文字

概述

• Swift中Large String
  • Native
    • 真正字符串的地址=objectAddr+0x20
  • Shared/Foreign
    • 真正字符串的地址=objectAddr

详解

• Swift的String(64位)
  • 通用（Small String和Large String都有）
    • discriminator
      • 关于名称和
        • 位置
          • == leading nibble == top nibble = 64bit的最顶部的4个bit：b63:b60 == [bit63 ~ bit60]
            • 注：nibble=半个字节 = 4bit
        • 名称
          • （此处有个专门的名字）discriminator = 辨别器：用于区分字符串的具体类型
            • 其实叫做：String type 更加容易理解
        • 所以代码和注释中下面的名称是一个意思：
          • b63:b60 = discriminator = leading nibble == top nibble
      • 具体字段和含义
        • b63 = isImmortal
          • Should the Swift runtime skip ARC
            • Small strings are just values, always immortal
            • Large strings can sometimes be immortal, e.g. literals
        • b62 = (large) isBridged / (small) isASCII
          • For large strings, this means lazily-bridged NSString: perform ObjC ARC
          • Small strings repurpose this as a dedicated bit to remember ASCII-ness
        • b61 = isSmall
          • Dedicated bit to denote small strings
        • b60 = isForeign = isSlow
          • cannot provide access to contiguous UTF-8
      • 完整的映射表 = 不同字段的组合，表示不同类型字符串
        • 
  • Small String
    • [+0x00-0x0E] = 8+7个字节 =15个字节 = 64+56位 = 120位 : realString
    • [+0xF] = 第16个字节=共8位: discriminator + count
      • byte15的[b4:b7] = discriminator
      • byte15的[b0:b3] = count
  • Large String
    • [+0x00-0x07 ] = 8个字节 = 64位 : flag
      • b63 = ASCII = isASCII
      • b62 = NFC = isNFC
      • b61 = native = isNativelyStored
      • b60 = tail = isTailAllocated
      • b59 = UTF8 = isForeignUTF8
      • b58:48 = reserved
      • b47:0 = count
    • [+0x08-0x0F] = 8个字节 = 64位 : discriminator + objectAddr
      • b63:b60 = discriminator
        • 详见上述解释：b63 = isImmortal、b62 = (large) isBridged / (small) isASCII、b61 = isSmall、b60 = isForeign = isSlow
      • b60:b0 = objectAddr
        • realStrAddr = objectAddr + nativeBias = objectAddr + 32 = objectAddr + 0x20