JITting a function that writes the letter 'H' to a bytes buffer

go_jit.go

package main

import (
	"bytes"
	"fmt"

	"github.com/nelhage/gojit"
	"github.com/nelhage/gojit/amd64"
)

var abi amd64.ABI

func main() {
	// mmap executable memory
	buf, e := gojit.Alloc(gojit.PageSize * 4)
	if e != nil {
		panic(e)
	}

	// Output buffer
	wr := bytes.NewBuffer([]byte{})

	// Set up amd64 assembler
	asm := &amd64.Assembler{Buf: buf, ABI: abi}
	asm.Mov(amd64.Indirect{amd64.Rdi, 0, 64}, amd64.Rax)

	// Move 0x48 ('H' in hex) to %rax
	asm.Addb(amd64.Imm{int32(0x48)}, amd64.Indirect{amd64.Rax, 0, 8})

	// Save %rax
	asm.Push(amd64.Rax)
	// Setup arguments for call to `wr.Write` on stack
	asm.Sub(amd64.Imm{48}, amd64.Rsp)
	asm.Mov(amd64.Imm{1}, amd64.Indirect{amd64.Rsp, 16, 64})
	asm.Mov(amd64.Imm{1}, amd64.Indirect{amd64.Rsp, 8, 64})
	asm.Mov(amd64.Rax, amd64.Indirect{amd64.Rsp, 0, 64})
	asm.CallFunc(wr.Write)
	// Remove arguments from stack
	asm.Add(amd64.Imm{48}, amd64.Rsp)
	// Restore %rax
	asm.Pop(amd64.Rax)
	// Return from jitted function
	asm.Ret()

	// Cast executable memory into function
	var jittedFunc func()
	asm.BuildTo(&jittedFunc)

	// Call function
	jittedFunc()

	fmt.Println(wr.String())
}