Understanding Swift’s value type thread safety - 代碼分析（二）

示範代碼

`

func testScenarioA2() throws {
    var store: Int = 100
    DispatchQueue.concurrentPerform(iterations: 1_000_000) { _ in
        store.negate()
        _ = store
    }
}

`
開啓線程race診斷後，出現以下錯誤

Swift access race in closure #1 (Swift.Int) -> () in UnderstandStruct.testScenarioA2() throws -> ()

查看彙編
`
0x100002a30 <+0>: pushq %rbp
0x100002a31 <+1>: movq %rsp, %rbp
0x100002a34 <+4>: subq $0x20, %rsp
0x100002a38 <+8>: movq 0x8(%rbp), %rdi
0x100002a3c <+12>: movq %rsi, -0x10(%rbp)
0x100002a40 <+16>: callq 0x100003bac ; symbol stub for: __tsan_func_entry
0x100002a45 <+21>: xorl %esi, %esi
0x100002a47 <+23>: leaq -0x8(%rbp), %rax
0x100002a4b <+27>: movq %rax, %rdi
0x100002a4e <+30>: movl $0x8, %edx
0x100002a53 <+35>: callq 0x100003c06 ; symbol stub for: memset
0x100002a58 <+40>: xorl %ecx, %ecx
0x100002a5a <+42>: movl %ecx, %esi
0x100002a5c <+44>: movq -0x10(%rbp), %rdx
0x100002a60 <+48>: movq %rdx, -0x8(%rbp)
-> 0x100002a64 <+52>: movq %rdx, %rdi
0x100002a67 <+55>: movl $0x1, %edx
0x100002a6c <+60>: movq %rax, -0x18(%rbp)
0x100002a70 <+64>: callq 0x100003ba6 ; symbol stub for: __tsan_external_write
0x100002a75 <+69>: movq -0x10(%rbp), %rax
0x100002a79 <+73>: movq %rax, %rdi
0x100002a7c <+76>: callq 0x100003bbe ; symbol stub for: __tsan_read8
0x100002a81 <+81>: xorl %ecx, %ecx
0x100002a83 <+83>: movl %ecx, %eax
0x100002a85 <+85>: movq -0x10(%rbp), %rdx
0x100002a89 <+89>: subq (%rdx), %rax
0x100002a8c <+92>: seto %r8b
0x100002a90 <+96>: testb $0x1, %r8b
0x100002a94 <+100>: movq %rax, -0x20(%rbp)
0x100002a98 <+104>: jne 0x100002ac1 ; <+145> [inlined] Swift runtime failure: arithmetic overflow at main.swift:13
0x100002a9a <+106>: movq -0x10(%rbp), %rdi
0x100002a9e <+110>: callq 0x100003bc4 ; symbol stub for: __tsan_write8
0x100002aa3 <+115>: movq -0x10(%rbp), %rax
0x100002aa7 <+119>: movq -0x20(%rbp), %rcx
0x100002aab <+123>: movq %rcx, (%rax)
0x100002aae <+126>: movq %rax, %rdi
0x100002ab1 <+129>: callq 0x100003bbe ; symbol stub for: __tsan_read8
0x100002ab6 <+134>: callq 0x100003bb2 ; symbol stub for: __tsan_func_exit
0x100002abb <+139>: addq $0x20, %rsp
0x100002abf <+143>: popq %rbp
0x100002ac0 <+144>: retq
0x100002ac1 <+145>: ud2
0x100002ac3 <+147>: nopw %cs:(%rax,%rax)
0x100002acd <+157>: nopl (%rax)

`
0x100002aab <+123>: movq %rcx, (%rax)
rax是store的地址，直接將計算結果賦給指向的地址。

結論

• 對於堆中的變量 store，系統會自動生成 begin_access 和 end_access插樁的檢測競爭代碼，該代碼不影響實際邏輯的執行
• 對於store的值的改變，不同線程中都是一條mov指令，所以這裏存在的線程競爭不會導致crash

參考
`

bb0(%0 : $Int, %1 : $Int):
debug_value %0 : $Int // id: %2
debug_value_addr %1 : $Int, var, name "store", argno 2 // id: %3
%4 = begin_access [modify] [unknown] %1 : $Int // users: %7, %6
// function_ref SignedNumeric.negate()
%5 = function_ref @$ss13SignedNumericPsE6negateyyF : $@convention(method) <τ_0_0 where τ_0_0 : SignedNumeric> (@inout τ_0_0) -> () // user: %6
%6 = apply %5(%4) : $@convention(method) <τ_0_0 where τ_0_0 : SignedNumeric> (@inout τ_0_0) -> ()
end_access %4 : $Int // id: %7
%8 = begin_access [read] [unknown] %1 : $Int // users: %10, %9
%9 = load [trivial] %8 : $Int
end_access %8 : $*Int // id: %10
%11 = tuple () // user: %12
return %11 : $() // id: %12
}
`