Understanding Swift’s value type thread safety - 代码分析（一）

• 结构体并不代表线程安全，swift在此上未做保证

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func testScenarioA() throws {     var store: Int = 0     DispatchQueue.concurrentPerform(iterations: 1_000_000) { i in         store = i         _ = store     } }

　　执行后下断点，对比寄存器和反汇编代码

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(lldb) dis UnderstandStruct`closure #1 in testScenarioA():     0x100002b60 <+0>:  pushq  %rbp     0x100002b61 <+1>:  movq   %rsp, %rbp     0x100002b64 <+4>:  movq   $0x0, -0x8(%rbp)     0x100002b6c <+12>: movq   $0x0, -0x10(%rbp)     0x100002b74 <+20>: movq   %rdi, -0x8(%rbp)     0x100002b78 <+24>: movq   %rsi, -0x10(%rbp) ->  0x100002b7c <+28>: movq   %rdi, (%rsi)     0x100002b7f <+31>: popq   %rbp     0x100002b80 <+32>: retq       0x100002b81 <+33>: nopw   %cs:(%rax,%rax)     0x100002b8b <+43>: nopl   (%rax,%rax) (lldb) register read General Purpose Registers:        rax = 0x00000001006236a0        rbx = 0x0000000000000000        rcx = 0x00007ffeefbff028        rdx = 0x00007fff6ac713a0  libswiftDispatch.dylib`___swift_dispatch_apply_current_block_invoke        rdi = 0x0000000000000000        rsi = 0x00007ffeefbff1f0        rbp = 0x00007ffeefbfefb0        rsp = 0x00007ffeefbfefb0         r8 = 0x00007ffeefbff158         r9 = 0x0000000101400080        r10 = 0x00000000000020ff        r11 = 0x0000000000000246        r12 = 0x00007ffeefbff158        r13 = 0x00007ffeefbff1d0        r14 = 0x0000000000000000        r15 = 0x00007fff6ac713a0  libswiftDispatch.dylib`___swift_dispatch_apply_current_block_invoke        rip = 0x0000000100002b7c  UnderstandStruct`closure #1 (Swift.Int) -> () in UnderstandStruct.testScenarioA() throws -> () + 28 at main.swift:14:15     rflags = 0x0000000000000246         cs = 0x000000000000002b         fs = 0x0000000000000000         gs = 0x0000000000000000c

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1	<strong><br><br>movq   %rdi, (%rsi)<br></strong><br><br>将寄存器rdi中的值赋给 rsi寄存器指向的地址所在的内存<br>rsi的内存指向的是 <strong>0x00007ffeefbff1f0 ， 这个地址是指向 store变量<br>这个地址在多个线程中这个地址不变<br></strong>

• 综上，Int是值类型，但是从上面的代码看起来依然存在线程安全问题，多线程直接操作的是store的内存，虽然赋值是一条指令，但是可能产生ABA的问题。
• 从数据时序上看，不是安全的
  
• https://forums.swift.org/t/understanding-swifts-value-type-thread-safety/41406