Where is the implementation of func append in Go?

Question

I'm very interested in go, and trying to read go function's implementations. I found some of these function doesn't have implementations there.

Such as append or call:

// The append built-in function appends elements to the end of a slice. If
// it has sufficient capacity, the destination is resliced to accommodate the
// new elements. If it does not, a new underlying array will be allocated.
// Append returns the updated slice. It is therefore necessary to store the
// result of append, often in the variable holding the slice itself:
//  slice = append(slice, elem1, elem2)
//  slice = append(slice, anotherSlice...)
// As a special case, it is legal to append a string to a byte slice, like this:
//  slice = append([]byte("hello "), "world"...)
func append(slice []Type, elems ...Type) []Type

// call calls fn with a copy of the n argument bytes pointed at by arg.
// After fn returns, reflectcall copies n-retoffset result bytes
// back into arg+retoffset before returning. If copying result bytes back,
// the caller must pass the argument frame type as argtype, so that
// call can execute appropriate write barriers during the copy.
func call(argtype *rtype, fn, arg unsafe.Pointer, n uint32, retoffset uint32)

It seems not calling a C code, because using cgo needs some special comments. Where is these function's implementations?

Answer 1

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 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. 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.).