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docs: clang 插件开发
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杭城小刘
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# 虚拟内存
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- 进程隔离的必要性
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- 虚拟内存是如何实现进程隔离的
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- 线性地址和物理地址是如何转换的
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# Swift 枚举值内存布局
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> enum 使用很简单，那大家有没有思考过系统针对枚举的实现是怎么样的？接下去会针对不同情况的枚举，结合汇编来窥探下系统实现原理。
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### 基础枚举
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```swift
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enum Season {
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case spring
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case summer
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case antumn
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case winter
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}
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var season: Season = Season.spring
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print(Mems.ptr(ofVal: &season))
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season = Season.summer
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season = Season.antumn
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print("over")
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```
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- `var season: Season = Season.spring` 基础枚举，默认值是0。
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/EnumBaseMemoryLayoutDemo1.png" style="zoom:25%">
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- `season = Season.summer`，此时可以看到第一个字节的位置是1.
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/EnumBaseMemoryLayoutDemo2.png" style="zoom:25%">
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- `season = Season.antumn` ，此时可以看到第一个字节的位置是2
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/EnumBaseMemoryLayoutDemo3.png" style="zoom:25%">
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结论：查看内存信息，可以看到基础枚举，只占1个字节大小空间，且值为默认值。
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### 只有原始值
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```swift
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enum Season:Int {
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case spring = 1
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case summer = 2
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case antumn = 3
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case winter = 4
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}
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//print(MemoryLayout<Season>.size)
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//print(MemoryLayout<Season>.stride)
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//print(MemoryLayout<Season>.alignment)
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var season: Season = Season.spring
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print(Mems.ptr(ofVal: &season))
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season = .summer
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season = .winter
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print("over")
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```
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- `var season: Season = Season.spring` 基础枚举，变量默认值，可以看到第一个字节的位置是0
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/EnumWithRawValueMemoryLayoutDemo1.png" style="zoom:25%">
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- `season = .winter` 基础枚举，当赋值为 winter 的时候，可以看到第一个字节的位置是3
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/EnumWithRawValueMemoryLayoutDemo2.png" style="zoom:25%">
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结论：带有原始值的枚举，同样只占用1个字节，该字节的值为枚举的位置（比如 case1 case2）
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### 带有关联值的枚举
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```swift
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enum Season {
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case spring(Int, Int, Int)
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case summer(Int, Int)
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case antumn(Int)
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case winter(Bool)
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case unknown
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}
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print(MemoryLayout<Season>.size)
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print(MemoryLayout<Season>.stride)
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print(MemoryLayout<Season>.alignment)
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var season: Season = Season.spring(1, 2, 3)
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print(Mems.ptr(ofVal: &season))
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season = Season.summer(4, 5)
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season = Season.antumn(6)
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season = Season.winter(true)
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season = Season.unknown
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print("over")
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```
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- `var season: Season = Season.spring(1, 2, 3)` 带有关联值的枚举，`.spring` 有3个 Int，单个 Int 占8个字节空间，所以红色框代表 spring 的1，蓝色框代表 spring 的2，绿色框代表 spring 的3，黄色框代表枚举的第1个 case，剩余7个字节，为空。
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/AssociatedEnumMemoryLayoutDemo1.png" style="zoom:25%">
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其内存信息如下（8字节为1组，对应上图）
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```shell
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01 00 00 00 00 00 00 00
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02 00 00 00 00 00 00 00
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03 00 00 00 00 00 00 00
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00
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00 00 00 00 00 00 00
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```
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这段内存信息怎么看？我划分了下
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```
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关联值： 01 00 00 00 00 00 00 00
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关联值： 02 00 00 00 00 00 00 00
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关联值： 03 00 00 00 00 00 00 00
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位置值： 00
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内存对齐占用：00 00 00 00 00 00 00
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```
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下面的几组一样
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- `season = Season.summer(4, 5)` 带有关联值的枚举，`.summer` 有2个 Int，单个 Int 占8个字节空间，所以红色框代表 summer 的4，蓝色框代表 summer 的5，绿色框为空，黄色框代表枚举的第2个 case，剩余7个字节，为空。
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/AssociatedEnumMemoryLayoutDemo3.png" style="zoom:25%">
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其内存信息如下（8字节为1组，对应上图）
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```shell
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04 00 00 00 00 00 00 00
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05 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00
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01
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00 00 00 00 00 00 00
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```
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- `season = Season.antumn(6) 带有关联值的枚举，`. `antumn` 有1个 Int，单个 Int 占8个字节空间，所以红色框代表 antumn 的6，蓝色框为空，绿色框为空，黄色框代表枚举的第3个 case，剩余7个字节，为空。
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/AssociatedEnumMemoryLayoutDemo4.png" style="zoom:25%">
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其内存信息如下（8字节为1组，对应上图）
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```shell
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06 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00
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02
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00 00 00 00 00 00 00
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```
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- `season = Season.winter(true)` 带有关联值的枚举，`. `winter` 有1个 Bool，单个 Int 占1个字节空间，所以红色框代表 winter 的 true，蓝色框为空，绿色框为空，黄色框代表枚举的第4个 case，剩余7个字节，为空。
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/AssociatedEnumMemoryLayoutDemo5.png" style="zoom:25%">
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其内存信息如下（8字节为1组，对应上图）
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```shell
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01 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00
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03
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00 00 00 00 00 00 00
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```
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- `season = Season.unknown` 带有关联值的枚举，`unknown` 没有关联值，所以红色框为空，蓝色框为空，绿色框为空，黄色框代表枚举的第5个 case，剩余7个字节，为空。
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/AssociatedEnumMemoryLayoutDemo6.png" style="zoom:25%">
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其内存信息如下（8字节为1组，对应上图）
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```shell
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00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00
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04
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00 00 00 00 00 00 00
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```
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- `MemoryLayout<Season>.size` ：3个 Int 最大为3*8，1个字节用来表达位置信息，`3*8 + 1 = 25`
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- `MemoryLayout<Season>.stride` ：获取系统分配给数据类型的内存大小，也就是实际内存大小（对齐后的）
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- `MemoryLayout<Season>.alignment` 内存对齐系数，以8 Byte 为单位，对象分配的内存必须是该值的整数倍
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### 只有一个 case 的枚举
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```swift
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enum SimpleEnum {
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case one
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}
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var caseOne = SimpleEnum.one
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print(MemoryLayout<SimpleEnum>.size) // 0
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print(MemoryLayout<SimpleEnum>.stride) // 1
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print(MemoryLayout<SimpleEnum>.alignment) // 1
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```
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为什么 size 为0？看上去是一个变量，但根本不占内存。因为枚举里面就一个 case，所以里面根本不需要存储值来区分是哪个 case。
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```swift
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enum SimpleEnum {
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case one
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case two
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}
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var caseOne = SimpleEnum.one
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print(MemoryLayout<SimpleEnum>.size) // 1
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print(MemoryLayout<SimpleEnum>.stride) // 1
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print(MemoryLayout<SimpleEnum>.alignment) // 1
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```
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现在好理解，2个 case 需要存储1个 Byte 的值来区分是哪个 case，1 Byte 可以代表最多256个 case
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### 只有1个 case 且带关联值的枚举
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```swift
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enum SimpleEnum {
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case one(Int)
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}
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var caseOne = SimpleEnum.one(4)
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print(MemoryLayout<SimpleEnum>.size) // 8
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print(MemoryLayout<SimpleEnum>.stride) // 8
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print(MemoryLayout<SimpleEnum>.alignment) // 8
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```
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带有关联值且只有1个 case 的枚举，因为有1个 Int 的关联值，但只有1个 case，所以只需要8 Byte 存储关联值即可。
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请看下面的对照实验
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```swift
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enum SimpleEnum {
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case one(Int)
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case two
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}
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var caseOne = SimpleEnum.one(4)
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print(MemoryLayout<SimpleEnum>.size) // 9
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print(MemoryLayout<SimpleEnum>.stride) // 16
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print(MemoryLayout<SimpleEnum>.alignment) // 8
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```
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2个 case，其中一个 case 有关联值 Int，所以需要8 Byte 存 Int 值，1 Byte 区分是哪个 case，实际需要占用 8 + 1 = 9 Byte，内存对齐单位是8，9向上为16.
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### 用汇编验证下内存
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```
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enum Season {
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case spring(Int, Int, Int)
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case summer(Int, Int)
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case antumn(Int)
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case winter(Bool)
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case unknown
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}
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var season: Season = Season.spring(1, 2, 3)
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print(Mems.ptr(ofVal: &season))
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season = Season.summer(4, 5)
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season = Season.antumn(6)
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season = Season.winter(true)
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season = Season.unknown
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print("over")
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```
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断点停到 `var season: Season = Season.spring(1, 2, 3)` 位置
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/AssociatedEnumMemoryLayoutExplore.png" style="zoom:25%">
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将断点处的汇编单独摘出来研究
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```assembly
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0x10000334b <+11>: movq $0x1, 0x8eaa(%rip) ; demangling cache variable for type metadata for Swift.Array<Swift.UInt8> + 4
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0x100003356 <+22>: movq $0x2, 0x8ea7(%rip) ; SwiftDemo.season : SwiftDemo.Season + 4
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0x100003361 <+33>: movq $0x3, 0x8ea4(%rip) ; SwiftDemo.season : SwiftDemo.Season + 12
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0x10000336c <+44>: movb $0x0, 0x8ea5(%rip) ; SwiftDemo.season : SwiftDemo.Season + 23
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0x100003373 <+51>: movl $0x1, %edi
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```
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`rip` 存储的说指令的地址。CPU 要执行的下一条指令地址就存储在 rip 中。所以在执行第一行的时候，rip 寄存器的值。
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所以第一句汇编代码的意思是：rip 为 `0x100003356`，再加上 `0x8eaa`，得到一个地址值（用 Mac 自带的计算器可以算出）`0X10000C200`，然后 movq 是将十六进制的1赋值给 `0X10000C200` 这个地址。
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第二句汇编代码类似，此时 rip 为 `0x100003361`，再加上 `0x8ea7`，得到一个地址值 `0X10000C208`，然后 movq 将十六进制的2赋值给 `0X10000C208` 这个地址。
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第三句汇编代码类似，此时 rip 为 `0x10000336c`，再加上 `0x8ea4`，得到一个地址值 `0X10000C210`，然后 movq 将十六进制的3赋值给 `0X10000C210` 这个地址。
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第四句汇编代码类似，此时 rip 为 `0x100003373`，再加上 `0x8ea5`，得到一个地址值 `0X10000C218`，然后 movq 将十六进制的0赋值给 `0X10000C218` 这个地址。
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此时断点走到下一行，拿到 season 的内存地址 `0X10000C200` ，查看内存发现和上面理论分析一直
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```shell
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01 00 00 00 00 00 00 00
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02 00 00 00 00 00 00 00
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03 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00
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```
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结论：如果枚举存在关联值，内存大小为：
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- 1个字节用来存储成员值
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- n个字节用来存储关联值（n取占用内存最大的关联值），任何一个 case 的关联值都共用这 n 个字节
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- 且存在内存对齐，所以占用大小为 n 和 1 的最大值，再结合内存对齐。
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- 如果枚举的定义非常简单，系统会用1个字节来存放值，最大范围是256个 case。
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- 枚举定义如果有原始值，也不会影响内存布局。
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