从Mach-O角度谈谈Swift和OC的存储差异

class MyClass {    var p:Int = 0    init() {        print("init")    }    func helloSwift() -> Int {        print("helloSwift")        return 100    }    func helloSwift1() -> Int {        print("helloSwift1")        return 100    }    func helloSwift2() -> Int {        print("helloSwift2")        return 100    }}

/**假设MyClass由OC实现*/@interface MyClass : NSObject
@property(nonatomic,assign)int p;
@end
@implementation MyClass
- (instancetype)init{    if (self = [super init]) {        NSLog(@"init");    }    return self;}- (int)helloSwift{    NSLog(@"helloSwift");    return  100;}- (int)helloSwift1{    NSLog(@"helloSwift1");    return  100;}- (int)helloSwift2{    NSLog(@"helloSwift2");    return  100;}
@end

/**那么通过runtime可以获取到任意的方法IMP*/  Class class = NSClassFromString(@"MyClass"); unsigned int count = 0; Method *list = class_copyMethodList(class,&count); for (int i = 0; i < count; i++) {      Method method = list[i];      NSLog(@"- [%@ %@]",class,NSStringFromSelector(method_getName(method))); } NSLog(@"%@ count = %u",class,count);
 //模拟通过IMP调用更直观 Method method = class_getInstanceMethod(class, @selector(helloSwift)); IMP imp = method_getImplementation(method); imp();

打印结果如下2020-11-19 17:16:08.885763+0800 SwiftToolDemo[45037:17709798] - [MyClass init]2020-11-19 17:16:08.886219+0800 SwiftToolDemo[45037:17709798] - [MyClass helloSwift]2020-11-19 17:16:08.886389+0800 SwiftToolDemo[45037:17709798] - [MyClass helloSwift1]2020-11-19 17:16:08.886537+0800 SwiftToolDemo[45037:17709798] - [MyClass helloSwift2]2020-11-19 17:16:08.886680+0800 SwiftToolDemo[45037:17709798] - [MyClass p]2020-11-19 17:16:08.886823+0800 SwiftToolDemo[45037:17709798] - [MyClass setP:]2020-11-19 17:16:08.886932+0800 SwiftToolDemo[45037:17709798] MyClass count = 62020-11-19 17:16:08.887166+0800 SwiftToolDemo[45037:17709798] helloSwift

 Class class = NSClassFromString(@"SwiftDynamicRun.MyClass");  unsigned int count = 0;  Method *list = class_copyMethodList(class,&count);  for (int i = 0; i < count; i++) {       Method method = list[i];       NSLog(@"- [%@ %@]",class,NSStringFromSelector(method_getName(method)));   }
打印结果如下：2020-11-11 16:08:30.714057+0800 SwiftDynamic[71869:13232511] SwiftDynamic.MyClass count = 0

struct class64{    unsigned long long isa;    unsigned long long superClass;    unsigned long long cache;    unsigned long long vtable;    unsigned long long data;};

if (address >= segmentCommand.vmaddr && address <= segmentCommand.vmaddr + segmentCommand.vmsize) {       return address - (segmentCommand.vmaddr - segmentCommand.fileoff); }

type ClassDescriptor struct {    Flags                       uint32    Parent                      int32    Name                        int32    AccessFunction              int32    FieldDescriptor             int32    SuperclassType              int32    MetadataNegativeSizeInWords uint32    MetadataPositiveSizeInWords uint32    NumImmediateMembers         uint32    NumFields                   uint32}

struct _ClassContextDescriptor: _ContextDescriptorProtocol {    var flags: Int32    var parent: Int32    var mangledNameOffset: Int32    var fieldTypesAccessor: Int32    var reflectionFieldDescriptor: Int32    var superClsRef: Int32    var metadataNegativeSizeInWords: Int32    var metadataPositiveSizeInWords: Int32    var numImmediateMembers: Int32    var numberOfFields: Int32    var fieldOffsetVector: Int32}

class ClassContextDescriptorBuilder//重写了addLayoutInfovoid layout() {      super::layout();      addVTable();      addOverrideTable();      addObjCResilientClassStubInfo();      maybeAddCanonicalMetadataPrespecializations();    }}

ClassContextDescriptorBuilder //重写父类addLayoutInfo方法，从而添加SuperclassType 、MetadataNegativeSizeInWords、MetadataPositiveSizeInWords、NumImmediateMembers 、NumFields、FieldOffsetVectorOffset、VTable、OverrideTable等               ^               |TypeContextDescriptorBuilderBase // 添加Name、AccessFunction、FieldDescriptor、NumFields、FieldOffsetVectorOffset               ^               |ContextDescriptorBuilderBase  //添加Flag 、Parent

class TypeContextDescriptorBuilderBasevoid layout() {      asImpl().computeIdentity();
      super::layout();      asImpl().addName();      asImpl().addAccessFunction();      asImpl().addReflectionFieldDescriptor();      asImpl().addLayoutInfo();      asImpl().addGenericSignature();      asImpl().maybeAddResilientSuperclass();      asImpl().maybeAddMetadataInitialization();    }
     void addLayoutInfo() {      auto properties = getType()->getStoredProperties();
      // uint32_t NumFields;      B.addInt32(properties.size());
      // uint32_t FieldOffsetVectorOffset;      B.addInt32(FieldVectorOffset / IGM.getPointerSize());    }}

 class ContextDescriptorBuilderBase {  void layout() {      asImpl().addFlags();      asImpl().addParent();    }}

 void addVTable() {       ...           B.addInt32(VTableEntries.size());       for (auto fn : VTableEntries)        emitMethodDescriptor(fn);  }

 ------------------------------------------------------------------------------------------------- |  TypeFlag(16bit)  |  version(8bit) | generic(1bit) | unique(1bit) | unknown (1bit) | Kind(5bit) | -------------------------------------------------------------------------------------------------

// Kinds of context descriptor.enum class ContextDescriptorKind : uint8_t {/// This context descriptor represents a module.Module = 0,
/// This context descriptor represents an extension.Extension = 1,
/// This context descriptor represents an anonymous possibly-generic context/// such as a function body.Anonymous = 2,
/// This context descriptor represents a protocol context.Protocol = 3,
/// This context descriptor represents an opaque type alias.OpaqueType = 4,
/// First kind that represents a type of any sort.Type_First = 16,
/// This context descriptor represents a class.Class = Type_First,
/// This context descriptor represents a struct.Struct = Type_First + 1,
/// This context descriptor represents an enum.Enum = Type_First + 2,
/// Last kind that represents a type of any sort.Type_Last = 31,};sVTable = 15,  };

/// Flags for nominal type context descriptors. These values are used as the/// kindSpecificFlags of the ContextDescriptorFlags for the type.class TypeContextDescriptorFlags : public FlagSet<uint16_t> {  enum {    // All of these values are bit offsets or widths.    // Generic flags build upwards from 0.    // Type-specific flags build downwards from 15.
    /// Whether there's something unusual about how the metadata is    /// initialized.    ///    /// Meaningful for all type-descriptor kinds.    MetadataInitialization = 0,    MetadataInitialization_width = 2,
    /// Set if the type has extended import information.    ///    /// If true, a sequence of strings follow the null terminator in the    /// descriptor, terminated by an empty string (i.e. by two null    /// terminators in a row).  See TypeImportInfo for the details of    /// these strings and the order in which they appear.    ///    /// Meaningful for all type-descriptor kinds.    HasImportInfo = 2,
    /// Set if the type descriptor has a pointer to a list of canonical    /// prespecializations.    HasCanonicalMetadataPrespecializations = 3,
    // Type-specific flags:
    /// The kind of reference that this class makes to its resilient superclass    /// descriptor.  A TypeReferenceKind.    ///    /// Only meaningful for class descriptors.    Class_ResilientSuperclassReferenceKind = 9,    Class_ResilientSuperclassReferenceKind_width = 3,
    /// Whether the immediate class members in this metadata are allocated    /// at negative offsets.  For now, we don't use this.    Class_AreImmediateMembersNegative = 12,
    /// Set if the context descriptor is for a class with resilient ancestry.    ///    /// Only meaningful for class descriptors.    Class_HasResilientSuperclass = 13,
    /// Set if the context descriptor includes metadata for dynamically    /// installing method overrides at metadata instantiation time.    Class_HasOverrideTable = 14,
    /// Set if the context descriptor includes metadata for dynamically    /// constructing a class's vtables at metadata instantiation time.    ///    /// Only meaningful for class descriptors.    Class_HasVTable = 15,  };

函数的Flag解释如下，感兴趣的可以关注下/** ------------------------------------------------------------------------------------ |  ExtraDiscriminator(16bit) | .. | isDynamic(1bit) | isInstance(1bit) | Kind(4bit) | ------------------------------------------------------------------------------------
enum class Kind {    Method,    Init,    Getter,    Setter,    ModifyCoroutine,    ReadCoroutine,  }; */

//OverrideTable结构如下，紧随VTable后4字节为OverrideTable数量，再其后为此结构数组struct SwiftOverrideMethod {    struct SwiftClassType *OverrideClass;    struct SwiftMethod *OverrideMethod;    struct SwiftMethod *Method;};