Question

Is it possible to include inline assembly in Go code?

This blog post shows compiling Go to a separate .s file and editing it, but not inline asm as part of a Go function like many C compilers support.

Answer 1

There is no support for inline assembly, but you can link with code written in assembly through C, compiling with cgo and using import "C", like in gmp.go. You could alternatively write in a style of assembly that is directly compatible with Go, like in asm_linux_amd64.s, that requires that function names start with “·”.

Or, you can use nasm and gccgo, my favorite way so far. (Note that Nasm does not seem to support functions starting with “·”).

Here’s a working “hello world” example:

hello.asm:

; Based on hello.asm from nasm

    SECTION .data       ; data section
msg:    db "Hello World",10 ; the string to print, 10=cr
len:    equ $-msg       ; "$" means "here"
                ; len is a value, not an address

    SECTION .text       ; code section

global go.main.hello        ; make label available to linker (Go)
go.main.hello:

    ; --- setup stack frame
    push rbp            ; save old base pointer
    mov rbp,rsp   ; use stack pointer as new base pointer

    ; --- print message
    mov edx,len     ; arg3, length of string to print
    mov ecx,msg     ; arg2, pointer to string
    mov ebx,1       ; arg1, where to write, screen
    mov eax,4       ; write sysout command to int 80 hex
    int 0x80        ; interrupt 80 hex, call kernel

    ; --- takedown stack frame
    mov rsp,rbp  ; use base pointer as new stack pointer
    pop rbp      ; get the old base pointer

    ; --- return
    mov rax,0       ; error code 0, normal, no error
    ret         ; return

main.go:

package main

func hello();

func main() {
    hello()
    hello()
}

And a handy Makefile:

main: main.go hello.o
    gccgo hello.o main.go -o main

hello.o: hello.asm
    nasm -f elf64 -o hello.o hello.asm

clean:
    rm -rf _obj *.o *~ *.6 *.gch a.out main

I call hello() twice in main.go, just to double check that hello() returns properly.

Note that calling interrupt 80h directly is not considered good style on Linux, and calling functions written is C is more “future proof”. Also note that this is assembly specifically for 64-bit Linux, and is not platform-independent in any way, shape or form.

I know it’s not a direct answer to your question, but it’s the easiest route I know for using assembly with Go, in lack of inlining. If you really need inlining, it’s possible to write a script that extracts inline assembly from source files and prepares it in a way that follows the pattern above. Close enough? 🙂

Quick example for Go, C and Nasm: gonasm.tgz

Update: Later versions of gccgo needs the -g flag and only “main.hello” is needed instead of “go.main.hello”. Here is an updated example for Go, C and Yasm: goyasm.tgz