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How does For-Range work with assembly code in golang?

王林

Feb 09, 2024 pm 05:18 PM

For-Range 如何在 golang 中与汇编代码一起工作？

php小编草莓为您介绍一种在golang中与汇编代码一起工作的方法——For-Range。For-Range是golang中的一个循环结构，可以与汇编代码结合使用，提供更高效的性能和灵活性。通过使用For-Range，您可以在golang中轻松处理大量的数据，并且可以借助汇编代码的优势，提升程序的执行效率。在本文中，我们将详细介绍For-Range的使用方法，并讲解如何与汇编代码进行协作，以实现更高效的程序运行。

问题内容

当源代码被汇编时，我对 golang 中 for-range 内的指针用法感到困惑。例如，我们知道下面的变量value将始终位于相同的内存地址中，并且相应的汇编代码显示了相同的逻辑。

// Source Code
func main() {
a := []int{1, 3, 5}

    for _, value := range a {
        foo(&value)
    }

}

func foo(a *int) int {
b := *a * 42
fmt.Println(b)
return b
}

// Assembly Code
"".main STEXT size=126 args=0x0 locals=0x38 funcid=0x0
0x0000 00000 (main.go:15)       TEXT    "".main(SB), ABIInternal, $56-0
0x0000 00000 (main.go:15)       CMPQ    SP, 16(R14)
0x0004 00004 (main.go:15)       PCDATA  $0, $-2
0x0004 00004 (main.go:15)       JLS     119
0x0006 00006 (main.go:15)       PCDATA  $0, $-1
0x0006 00006 (main.go:15)       SUBQ    $56, SP
0x000a 00010 (main.go:15)       MOVQ    BP, 48(SP)
0x000f 00015 (main.go:15)       LEAQ    48(SP), BP
0x0014 00020 (main.go:15)       FUNCDATA        $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0014 00020 (main.go:15)       FUNCDATA        $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0014 00020 (main.go:16)       MOVQ    $0, ""..autotmp_4+24(SP)
0x001d 00029 (main.go:16)       LEAQ    ""..autotmp_4+32(SP), CX
0x0022 00034 (main.go:16)       MOVUPS  X15, (CX)
0x0026 00038 (main.go:16)       MOVQ    $1, ""..autotmp_4+24(SP)
0x002f 00047 (main.go:16)       MOVQ    $3, ""..autotmp_4+32(SP)
0x0038 00056 (main.go:16)       MOVQ    $5, ""..autotmp_4+40(SP)
0x0041 00065 (main.go:16)       XORL    AX, AX
0x0043 00067 (main.go:18)       JMP     103
0x0045 00069 (main.go:18)       MOVQ    AX, ""..autotmp_10+16(SP)
0x004a 00074 (main.go:18)       MOVQ    ""..autotmp_4+24(SP)(AX\*8), CX
0x004f 00079 (main.go:18)       MOVQ    CX, "".value+8(SP)
0x0054 00084 (main.go:19)       LEAQ    "".value+8(SP), AX        // Here we see we always use "".value+8(SP) as the argument into foo()
0x0059 00089 (main.go:19)       PCDATA  $1, $0
0x0059 00089 (main.go:19)       CALL    "".foo(SB)
0x005e 00094 (main.go:18)       MOVQ    ""..autotmp_10+16(SP), CX
0x0063 00099 (main.go:18)       LEAQ    1(CX), AX
0x0067 00103 (main.go:18)       CMPQ    AX, $3
0x006b 00107 (main.go:18)       JLT     69
0x006d 00109 (main.go:21)       PCDATA  $1, $-1
0x006d 00109 (main.go:21)       MOVQ    48(SP), BP
0x0072 00114 (main.go:21)       ADDQ    $56, SP
0x0076 00118 (main.go:21)       RET
0x0077 00119 (main.go:21)       NOP
0x0077 00119 (main.go:15)       PCDATA  $1, $-1
0x0077 00119 (main.go:15)       PCDATA  $0, $-2
0x0077 00119 (main.go:15)       CALL    runtime.morestack_noctxt(SB)
0x007c 00124 (main.go:15)       PCDATA  $0, $-1
0x007c 00124 (main.go:15)       JMP     0
0x0000 49 3b 66 10 76 71 48 83 ec 38 48 89 6c 24 30 48  I;f.vqH..8H.l$0H
0x0010 8d 6c 24 30 48 c7 44 24 18 00 00 00 00 48 8d 4c  .l$0H.D$.....H.L
0x0020 24 20 44 0f 11 39 48 c7 44 24 18 01 00 00 00 48  $ D..9H.D$.....H
0x0030 c7 44 24 20 03 00 00 00 48 c7 44 24 28 05 00 00  .D$ ....H.D$(...
0x0040 00 31 c0 eb 22 48 89 44 24 10 48 8b 4c c4 18 48  .1.."H.D$.H.L..H
0x0050 89 4c 24 08 48 8d 44 24 08 e8 00 00 00 00 48 8b  .L$.H.D$......H.
0x0060 4c 24 10 48 8d 41 01 48 83 f8 03 7c d8 48 8b 6c  L$.H.A.H...|.H.l
0x0070 24 30 48 83 c4 38 c3 e8 00 00 00 00 eb 82        $0H..8........
rel 90+4 t=7 "".foo+0
rel 120+4 t=7 runtime.morestack_noctxt+0

但是，当我更改源代码并尝试查看汇编代码中的变化时，我发现没有任何变化。

// Changed Source Code
func main() {
a := []int{1, 3, 5}

    for _, value := range a {
        v := value
        foo(&v)
    }

}

func foo(a *int) int {
b := *a * 42
fmt.Println(b)
return b
}

// Changed Assembly Code
"".main STEXT size=126 args=0x0 locals=0x38 funcid=0x0
0x0000 00000 (main.go:15)       TEXT    "".main(SB), ABIInternal, $56-0
0x0000 00000 (main.go:15)       CMPQ    SP, 16(R14)
0x0004 00004 (main.go:15)       PCDATA  $0, $-2
0x0004 00004 (main.go:15)       JLS     119
0x0006 00006 (main.go:15)       PCDATA  $0, $-1
0x0006 00006 (main.go:15)       SUBQ    $56, SP
0x000a 00010 (main.go:15)       MOVQ    BP, 48(SP)
0x000f 00015 (main.go:15)       LEAQ    48(SP), BP
0x0014 00020 (main.go:15)       FUNCDATA        $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0014 00020 (main.go:15)       FUNCDATA        $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0014 00020 (main.go:16)       MOVQ    $0, ""..autotmp_5+24(SP)
0x001d 00029 (main.go:16)       LEAQ    ""..autotmp_5+32(SP), CX
0x0022 00034 (main.go:16)       MOVUPS  X15, (CX)
0x0026 00038 (main.go:16)       MOVQ    $1, ""..autotmp_5+24(SP)
0x002f 00047 (main.go:16)       MOVQ    $3, ""..autotmp_5+32(SP)
0x0038 00056 (main.go:16)       MOVQ    $5, ""..autotmp_5+40(SP)
0x0041 00065 (main.go:16)       XORL    AX, AX
0x0043 00067 (main.go:18)       JMP     103
0x0045 00069 (main.go:18)       MOVQ    AX, ""..autotmp_11+16(SP)
0x004a 00074 (main.go:18)       MOVQ    ""..autotmp_5+24(SP)(AX\*8), CX
0x004f 00079 (main.go:19)       MOVQ    CX, "".v+8(SP)
0x0054 00084 (main.go:20)       LEAQ    "".v+8(SP), AX    // Here we see the logic of argument is the same as above. This makes me confused.
0x0059 00089 (main.go:20)       PCDATA  $1, $0
0x0059 00089 (main.go:20)       CALL    "".foo(SB)
0x005e 00094 (main.go:18)       MOVQ    ""..autotmp_11+16(SP), CX
0x0063 00099 (main.go:18)       LEAQ    1(CX), AX
0x0067 00103 (main.go:18)       CMPQ    AX, $3
0x006b 00107 (main.go:18)       JLT     69
0x006d 00109 (main.go:22)       PCDATA  $1, $-1
0x006d 00109 (main.go:22)       MOVQ    48(SP), BP
0x0072 00114 (main.go:22)       ADDQ    $56, SP
0x0076 00118 (main.go:22)       RET
0x0077 00119 (main.go:22)       NOP
0x0077 00119 (main.go:15)       PCDATA  $1, $-1
0x0077 00119 (main.go:15)       PCDATA  $0, $-2
0x0077 00119 (main.go:15)       CALL    runtime.morestack_noctxt(SB)
0x007c 00124 (main.go:15)       PCDATA  $0, $-1
0x007c 00124 (main.go:15)       JMP     0
0x0000 49 3b 66 10 76 71 48 83 ec 38 48 89 6c 24 30 48  I;f.vqH..8H.l$0H
0x0010 8d 6c 24 30 48 c7 44 24 18 00 00 00 00 48 8d 4c  .l$0H.D$.....H.L
0x0020 24 20 44 0f 11 39 48 c7 44 24 18 01 00 00 00 48  $ D..9H.D$.....H
0x0030 c7 44 24 20 03 00 00 00 48 c7 44 24 28 05 00 00  .D$ ....H.D$(...
0x0040 00 31 c0 eb 22 48 89 44 24 10 48 8b 4c c4 18 48  .1.."H.D$.H.L..H
0x0050 89 4c 24 08 48 8d 44 24 08 e8 00 00 00 00 48 8b  .L$.H.D$......H.
0x0060 4c 24 10 48 8d 41 01 48 83 f8 03 7c d8 48 8b 6c  L$.H.A.H...|.H.l
0x0070 24 30 48 83 c4 38 c3 e8 00 00 00 00 eb 82        $0H..8........
rel 90+4 t=7 "".foo+0
rel 120+4 t=7 runtime.morestack_noctxt+0

那么局部变量 v 如何影响 for-range 呢？

正如上面的细节，我认为汇编代码应该显示新的局部变量的引入是如何工作的，但事实并非如此。

解决方法

几点。

规范仅针对您的情况说明了以下内容：

这就是全部：它说将会有变量，并且它们将会重新使用。后一点意味着，比如说，如果您创建一个闭包（使用函数文字的匿名函数），它将关闭一个或多个迭代变量，并将其返回/保存在某处并在循环结束后调用或者与循环同时（例如，在一个单独的 goroutine 中），该闭包将在循环的每次迭代更新（或正在更新）时访问这些完全相同的变量。

如果您不做任何此类奇特的事情 - 例如，仅从循环体代码中的那些变量中读取，那么这些变量被重用的事实是无关紧要的。

让我们重申一下：规范没有对迭代变量的内存地址提供任何保证。

为什么这很重要？因为编译器可以自由地生成它希望的任何代码，只要结果以遵循规范的方式工作，并且编译器的作用至少取决于以下内容：

目标硬件（GOARCH）。
Go 的版本和 make（实现）。不要低估这一点：例如，规范并没有精确定义 GC 的工作方式，因此任何品牌和任何版本的 Go 都可以自由地实现移动 GC，这将移动内存中的任意变量并更新指向的所有指针他们。
目前，流行的 Go 版本（您应该使用的版本和 GCC 前端）并没有这样做，但没有什么可以阻止它们或任何其他实现这样做。

因此，最后，您会问为什么特定的编译器会生成看起来特别的代码，并且既不说明 Go 的品牌，也不说明其版本，也不说明您的 GOARCH （尽管可以猜测它可能是 amd64）。因此，您的问题实际上是无法回答的，并且精确的答案不会太有用，因为它们很快就会过时。
因此，此类问题与 SO 无关。我决定将所有这些作为答案，只是因为对于评论来说太多了。

在您的特定情况下，编译器可能分析了 foo 的代码，并发现它不会更新通过指针参数传递给它的变量，并且更重要的是不会将其进一步传递到调用堆栈。由于循环体中的 v 没有做任何其他事情，编译器可能会认为语义 v 可以被视为 variable 的纯粹别名，并且它只是跳过了该单独变量的创建。我可以想象，如果您将这些变量的指针传递给 foo，并且让它打印这些地址，编译器将被迫实现 v。

请注意，“关于”只是一个猜测。如果您想了解所有细节，您可以随时研究编译器的工作原理（Go 的两种流行实现都是 F/OSS 的一部分）和/或检测其代码。

另请注意，您可以要求编译器告诉您有关其功能的更多信息。最常用的 Go 实现（最初称为 gc ）在其 go build 和 go install 调用中支持 -gcflags 命令行参数，该调用将参数传递给编译器（请参阅 go 工具编译 -help ）。特别是，它的 -m 和 -N （还有 -S 和 -live）标志可能值得玩一下。

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