duanmu6231 2016-04-05 15:18
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Go中导出和未导出的说明

I skimmed through math package in Golang GitHub repo. Some of them have two difference function declarations. In the code below, sqrt.go has Sqrt and sqrt func. My questions are: Why do they implement it this way? What are the benefits? Is this because of the exported and unexported identifier (lowercase vs uppercase first letter)?

func Sqrt(x float64) float64

// Note: Sqrt is implemented in assembly on some systems.
// Others have assembly stubs that jump to func sqrt below.
// On systems where Sqrt is a single instruction, the compiler
// may turn a direct call into a direct use of that instruction instead.

func sqrt(x float64) float64 {

math.Sqrt()

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  • doushi1510 2016-04-05 15:49
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    According to the spec, function declarations without a body, as you show in the first line of the code you quoted, provide a declaration for code implemented outside of Go; usually in assembly or another language.

    In this case, most implementations for that func Sqrt(x float64) float64 are in assembly; for example 386, amd64, arm, etc.

    Sample from amd64:

    // func Sqrt(x float64) float64
    TEXT ·Sqrt(SB),NOSPLIT,$0
        SQRTSD x+0(FP), X0
        MOVSD X0, ret+8(FP)
        RET
    

    For architectures that have no assembly implementation the assembly simply refers to the unexported / private func sqrt(x float64) float64 version that is in Go. For example the mips64 architecture.

    TEXT ·Sqrt(SB),NOSPLIT,$0
        JMP ·sqrt(SB)
    
    本回答被题主选为最佳回答 , 对您是否有帮助呢?
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