Go语言中append()函数的源码实现在哪里？

今天在学习Go的基础数据结构——slice，期间阅读了slice的内部实现，扩容策略等。然后想到，我们在调用append()函数进行追加元素是究竟发生了什么？于是就想看看append()函数的内部实现，结果源码里并没有找到，搜索一番，还是在StackOverflow上找到了答案。记录如下。

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(词法分析程序) will detect 'append(...)' as APPEND token, parser will translate APPEND, depending on the circumstances/parameters/environment to code, code is written as assembly and assembled(代码被写成汇编语言，然后进行编译). The middle step - the implementation of append - can be found in the compiler here.

What happens to an append call 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

The append call 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, no CALL statement to a function called append can be seen. This is the full implementation of the append call in the example code. Another call with different parameters will look differently (other registers, different parameters depending on the slice type, etc.).

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参考：

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