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

    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

(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

movq   %rdi, (%rsi)


将寄存器rdi中的值赋给 rsi寄存器指向的地址所在的内存
rsi的内存指向的是 0x00007ffeefbff1f0 ， 这个地址是指向 store变量
这个地址在多个线程中这个地址不变

• 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