Go Assembly by Example: Hello

The following example is a bad example. It was obtained by using the following command on a simple Go program printing the string “hello world”:

go tool compile -S helloworld.go

package main
//The underscore _ is to avoid an “unusued package” compiler error.
import _ "fmt"
func hello()
func main() {
    hello()
}

This #include is needed to use Go Assembly’s constants (for example the RODATA that we use later on). Other pre-processor macros can be used in Go Assembly (like #define).

#include "textflag.h"

DATA allows you to store global words in memory 1, 2, 4 or 8 bytes at a time. The <> after the symbol name restricts the data to the current file.

DATA world<>+0(SB)/8, $"hello wo"
DATA world<>+8(SB)/4, $"rld "
//GLOBL makes the data global and read-only (RODATA) for the relevant length (12).
GLOBL world<>+0(SB), RODATA, $12
TEXT ·hello(SB),$88-0
    SUBQ    $88, SP
    MOVQ    BP, 80(SP)
    LEAQ    80(SP), BP
//A Go string is created in my_string (a pointer to the string followed by the string’s length).
    LEAQ    world<>+0(SB), AX 
    MOVQ    AX, my_string+48(SP)        
    MOVQ    $11, my_string+56(SP)
    MOVQ    $0, autotmp_0+64(SP)
    MOVQ    $0, autotmp_0+72(SP)
    LEAQ    type·string(SB), AX
    MOVQ    AX, (SP)
    LEAQ    my_string+48(SP), AX        
    MOVQ    AX, 8(SP)
//convT2E from the runtime package is used to create an interface.
    CALL    runtime·convT2E(SB)           
    MOVQ    24(SP), AX
    MOVQ    16(SP), CX                    
    MOVQ    CX, autotmp_0+64(SP)        
    MOVQ    AX, autotmp_0+72(SP)
    LEAQ    autotmp_0+64(SP), AX        
    MOVQ    AX, (SP)                      
    MOVQ    $1, 8(SP)                      
    MOVQ    $1, 16(SP)
//fmt.Println is called with the interface created
    CALL    fmt·Println(SB)
//This is pretty complicated, the lesson here is: don’t try to call functions from Go’s assembly.
    MOVQ 80(SP), BP
    ADDQ $88, SP
    RET

Next example: Arcsin.