Go Assembly by Example: Add

package main
import "fmt"
// the function’s body is empty
func add(x, y int64) int64
func main() {
    fmt.Println(add(2, 3))
}

We declare the add function via the TEXT package_name · function_name(SB),$frame_size-arguments_size pattern. Notice that the package name is empty here, corresponding to the current package, and a middle point · is used (U+00B7) not a period. The frame size of $0 at the end indicates the stack space needed (none, we’ll just use registers), while the arguments to the function and the return value take 3*8 bytes in total.

TEXT ·add(SB),$0-24

The MOVQ instruction is used to move a 64-bit value (Q stands for QUADWORD) around. Here from an offset of the frame pointer FP (used to refer to function arguments) to a register (BX and BP). The syntax symbol+offset(register) is used, where (register) is the address pointed by the register. For example, on the second line the content at *(FP + 8) is moved into BP. Note that x and y are the arguments’ names from the function’s prototype.

    MOVQ x+0(FP), BX
    MOVQ y+8(FP), BP

The ADDQ instruction is used to add the two 64-bit registers together, it then stores the result in BX.

    ADDQ BP, BX

The result is moved at address *(FP + 16) which is the address of the return value (positioned after all the arguments). Note that we named the symbol ret, it might seem useless but a symbol is always required by the compiler.

    MOVQ BX, ret+16(FP)

The last instruction simply returns to the caller.

    RET

Next example: Hello.