一，研究Clang
1 ， Clang了解
- Clang是一个
C语言、C++、Objective-C语言的轻量级编译器。源代码发布于BSD协议下。Clang将支持其普通lambda表达式、返回类型的简化处理以及更好的处理constexpr关键字。 - Clang是一个由
Apple主导编写，基于LLVM的C/C++/Objective-C编译器 2013年4月,Clang已经全面支持C++11标准，并开始实现C++1y特性（也就是C++14，这是C++的下一个小更新版本）。Clang将支持其普通lambda表达式、返回类型的简化处理以及更好的处理constexpr关键字。Clang是一个C++编写、基于LLVM、发布于LLVMBSD许可证下的C/C++/Objective-C/Objective-C++编译器。它与GNU C语言规范几乎完全兼容（当然，也有部分不兼容的内容，包括编译命令选项也会有点差异），并在此基础上增加了额外的语法特性，比如C函数重载（通过__attribute__((overloadable))来修饰函数），其目标（之一）就是超越GCC。
2，Clang特色
终端用户特色：快速编译和较少内存占用；有诊断功能；兼容GCC；实用工具和应用：基础架构模块库；可满足多样的客户需求（代码重构，动态分析，代码生成，等）；允许集成到各种IDE中；使用LLVM'BSD'协议；内部设计与实现：一个不可移植的代码库；一个为C语言、Objective-C、C++、Objective-C++设计的非通用(Non general-purpose)的语法解析器；与C/C++/Objective-C及它们的衍生语言具有高度一致性；
3，C语言关联
函式库：C 标准函式库、glibc、Dietlibc、uClibc、Newlib、EGLIBC、Bionic特性：String、Syntax、Preprocessor、Variable types and declarations、Functions延伸的相关编程语言：C++、Objective-C、D、C#C 与其他的编程语言：Compatibility、运算子、Comparison of Pascal and C、 C to Java byte-code compiler编译器：Borland Turbo C 、Clang、GCC、 LCC、 Pelles C、 PCC、TCC、Visual C++、 C++/CLI、C++/CX、 Watcom C/C++ compiler
4，Clang使用
新建项目然后到main.m所在文件目录下
//默认将main.m生成main.cpp
clang -rewrite-objc main.m
//发现当前文件目录下生成main.cpp
//我们也可以生成区分版本生成debug或者release.cpp方便做对比
//对于版本多的项目还是有必要的
clang -rewrite-objc main.m -o mainDebug.cpp
clang -rewrite-objc main.m -o mainRelease.cpp
- 遇到
fatal error: 'UIKit/UIKit.h' file not found时 - 方法1
//默认
clang -x objective-c -rewrite-objc -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk main.m
//同理版本区分
clang -x objective-c -rewrite-objc -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk main.m -o mainclang.cpp
- 方法2
xcrun -sdk iphonesimulator clang -rewrite-objc main.m
//版本区分
xcrun -sdk iphonesimulator clang -rewrite-objc main.m -o maixcrun.cpp
- 不同场景根据需要自行替换路径
//apple为开发者提供了7个不同场景的sdk
//1 TV真机
AppleTVOS.platform
//2 TV真机模拟器
AppleTVSimulator.platform
//3 手机真机
iPhoneOS.platform
//4 手机模拟器
iPhoneSimulator.platform
//5 MacBook
MacOSX.platform
//6 watch真机
WatchOS.platform
//7 watch模拟器
WatchSimulator.platform
遇到fatal error: 'libkern/machine/OSByteOrder.h' file not found
把
-rewrite-objc
替换成
-arch arm64 -rewrite-objc
Clang目前到这里了，后续如果需要可以补充
二，联合体和位域
1，联合体
- 新建联合体
union unionA{
int a;//4
short b;//2
char c;//1
};
- 运行代码
union unionA person;
person.a = 97;
NSLog(@"a=%d---b=%d---c=%c",person.a,person.b,person.c);
person.b = 98;
NSLog(@"a=%d---b=%d---c=%c",person.a,person.b,person.c);
person.c = 'c';
NSLog(@"a=%d---b=%d---c=%c",person.a,person.b,person.c);
NSLog(@"%lu---%lu",sizeof(person),sizeof(union unionA));
- 输出结果
2021-07-22 19:12:59.705962+0800 对象与isa之间的关系[9115:282714] a=97---b=97---c=a
2021-07-22 19:12:59.706048+0800 对象与isa之间的关系[9115:282714] a=98---b=98---c=b
2021-07-22 19:12:59.706077+0800 对象与isa之间的关系[9115:282714] a=99---b=99---c=c
2021-07-22 19:12:59.706097+0800 对象与isa之间的关系[9115:282714] 4---4
//97-99 对应ASSCII码为 a-c
//日志可以看出联合体的三个元素三次取值都是一样的
//联合体内存为4字节和a的内存一致
总结
- 联合体所有元素
共用内存，给任何元素赋值都能覆盖内存 - 联合体可以定义多个不同类型的成员，联合体的
内存大小由其中最大的成员的大小决定
联合体优缺点
- 优点：内存使用更为灵活，节省内存。
- 缺点：不够包容。
ASSCII码对比
ASSCII码10进制0-255，16进制00-FF，参考其中片段
2，位域
- 位域有必要先掌握一下 x/nuf addr
n表示要现实的内存单元的个数
---------------------------
u表示每个单元字节长度
b单字节
h双字节
w四字节
g八字节
---------------------------
f表示显示方式，可取如下值：
x 16进制
d 10进制
u 10进制无符号
o 8进制
t 2进制
a 16进制
i 指令
c 字符
f 浮点
基础还是要牢靠一点
- 进入探索 新建两个结构体
//结构体
struct struct1 {
BOOL A;
BOOL B;
BOOL C;
BOOL D;
BOOL E;
};
//位域
struct struct2 {
BOOL A:1;
BOOL B:1;
BOOL C:1;
BOOL D:1;
BOOL E:1;
};
输出日志以及断点调试
struct struct1 truct1;
struct struct2 truct2;
NSLog(@"----%lu----%lu",sizeof(truct1),sizeof(truct2));
truct1.A = YES;
truct1.B = YES;
truct2.A = YES;
truct2.B = YES;
//输出结果 truct1为5个字节 truct2 为1个字节
2021-07-23 10:43:14.583061+0800 对象与isa之间的关系[12570:373355] ----5----1
//查看truct1 看8个字节用16进制
//输出结果 低地址到高地址 ABCDE 分别为0x01 0x01 0x00 0x00 0x00(也就是11000)
(lldb) p &truct1
(struct1 *) $0 = 0x00007ffeef028308
(lldb) x/gx 0x00007ffeef028308
0x7ffeef028308: 0x0000000000000101
//truct2 我查看一个字节 用2进制
//输出结果 低地址到高地址 ABCDE 分别为0b1 0b1 0b0 0b0 0b0(也就是11000)
(lldb) p &truct2
(struct2 *) $1 = 0x00007ffeef028300
(lldb) x/bt 0x00007ffeef028300
0x7ffeef028300: 0b00000011
结构体：0x0000000000000101位域：0b00000011- 很明显位域将二进制发挥的淋漓尽致，在数据
存储与传输方面占尽优势
三，对象研究
- 在main.m中新建对象
@interface NBPerson : NSObject{
NSString * height;
}
@property(nonatomic, copy)NSString *name;
@property(nonatomic,assign)NSInteger age;
@end
@implementation NBPerson
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
// insert code here...
NSLog(@"Hello, World!");
}
return 0;
}
生成main.cpp 查找NBPerson
#ifndef _REWRITER_typedef_NBPerson
#define _REWRITER_typedef_NBPerson
//类和对象在cpp文件里都是以结构体形式存在
//NBPerson继承objc_object
//objc_object 为NSObject底层实现
typedef struct objc_object NBPerson;
typedef struct {} _objc_exc_NBPerson;
#endif
extern "C" unsigned long OBJC_IVAR_$_NBPerson$_name;
extern "C" unsigned long OBJC_IVAR_$_NBPerson$_age;
struct NBPerson_IMPL {
struct NSObject_IMPL NSObject_IVARS;//嵌套结构体继承成员变量
NSString *height;//NBPerson成员变量
NSString *_name;//NBPerson属性
NSInteger _age;//NBPerson属性
};
// @property(nonatomic, copy)NSString *name;
// @property(nonatomic,assign)NSInteger age;
/* @end */
//引用结构体
struct NSObject_IMPL {
Class isa;
};
- 查找到方法
// @implementation NBPerson
//_name getter方法
static NSString * _I_NBPerson_name(NBPerson * self, SEL _cmd) { return (*(NSString **)((char *)self + OBJC_IVAR_$_NBPerson$_name)); }
extern "C" __declspec(dllimport) void objc_setProperty (id, SEL, long, id, bool, bool);
//_name setter方法
static void _I_NBPerson_setName_(NBPerson * self, SEL _cmd, NSString *name) { objc_setProperty (self, _cmd, __OFFSETOFIVAR__(struct NBPerson, _name), (id)name, 0, 1); }
//_age getter方法
static NSInteger _I_NBPerson_age(NBPerson * self, SEL _cmd) { return (*(NSInteger *)((char *)self + OBJC_IVAR_$_NBPerson$_age)); }
//_age setter方法
static void _I_NBPerson_setAge_(NBPerson * self, SEL _cmd, NSInteger age) { (*(NSInteger *)((char *)self + OBJC_IVAR_$_NBPerson$_age)) = age; }
// @end
这段代码中为name和age生成了setter和getter方法
- 常用类型
//Class为objc_class的结构体类型指针
typedef struct objc_class *Class;
//objc_object只有一个成员变量
struct objc_object {
Class _Nonnull isa __attribute__((deprecated));
};
//同理id为objc_object结构体指针
typedef struct objc_object *id;
//SEL为objc_selector结构体指针
typedef struct objc_selector *SEL;
四，关联isa
- 代码断点
NBPerson *nb = [[NBPerson alloc]init];
nb.name = @"nb";
NSLog(@"%@",nb.name);
- 查找源码
//1
+ (id)alloc
//2
_objc_rootAlloc(Class cls)
//3
callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
//4
_objc_rootAllocWithZone(cls, nil);
//5
_class_createInstanceFromZone(cls, 0, nil,OBJECT_CONSTRUCT_CALL_BADALLOC);
//6
initInstanceIsa(cls, hasCxxDtor)
inline void
objc_object::initInstanceIsa(Class cls, bool hasCxxDtor)
{
ASSERT(!cls->instancesRequireRawIsa());
ASSERT(hasCxxDtor == cls->hasCxxDtor());
initIsa(cls, true, hasCxxDtor);
}
inline void
objc_object::initIsa(Class cls, bool nonpointer, UNUSED_WITHOUT_INDEXED_ISA_AND_DTOR_BIT bool hasCxxDtor)
{
ASSERT(!isTaggedPointer());
isa_t newisa(0);
if (!nonpointer) {
newisa.setClass(cls, this);
} else {
ASSERT(!DisableNonpointerIsa);
ASSERT(!cls->instancesRequireRawIsa());
#if SUPPORT_INDEXED_ISA
ASSERT(cls->classArrayIndex() > 0);
newisa.bits = ISA_INDEX_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
newisa.has_cxx_dtor = hasCxxDtor;
newisa.indexcls = (uintptr_t)cls->classArrayIndex();
#else
newisa.bits = ISA_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
# if ISA_HAS_CXX_DTOR_BIT
newisa.has_cxx_dtor = hasCxxDtor;
# endif
newisa.setClass(cls, this);
#endif
newisa.extra_rc = 1;
}
// This write must be performed in a single store in some cases
// (for example when realizing a class because other threads
// may simultaneously try to use the class).
// fixme use atomics here to guarantee single-store and to
// guarantee memory order w.r.t. the class index table
// ...but not too atomic because we don't want to hurt instantiation
isa = newisa;
}
- 类关联的关键
inline void
isa_t::setClass(Class newCls, UNUSED_WITHOUT_PTRAUTH objc_object *obj)
{
// Match the conditional in isa.h.
#if __has_feature(ptrauth_calls) || TARGET_OS_SIMULATOR
# if ISA_SIGNING_SIGN_MODE == ISA_SIGNING_SIGN_NONE
// No signing, just use the raw pointer.
uintptr_t signedCls = (uintptr_t)newCls;
# elif ISA_SIGNING_SIGN_MODE == ISA_SIGNING_SIGN_ONLY_SWIFT
// We're only signing Swift classes. Non-Swift classes just use
// the raw pointer
uintptr_t signedCls = (uintptr_t)newCls;
if (newCls->isSwiftStable())
signedCls = (uintptr_t)ptrauth_sign_unauthenticated((void *)newCls, ISA_SIGNING_KEY, ptrauth_blend_discriminator(obj, ISA_SIGNING_DISCRIMINATOR));
# elif ISA_SIGNING_SIGN_MODE == ISA_SIGNING_SIGN_ALL
// We're signing everything
uintptr_t signedCls = (uintptr_t)ptrauth_sign_unauthenticated((void *)newCls, ISA_SIGNING_KEY, ptrauth_blend_discriminator(obj, ISA_SIGNING_DISCRIMINATOR));
# else
# error Unknown isa signing mode.
# endif
//这里是将对象地址右移3位赋值给shiftcls
shiftcls_and_sig = signedCls >> 3;
#elif SUPPORT_INDEXED_ISA
// Indexed isa only uses this method to set a raw pointer class.
// Setting an indexed class is handled separately.
cls = newCls;
#else // Nonpointer isa, no ptrauth
//这里是将对象地址右移3位赋值给shiftcls
shiftcls = (uintptr_t)newCls >> 3;
#endif
}
- 最终发现isa其实个联合体 isa_t
union isa_t {
isa_t() { }
isa_t(uintptr_t value) : bits(value) { }
uintptr_t bits;
private:
// Accessing the class requires custom ptrauth operations, so
// force clients to go through setClass/getClass by making this
// private.
Class cls;
public:
#if defined(ISA_BITFIELD)
struct {
ISA_BITFIELD; // defined in isa.h
};
bool isDeallocating() {
return extra_rc == 0 && has_sidetable_rc == 0;
}
void setDeallocating() {
extra_rc = 0;
has_sidetable_rc = 0;
}
#endif
void setClass(Class cls, objc_object *obj);
Class getClass(bool authenticated);
Class getDecodedClass(bool authenticated);
};
-
isa_t包含了
bits、cls（私有）和一个匿名结构体，所以这3个其实是共用内存,占用8字节64位。 -
ISA_BITFIELD源码分析
//这里是arm64
# if __arm64__
// ARM64 simulators have a larger address space, so use the ARM64e
// scheme even when simulators build for ARM64-not-e.
//模拟器------------------------------------
# if __has_feature(ptrauth_calls) || TARGET_OS_SIMULATOR
# define ISA_MASK 0x007ffffffffffff8ULL
# define ISA_MAGIC_MASK 0x0000000000000001ULL
# define ISA_MAGIC_VALUE 0x0000000000000001ULL
# define ISA_HAS_CXX_DTOR_BIT 0
# define ISA_BITFIELD \
uintptr_t nonpointer : 1; \
uintptr_t has_assoc : 1; \
uintptr_t weakly_referenced : 1; \
uintptr_t shiftcls_and_sig : 52; \
uintptr_t has_sidetable_rc : 1; \
uintptr_t extra_rc : 8
# define RC_ONE (1ULL<<56)
# define RC_HALF (1ULL<<7)
# else
//手机真机------------------------------------
# define ISA_MASK 0x0000000ffffffff8ULL
# define ISA_MAGIC_MASK 0x000003f000000001ULL
# define ISA_MAGIC_VALUE 0x000001a000000001ULL
# define ISA_HAS_CXX_DTOR_BIT 1
# define ISA_BITFIELD \
uintptr_t nonpointer : 1; \
uintptr_t has_assoc : 1; \
uintptr_t has_cxx_dtor : 1; \
uintptr_t shiftcls : 33; /*MACH_VM_MAX_ADDRESS 0x1000000000*/ \
uintptr_t magic : 6; \
uintptr_t weakly_referenced : 1; \
uintptr_t unused : 1; \
uintptr_t has_sidetable_rc : 1; \
uintptr_t extra_rc : 19
# define RC_ONE (1ULL<<45)
# define RC_HALF (1ULL<<18)
# endif
//这次主要研究MACOS版本
//MACOS------------------------------------
# elif __x86_64__
//这个定义可以当作一个位与用来取出shiftcls方便又给力
//转成2进制为 0000000000000000011111111111111111111111111111111111111111111000
//64位2进制从低位到高位分别为 3个0 + 44个1 + 11个0
# define ISA_MASK 0x00007ffffffffff8ULL
# define ISA_MAGIC_MASK 0x001f800000000001ULL
# define ISA_MAGIC_VALUE 0x001d800000000001ULL
# define ISA_HAS_CXX_DTOR_BIT 1
# define ISA_BITFIELD \
//表示是否对isa指针进行优化，0表示纯指针，1表示不止是类对象的地址，isa中包含了类信息、对象、引用计数等
uintptr_t nonpointer : 1; \
//关联对象标志位，`0`表示未关联，`1`表示关联
uintptr_t has_assoc : 1; \
//该对象是否`C ++` 或者`Objc`的析构器，如果有析构函数，则需要做析构逻辑，没有，则释放对象
uintptr_t has_cxx_dtor : 1; \
//储存类指针的值，开启指针优化的情况下，在`arm64`架构中有`33`位用来存储类指针，`x86_64`架构中占`44`位
uintptr_t shiftcls : 44; /*MACH_VM_MAX_ADDRESS 0x7fffffe00000*/ \
//用于调试器判断当前对象是`真的对象`还是`没有初始化`的空间
uintptr_t magic : 6; \
//指对象是否被指向或者曾经指向一个`ARC`的弱变量，没有弱引用的对象可以更快释放
uintptr_t weakly_referenced : 1; \
//标志对象是否正在释放
uintptr_t unused : 1; \
//当对象引用计数大于`10`时，则需要借用该变量存储进位
uintptr_t has_sidetable_rc : 1; \
//表示该对象的引用计数值，实际上引用计数值减`1`，例如，如果对象的引用计数为`10`，那么`extra_rc`为`9`，如果大于`10`，就需要用到上面的`has_sidetable_rc`
uintptr_t extra_rc : 8
# define RC_ONE (1ULL<<56)
# define RC_HALF (1ULL<<7)
# else
# error unknown architecture for packed isa
# endif
- 断点调试isa关联前后
//关联前
(lldb) p/t newisa
(isa_t) $28 = {
bits = 0b0000000000000000000000000000000000000000000000000000000000000000
cls = nil
= {
nonpointer = 0b0
has_assoc = 0b0
has_cxx_dtor = 0b0
shiftcls = 0b00000000000000000000000000000000000000000000
magic = 0b000000
weakly_referenced = 0b0
unused = 0b0
has_sidetable_rc = 0b0
extra_rc = 0b00000000
}
}
//关联后
(lldb) p/x newisa
(isa_t) $5 = {
bits = 0x011d80010000823d
cls = 0x011d80010000823d NBPerson
= {
nonpointer = 0x0000000000000001
has_assoc = 0x0000000000000000
has_cxx_dtor = 0x0000000000000001
shiftcls = 0x0000000020001047
magic = 0x000000000000003b
weakly_referenced = 0x0000000000000000
unused = 0x0000000000000000
has_sidetable_rc = 0x0000000000000000
extra_rc = 0x0000000000000001
}
}
验证isa
- isa指针验证
(lldb) x/4gx nb
0x101237d20: 0x011d80010000823d 0x0000000000000000
0x101237d30: 0x0000000100004018 0x0000000000000000
(lldb) p/x 0x011d80010000823d >> 3
(long) $11 = 0x0023b00020001047
(lldb) p/x 0x0023b00020001047 << 20
(long) $12 = 0x0002000104700000
(lldb) p/x 0x0002000104700000 >> 17
(long) $13 = 0x0000000100008238
(lldb) po 0x0000000100008238
NBPerson
- ISA_MAGIC_MASK验证
//isa指针式64位
//ISA_MAGIC_MASK也是64位并且[3-46]44位刚好位1与isa指针位与获取对象地址
(lldb) p/x 0x00007ffffffffff8&0x011d80010000823d
(long) $16 = 0x0000000100008238
(lldb) po 0x0000000100008238
NBPerson
- shiftcls获取
(lldb) p/x (long) 0x0000000020001047 << 3
(long) $24 = 0x0000000100008238
(lldb) po 0x0000000100008238
NBPerson
- 三种方式都表明了
shiftcls是isa关联对象的关键 - 在__x86_64__中MACH_VM_MAX_ADDRESS = 0x7fffffe00000 虚拟内存最大寻址空间是47位。
- 字节对齐是8字节对齐，也就是说指针的地址只能是8的倍数，那么指针地址的后3位只能是0，比如0x8，0x18，0x30转换成二进制后3位都是0。
- 在isa的关联中不仅用到了
位域还用到了联合体将一个isa指针内存优化到了8字节真的是做到了不浪费每一个位置
