今天在學習Go的基礎數據結構——slice，期間閱讀了slice的內部實現，擴容策略等。然后想到，我們在調用append()函數進行追加元素是究竟發生了什么？于是就想看看append()函數的內部實現，結果源碼里并沒有找到，搜索一番，還是在StackOverflow上找到了答案。記錄如下。

Q：Where is the implementation of func append in Go?

A：

The code you are reading and citing is just dummy code to have consistent documentation. The built-in functions are, well, built into the language and, as such, are included in the code processing step (the compiler).

Simplified what happens is: lexer(詞法分析程序) will detect 'append(...)' asAPPENDtoken, parser will translateAPPEND, depending on the circumstances/parameters/environment to code, code is written as assembly and assembled(代碼被寫成匯編語言，然后進行編譯). The middle step - the implementation ofappend- can be found in the compilerhere.

What happens to anappendcall is best seen when looking at the assembly of an example program. Consider this:

b := []byte{'a'}
b = append(b, 'b')
println(string(b), cap(b))

Running it will yield the following output:

ab 2

Theappendcall is translated to assembly like this:

// create new slice object
MOVQ    BX, "".b+120(SP)       // BX contains data addr., write to b.addr
MOVQ    BX, CX                 // store addr. in CX
MOVQ    AX, "".b+128(SP)       // AX contains len(b) == 1, write to b.len
MOVQ    DI, "".b+136(SP)       // DI contains cap(b) == 1, write to b.cap
MOVQ    AX, BX                 // BX now contains len(b)
INCQ    BX                     // BX++
CMPQ    BX, DI                 // compare new length (2) with cap (1)
JHI $1, 225                    // jump to grow code if len > cap
...
LEAQ    (CX)(AX*1), BX         // load address of newly allocated slice entry
MOVB    $98, (BX)              // write 'b' to loaded address

// grow code, call runtime.growslice(t *slicetype, old slice, cap int)
LEAQ    type.[]uint8(SB), BP
MOVQ    BP, (SP)               // load parameters onto stack
MOVQ    CX, 8(SP)
MOVQ    AX, 16(SP)
MOVQ    SI, 24(SP)
MOVQ    BX, 32(SP)
PCDATA  $0, $0
CALL    runtime.growslice(SB)  // call
MOVQ    40(SP), DI
MOVQ    48(SP), R8
MOVQ    56(SP), SI
MOVQ    R8, AX
INCQ    R8
MOVQ    DI, CX
JMP 108                        // jump back, growing done

As you can see, noCALLstatement to a function calledappendcan be seen. This is the full implementation of theappendcall in the example code. Another call with different parameterswilllook differently (other registers, different parameters depending on the slice type, etc.).

---

參考：

https://stackoverflow.com/questions/33405327/where-is-the-implementation-of-func-append-in-go

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