!ErrorOrAtom)
+      return ErrorOrAtom.getError();
+    if (auto *atom = ErrorOrAtom.get()) {
+      if (auto *definedAtom = dyn_cast<DefinedAtom>(atom))
+        return definedAtom;
+      return make_dynamic_error_code("ObjC atom must be defined");
+    }
+    return nullptr;
+  }
+
+  /// Looks up a reference at 'Offset' bytes after the header.
+  ErrorOr<const Atom *> getReferenceAtOffset(unsigned Offset) const {
+    const DefinedAtom *atom = AtomAndIs64Bit.getPointer();
+    for (const Reference *ref : *atom) {
+      if (ref->offsetInAtom() != Offset)
+        continue;
+      if (ref->addend())
+        return make_dynamic_error_code("ObjC RO data field"
+                                       " addend is not supported");
+      return ref->target();
+    }
+    return nullptr;
+  }
+
+private:
+
+  bool is64Bit() const {
+    return AtomAndIs64Bit.getInt();
+  }
+
+  /// On 32-bit targets, the header is 3 uint32_t's, while on 64-bit targets
+  /// we have 4 uint32_t's.  Return the size in bytes of the header here.
+  unsigned getHeaderSizeInBytes() const {
+    return is64Bit() ? 16 : 12;
+  }
+
+  llvm::PointerIntPair<const DefinedAtom *, 1> AtomAndIs64Bit;
+};
+
+/// Provides accessors to an in memory class method list atom.
+///
+/// struct _method_list_t {
+///   uint32_t SizeOfMethod = sizeof(method_t);
+///   uint32_t NumMethods;
+///   method_t Methods[NumMethods];
+/// }
+///
+class MethodList {
+public:
+  MethodList(const DefinedAtom *atom, MachOLinkingContext::Arch arch)
+    : atom(atom), is64Bit(MachOLinkingContext::is64Bit(arch)),
+      isBig(MachOLinkingContext::isBigEndian(arch)) {
+  }
+
+  /// Verifies that the method list is correct.  Otherwise returns an error.
+  std::error_code initAndverify() {
+    if (atom->size() < 8)
+      return make_dynamic_error_code("Method list requries size and count");
+
+    const uint8_t *atomData = atom->rawContent().data();
+    methodSize = normalized::read32(atomData, isBig);
+    methodCount = normalized::read32(atomData + 4, isBig);
+
+    if (methodSize != (3 * (is64Bit ? 8 : 4)))
+      return make_dynamic_error_code("ObjC method is the wrong size");
+
+    if (atom->size() != (8 + (methodSize * methodCount)))
+      return make_dynamic_error_code("ObjC method list size in incorrect");
+    return std::error_code();
+  }
+
+  ErrorOr<const Atom *> getReferenceAtOffset(unsigned Offset) const {
+    for (const Reference *ref : *atom) {
+      if (ref->offsetInAtom() != Offset)
+        continue;
+      if (ref->addend())
+        return make_dynamic_error_code("ObjC method list field"
+                                       " addend is not supported");
+      return ref->target();
+    }
+    return nullptr;
+  }
+
+  unsigned getMethodCount() const {
+    return methodCount;
+  }
+
+  unsigned getMethodSize() const {
+    return methodSize;
+  }
+
+private:
+
+  /// On 32-bit targets, the header is 3 uint32_t's, while on 64-bit targets
+  /// we have 4 uint32_t's.  Return the size in bytes of the header here.
+  unsigned getHeaderSizeInBytes() const {
+    return is64Bit ? 16 : 12;
+  }
+
+  const DefinedAtom *atom;
+  unsigned methodSize;
+  unsigned methodCount;
+  bool is64Bit;
+  bool isBig;
+};
+
+/// Returns a new method list atom.  This merges all of the methods in the
+/// given class method list, with all of the method lists from the categories.
+static ErrorOr<const DefinedAtom*>
+getMethodListAtom(StringRef classAtomName,
+                  const DefinedAtom *baseMethodsAtom,
+                  ArrayRef<Category> categories,
+                  const MachOLinkingContext &context,
+                  llvm::BumpPtrAllocator &allocator) {
+  MachOLinkingContext::Arch arch = context.arch();
+
+  unsigned methodCount = 0;
+  const File *file = nullptr;
+
+  // First work out how many methods we have and which file the new atom
+  // should be in.
+  auto processMethodList =
+    [&](const DefinedAtom *methodListAtom)->std::error_code {
+    if (!file)
+      file = &methodListAtom->file();
+    MethodList methodList(methodListAtom, arch);
+    auto EC = methodList.initAndverify();
+    if (EC)
+      return EC;
+    methodCount += methodList.getMethodCount();
+    // TODO: We should delete the method list atom once we are done with it.
+    return std::error_code();
+  };
+
+  if (baseMethodsAtom)
+    if (auto EC = processMethodList(baseMethodsAtom))
+      return EC;
+
+  for (const Category &category : categories) {
+    auto ErrorOrMethods = category.getInstanceMethods();
+    if (ErrorOrMethods.getError())
+      return ErrorOrMethods;
+    auto *methodListAtom = ErrorOrMethods.get();
+    if (!methodListAtom)
+      continue;
+    if (auto EC = processMethodList(methodListAtom))
+      return EC;
+  }
+
+  // Now create an atom to hold our method list.
+  auto *newMethodListAtom = new (allocator) ObjCMethodListAtom(*file,
+                                                               classAtomName,
+                                                               methodCount,
+                                                               context,
+                                                               allocator);
+
+  // Finally, add fixups for all of the methods in the lists.
+  unsigned slide = 0;
+  auto addFixups = [&](const DefinedAtom *methodListAtom) {
+    MethodList methodList(methodListAtom, arch);
+    auto EC = methodList.initAndverify();
+    assert(!EC && "Method list error should have been caught earlier");
+    for (const Reference *ref : *methodListAtom)
+      newMethodListAtom->addReference(ref->kindNamespace(),
+                                      ref->kindArch(),
+                                      ref->kindValue(),
+                                      ref->offsetInAtom() + slide,
+                                      ref->target(),
+                                      ref->addend());
+
+    slide += methodList.getMethodSize() * methodList.getMethodCount();
+  };
+
+  // We add these in reverse, to match the objc runtime.
+  for (const Category &category : llvm::reverse(categories)) {
+    auto *methodListAtom = category.getInstanceMethods().get();
+    if (methodListAtom)
+      addFixups(methodListAtom);
+  }
+
+  // And the base class method list comes last, if at all.
+  if (baseMethodsAtom)
+    addFixups(baseMethodsAtom);
+
+  return newMethodListAtom;
+}
+
+static void addOrReplaceFixup(const DefinedAtom *atom,
+                              uint64_t OffsetInAtom,
+                              const DefinedAtom *newTarget,
+                              mach_o::ArchHandler &handler) {
+  for (const Reference *ref : *atom) {
+    if (ref->offsetInAtom() != OffsetInAtom)
+      continue;
+    auto *mutableRef = const_cast<Reference*>(ref);
+    mutableRef->setTarget(newTarget);
+    mutableRef->setKindValue(handler.pointerKind());
+    return;
+  }
+  // Couldn't find a reference, so add a new one.
+  auto *mutableAtom = const_cast<DefinedAtom *>(atom);
+  mutableAtom->addReference(Reference::KindNamespace::mach_o,
+                            handler.kindArch(), handler.pointerKind(),
+                            OffsetInAtom, newTarget, 0);
+}
+
+std::error_code ObjCPass::optimizeCategories(SimpleFile &mergedFile) {
+  // Map from class atoms to the category atoms which point to them.
+  llvm::DenseMap<const Atom *, std::vector<Category>> classToCategories;
+
+  // Keep track of the order in which we add classes to the map
+  std::vector<const DefinedAtom *> classOrder;
+
+  // Keep track of the atoms we can now delete
+  std::set<const Atom *> deadAtoms;
+
+  // Find all of the classes referenced in the category list, and map from
+  // classes to all of the categories which reference them
+  for (const DefinedAtom *atom : mergedFile.defined()) {
+    if (atom->contentType() != DefinedAtom::typeObjC2CategoryList)
+      continue;
+    const DefinedAtom *categoryListAtom = cast<DefinedAtom>(atom);
+
+    // Category lists are atomized.  They should only contain a single reference
+    // to a category.
+    assert((++categoryListAtom->begin()) == categoryListAtom->end() &&
+           "Category lists should be atomized to a single reference");
+    const Reference *ref = *categoryListAtom->begin();
+    const DefinedAtom* categoryAtom = dyn_cast<DefinedAtom>(ref->target());
+
+    // TODO: We need to ignore categories in the __objc_nlcatlist section.
+
+    if (!categoryAtom)
+      return make_dynamic_error_code("ObjC category must be defined");
+
+    Category category(categoryAtom, _ctx.arch());
+    if (auto EC = category.verify())
+      return EC;
+
+    // Get the class this category points to.
+    const DefinedAtom *classAtom = nullptr;
+    if (auto ErrorOrClassAtom = category.getClass())
+      classAtom = ErrorOrClassAtom.get();
+    else
+      return ErrorOrClassAtom.getError();
+
+    // TODO: Check for aliasing classes.
+
+    // Keep track of this class, as well as the reference from the category
+    // back to the class.
+    auto PosAndInserted = classToCategories.insert({ classAtom, { }});
+    if (PosAndInserted.second)
+      classOrder.push_back(classAtom);
+    PosAndInserted.first->second.push_back(category);
+
+    // Delete this category list atom as we've now processed this element.
+    deadAtoms.insert(categoryListAtom);
+
+    // TODO: We can also delete the category atom once we've verified that its
+    // not in the nlcatlist.
+  }
+
+  if (classOrder.empty())
+    return std::error_code();
+
+  std::unique_ptr<mach_o::ArchHandler> handler
+    = ArchHandler::create(_ctx.arch());
+
+  // Walk the classes and merge in any new methods/protocols/etc from the
+  // categories.
+  for (auto *classAtom : classOrder) {
+    ArrayRef<Category> categories = classToCategories[classAtom];
+    assert(!categories.empty() && "Must have categories");
+
+    Class cls(classAtom, _ctx.arch());
+    if (auto EC = cls.verify())
+      return EC;
+
+    // Get the data atom from the class.  This data contains pointers to the
+    // method list and other lists we need to merge.
+    auto ErrorOrDataAtom = cls.getData();
+    if (!ErrorOrDataAtom)
+      return ErrorOrDataAtom.getError();
+
+    const DefinedAtom *dataAtom = ErrorOrDataAtom.get();
+
+    ClassROData data(dataAtom, _ctx.arch());
+    if (auto EC = data.verify())
+      return EC;
+
+    // Merge in instance methods from the categories.
+    const DefinedAtom *newInstanceMethods = nullptr;
+    if (llvm::any_of(categories,
+                     [](Category cat) { return cat.hasInstanceMethods(); })) {
+      auto ErrorOrBaseMethodsAtom = data.getBaseMethods();
+      if (!ErrorOrBaseMethodsAtom)
+        return ErrorOrBaseMethodsAtom.getError();
+
+      const DefinedAtom *baseMethodsAtom = ErrorOrBaseMethodsAtom.get();
+      auto ErrorOrAtom = getMethodListAtom(classAtom->name(),
+                                           baseMethodsAtom, categories,
+                                           _ctx, _file.allocator());
+      if (!ErrorOrAtom)
+        return ErrorOrAtom.getError();
+      newInstanceMethods = ErrorOrAtom.get();
+
+      mergedFile.addAtom(*newInstanceMethods);
+    }
+
+    // Patch the class RO data with the new lists.
+    if (newInstanceMethods)
+      addOrReplaceFixup(dataAtom, data.offsetOfBaseMethods(),
+                        newInstanceMethods, *handler);
+  }
+
+  // Delete any atoms we don't need any more.
+  mergedFile.removeDefinedAtomsIf([&deadAtoms](const DefinedAtom *atom) {
+    return deadAtoms.count(atom);
+  });
+
+  return std::error_code();
+}
 
 void addObjCPass(PassManager &pm, const MachOLinkingContext &ctx) {
   pm.add(llvm::make_unique<ObjCPass>(ctx));

test/mach-o/objc-method-list-merge.yaml

+# RUN: lld -flavor darwin -arch x86_64 -r -print_atoms %s -o %t1 -no_objc_category_merging | FileCheck %s --check-prefix=NO_OPT
+# RUN: lld -flavor darwin -arch x86_64 -r -print_atoms %t1 -o %t2 -no_objc_category_merging | FileCheck %s --check-prefix=NO_OPT
+# RUN: lld -flavor darwin -arch x86_64 -r -print_atoms %s -o %t3 | FileCheck %s --check-prefix=OPT
+# RUN: lld -flavor darwin -arch x86_64 -r -print_atoms %t3 -o %t4 | FileCheck %s --check-prefix=OPT
+
+
+--- !mach-o
+arch:            x86_64
+file-type:       MH_OBJECT
+flags:           [ MH_SUBSECTIONS_VIA_SYMBOLS ]
+compat-version:  0.0
+current-version: 0.0
+has-UUID:        false
+OS:              unknown
+sections:        
+  - segment:         __TEXT
+    section:         __text
+    type:            S_REGULAR
+    attributes:      [ S_ATTR_PURE_INSTRUCTIONS, S_ATTR_SOME_INSTRUCTIONS ]
+    alignment:       16
+    address:         0x0000000000000000
+    content:         [ 0x55, 0x48, 0x89, 0xE5, 0x48, 0x89, 0x7D, 0xF8, 
+                       0x48, 0x89, 0x75, 0xF0, 0x5D, 0xC3, 0x66, 0x90, 
+                       0x55, 0x48, 0x89, 0xE5, 0x48, 0x89, 0x7D, 0xF8, 
+                       0x48, 0x89, 0x75, 0xF0, 0x5D, 0xC3, 0x66, 0x90, 
+                       0x55, 0x48, 0x89, 0xE5, 0x48, 0x89, 0x7D, 0xF8, 
+                       0x48, 0x89, 0x75, 0xF0, 0x5D, 0xC3, 0x66, 0x90, 
+                       0x55, 0x48, 0x89, 0xE5, 0x48, 0x89, 0x7D, 0xF8, 
+                       0x48, 0x89, 0x75, 0xF0, 0x5D, 0xC3 ]
+  - segment:         __DATA
+    section:         __objc_data
+    type:            S_REGULAR
+    attributes:      [  ]
+    alignment:       8
+    address:         0x0000000000000040
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+    address:         0x00000000000003F8
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+                       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 ]
+    relocations:     
+      - offset:          0x00000008
+        type:            X86_64_RELOC_UNSIGNED
+        length:          3
+        pc-rel:          false
+        extern:          true
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+        type:            X86_64_RELOC_UNSIGNED
+        length:          3
+        pc-rel:          false
+        extern:          true
+        symbol:          9
+  - segment:         __DATA
+    section:         __objc_nlcatlist
+    type:            S_REGULAR
+    attributes:      [ S_ATTR_NO_DEAD_STRIP ]
+    alignment:       8
+    address:         0x0000000000000408
+    content:         [ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 ]
+    relocations:     
+      - offset:          0x00000000
+        type:            X86_64_RELOC_UNSIGNED
+        length:          3
+        pc-rel:          false
+        extern:          true
+        symbol:          9
+  - segment:         __DATA
+    section:         __objc_imageinfo
+    type:            S_REGULAR
+    attributes:      [ S_ATTR_NO_DEAD_STRIP ]
+    address:         0x0000000000000410
+    content:         [ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 ]
+  - segment:         __LD
+    section:         __compact_unwind
+    type:            S_REGULAR
+    attributes:      [  ]
+    alignment:       8
+    address:         0x0000000000000418
+    content:         [ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
+                       0x0E, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 
+                       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
+                       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
+                       0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
+                       0x0E, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 
+                       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
+                       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
+                       0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
+                       0x0E, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 
+                       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
+                       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
+                       0x30, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
+                       0x0E, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 
+                       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
+                       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 ]
+    relocations:     
+      - offset:          0x00000060
+        type:            X86_64_RELOC_UNSIGNED
+        length:          3
+        pc-rel:          false
+        extern:          false
+        symbol:          1
+      - offset:          0x00000040
+        type:            X86_64_RELOC_UNSIGNED
+        length:          3
+        pc-rel:          false
+        extern:          false
+        symbol:          1
+      - offset:          0x00000020
+        type:            X86_64_RELOC_UNSIGNED
+        length:          3
+        pc-rel:          false
+        extern:          false
+        symbol:          1
+      - offset:          0x00000000
+        type:            X86_64_RELOC_UNSIGNED
+        length:          3
+        pc-rel:          false
+        extern:          false
+        symbol:          1
+local-symbols:   
+  - name:            '+[A load]'
+    type:            N_SECT
+    sect:            1
+    value:           0x0000000000000000
+  - name:            '+[A(Cat) load]'
+    type:            N_SECT
+    sect:            1
+    value:           0x0000000000000010
+  - name:            '-[B load]'
+    type:            N_SECT
+    sect:            1
+    value:           0x0000000000000020
+  - name:            '-[B(Cat) load]'
+    type:            N_SECT
+    sect:            1
+    value:           0x0000000000000030
+  - name:            'l_OBJC_CLASS_RO_$_A'
+    type:            N_SECT
+    sect:            6
+    value:           0x00000000000001B0
+  - name:            'l_OBJC_METACLASS_RO_$_A'
+    type:            N_SECT
+    sect:            6
+    value:           0x0000000000000168
+  - name:            'l_OBJC_$_CLASS_METHODS_A'
+    type:            N_SECT
+    sect:            6
+    value:           0x0000000000000148
+  - name:            'l_OBJC_$_CATEGORY_CLASS_METHODS_A_$_Cat'
+    type:            N_SECT
+    sect:            6
+    value:           0x00000000000001F8
+  - name:            _CLASS_A_ALIAS
+    type:            N_SECT
+    sect:            2
+    desc:            [ N_NO_DEAD_STRIP ]
+    value:           0x0000000000000040
+  - name:            'l_OBJC_$_CATEGORY_A_$_Cat'
+    type:            N_SECT
+    sect:            6
+    value:           0x0000000000000218
+  - name:            'l_OBJC_CLASS_RO_$_B'
+    type:            N_SECT
+    sect:            6
+    value:           0x00000000000002B0
+  - name:            'l_OBJC_METACLASS_RO_$_B'
+    type:            N_SECT
+    sect:            6
+    value:           0x0000000000000248
+  - name:            'l_OBJC_$_INSTANCE_METHODS_B'
+    type:            N_SECT
+    sect:            6
+    value:           0x0000000000000290
+  - name:            'l_OBJC_$_CATEGORY_INSTANCE_METHODS_B_$_Cat'
+    type:            N_SECT
+    sect:            6
+    value:           0x00000000000002F8
+  - name:            'l_OBJC_$_CATEGORY_B_$_Cat'
+    type:            N_SECT
+    sect:            6
+    value:           0x0000000000000318
+  - name:            'l_OBJC_METACLASS_RO_$_C'
+    type:            N_SECT
+    sect:            6
+    value:           0x0000000000000348
+  - name:            'l_OBJC_CLASS_RO_$_C'
+    type:            N_SECT
+    sect:            6
+    value:           0x0000000000000390
+global-symbols:  
+  - name:            '_OBJC_CLASS_$_A'
+    type:            N_SECT
+    scope:           [ N_EXT ]
+    sect:            2
+    value:           0x0000000000000040
+  - name:            '_OBJC_CLASS_$_B'
+    type:            N_SECT
+    scope:           [ N_EXT ]
+    sect:            2
+    value:           0x0000000000000090
+  - name:            '_OBJC_CLASS_$_C'
+    type:            N_SECT
+    scope:           [ N_EXT ]
+    sect:            2
+    value:           0x0000000000000108
+  - name:            '_OBJC_METACLASS_$_A'
+    type:            N_SECT
+    scope:           [ N_EXT ]
+    sect:            2
+    value:           0x0000000000000068
+  - name:            '_OBJC_METACLASS_$_B'
+    type:            N_SECT
+    scope:           [ N_EXT ]
+    sect:            2
+    value:           0x00000000000000B8
+  - name:            '_OBJC_METACLASS_$_C'
+    type:            N_SECT
+    scope:           [ N_EXT ]
+    sect:            2
+    value:           0x00000000000000E0
+undefined-symbols: 
+  - name:            __objc_empty_cache
+    type:            N_UNDF
+    scope:           [ N_EXT ]
+    value:           0x0000000000000000
+page-size:       0x00000000
+...
+
+# NO_OPT:   - type:            objc-category-list
+# NO_OPT:     content:         [ 00, 00, 00, 00, 00, 00, 00, 00 ]
+# NO_OPT:     merge:           by-content
+# NO_OPT:     alignment:       8
+# NO_OPT:     references:      
+# NO_OPT:       - kind:            pointer64
+# NO_OPT:         offset:          0
+# NO_OPT:         target:          'l_OBJC_$_CATEGORY_A_$_Cat'
+# NO_OPT:   - type:            objc-category-list
+# NO_OPT:     content:         [ 00, 00, 00, 00, 00, 00, 00, 00 ]
+# NO_OPT:     merge:           by-content
+# NO_OPT:     alignment:       8
+# NO_OPT:     references:      
+# NO_OPT:       - kind:            pointer64
+# NO_OPT:         offset:          0
+# NO_OPT:         target:          'l_OBJC_$_CATEGORY_B_$_Cat'
+
+# Make sure that the class RO data for B points to the new method list.
+
+
+# OPT-DAG:   - name:            'l__OBJC_CLASS_$_B_objc_merged_method_list'
+# OPT:          scope:           global
+# OPT-NEXT:     type:            {{objc-method-list|unknown}}
+# OPT-NEXT:     content:         [ 18, 00, 00, 00, 02, 00, 00, 00, {{..}}, {{..}}, {{..}}, {{..}}, 
+# OPT-NEXT:                        {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, 
+# OPT-NEXT:                        {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, 
+# OPT-NEXT:                        {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, 
+# OPT-NEXT:                        {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}} ]
+# OPT-NEXT:     alignment:       8
+# OPT-NEXT:     section-choice:  custom-required
+# OPT-NEXT:     section-name:    __DATA/__objc_const
+# OPT-NEXT:     dead-strip:      never
+# OPT-NEXT:     references:      
+# OPT-NEXT:       - kind:            pointer64Anon
+# OPT-NEXT:         offset:          8
+# OPT-NEXT:         target:          L{{[0-9]*}}
+# OPT-NEXT:       - kind:            pointer64Anon
+# OPT-NEXT:         offset:          16
+# OPT-NEXT:         target:          L{{[0-9]*}}
+# OPT-NEXT:       - kind:            pointer64
+# OPT-NEXT:         offset:          24
+# OPT-NEXT:         target:          '-[B(Cat) load]'
+# OPT-NEXT:       - kind:            pointer64Anon
+# OPT-NEXT:         offset:          32
+# OPT-NEXT:         target:          L{{[0-9]*}}
+# OPT-NEXT:       - kind:            pointer64Anon
+# OPT-NEXT:         offset:          40
+# OPT-NEXT:         target:          L{{[0-9]*}}
+# OPT-NEXT:       - kind:            pointer64
+# OPT-NEXT:         offset:          48
+# OPT-NEXT:         target:          '-[B load]'
+
+# OPT-DAG:  - name:            'l_OBJC_CLASS_RO_$_B'
+# OPT:         type:            unknown
+# OPT-NEXT:    content:         [ 02, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 
+# OPT-NEXT:                       00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 
+# OPT-NEXT:                       {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, {{..}}, 
+# OPT-NEXT:                       {{..}}, {{..}}, {{..}}, {{..}}, 00, 00, 00, 00, 00, 00, 00, 00, 
+# OPT-NEXT:                       00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 
+# OPT-NEXT:                       00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00 ]
+# OPT-NEXT:    alignment:       8
+# OPT-NEXT:    section-choice:  custom-required
+# OPT-NEXT:    section-name:    __DATA/__objc_const
+# OPT-NEXT:    references:      
+# OPT-NEXT:      - kind:            pointer64Anon
+# OPT-NEXT:        offset:          24
+# OPT-NEXT:        target:          L{{[0-9]*}}
+# OPT-NEXT:      - kind:            pointer64
+# OPT-NEXT:        offset:          32
+# OPT-NEXT:        target:          'l__OBJC_CLASS_$_B_objc_merged_method_list'