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Optimized version of JIT interpreter using LuaJIT with FFI
local string = require('string')
local ffi = require('ffi')
local JUMP_IF_A = 1
local MOV_A_R = 2
local MOV_R_A = 3
local ADD_R_TO_A = 4
local DECR_A = 5
local RETURN_A = 6
local function jit_interpret(bytecode, a)
f_str = [[
local ffi = require('ffi')
function _jit(a)
local la = ffi.cast('int32_t',a)
]]
for r=1,16 do
f_str = f_str .. string.format('local reg%d = 0LL\n',r)
end
local pc = 1
while pc < #bytecode + 1 do
local opcode = bytecode[pc]
f_str = f_str .. string.format('::op_%d::\n', pc)
pc = pc + 1
if opcode == JUMP_IF_A then
local target = bytecode[pc]
pc = pc + 1
f_str = f_str .. string.format([[
if la == 0 then
goto op_%d
end
goto op_%d
]], pc, target)
elseif opcode == MOV_R_A then
local n = bytecode[pc]
pc = pc + 1
f_str = f_str .. string.format([[
la = reg%d
]], n)
elseif opcode == MOV_A_R then
local n = bytecode[pc]
pc = pc + 1
f_str = f_str .. string.format([[
reg%d = la
]], n)
elseif opcode == ADD_R_TO_A then
local n = bytecode[pc]
pc = pc + 1
f_str = f_str .. string.format([[
la = la + reg%d
]], n)
elseif opcode == DECR_A then
f_str = f_str .. string.format([[
la = la - 1
]])
elseif opcode == RETURN_A then
f_str = f_str .. string.format([[
return la
]])
end
end
f_str = f_str .. 'end\n'
print('Generated lua code\n', f_str)
loadstring(f_str)()
return _jit(a)
end
-- __________ Entry point __________
local function entry_point(argv)
-- the program we want to interpret
-- it computes the square of its argument
bytecode = {
MOV_A_R, 1, -- i = a
MOV_A_R, 2, -- copy of 'a'
-- 5:
MOV_R_A, 1, -- i--
DECR_A,
MOV_A_R, 1,
MOV_R_A, 3, -- res += a
ADD_R_TO_A, 2,
MOV_A_R, 3,
MOV_R_A, 1, -- if i!=0: goto 5
JUMP_IF_A, 5,
MOV_R_A, 3,
RETURN_A
}
-- result = interpret(bytecode, tonumber(argv[1]))
result = jit_interpret(bytecode, tonumber(argv[1]))
print(result)
end
-- main function
entry_point(arg)
@romix
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Author

romix commented Jul 16, 2013

There are two optimizations:

  1. Use 64bit integers for registers
  2. Treat a as an integer variable by explicitly casting it to an integer

This code performs way better than the version published here:
https://github.com/rjpower/luajit-interpreter/blob/master/toy-jit-ffi.lua

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