Understanding Swift’s value type thread safety - 代碼分析(一)

  • 結構體並不代表線程安全,swift在此上未做保證
    func testScenarioA() throws {
        var store: Int = 0
        DispatchQueue.concurrentPerform(iterations: 1_000_000) { i in
            store = i
            _ = store
        }
    }

  執行後下斷點,對比寄存器和反彙編代碼

(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)


將寄存器rdi中的值賦給 rsi寄存器指向的地址所在的內存
rsi的內存指向的是 0x00007ffeefbff1f0 , 這個地址是指向 store變量
這個地址在多個線程中這個地址不變
  • 綜上,Int是值類型,但是從上面的代碼看起來依然存在線程安全問題,多線程直接操作的是store的內存,雖然賦值是一條指令,但是可能產生ABA的問題。
    • 從數據時序上看,不是安全的
  • https://forums.swift.org/t/understanding-swifts-value-type-thread-safety/41406
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