jit.lua

local ffi = require("ffi")

ffi.cdef[[
    char *mmap(void *addr, size_t length, int prot, int flags, int fd, long int offset);
    int munmap(void *addr, size_t length);
]]

local PROT_READ = 0x1 -- Page can be read.
local PROT_WRITE = 0x2 -- Page can be written.
local PROT_EXEC = 0x4 -- Page can be executed.
local PROT_NONE = 0x0 -- Page can not be accessed.
local PROT_GROWSDOWN = 0x01000000 -- Extend change to start of growsdown vma (mprotect only).
local PROT_GROWSUP = 0x02000000 -- Extend change to start of growsup vma (mprotect only).
local MAP_SHARED = 0x01 -- Share changes.
local MAP_PRIVATE = 0x02
local MAP_ANONYMOUS = 0x20

local memory = ffi.C.mmap(nil, 4096, bit.bor(PROT_READ, PROT_WRITE, PROT_EXEC), bit.bor(MAP_PRIVATE, MAP_ANONYMOUS), -1, 0)

if memory == nil then
    error("failed to map memory")
end

local pos = 0

local function write(bytes)
    ffi.copy(memory + pos, bytes, #bytes)
    pos = pos + #bytes
end

write("\x48\x8b\xc7") -- mov %rdi, %rax
write("\xc3") -- ret

local func = ffi.cast("long (*)(long)", memory)

for i = 0, 9 do
    logf("f(%d) = %d\n", i, tonumber(func(i)))
end

ffi.C.munmap(memory, 4096)