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1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
2<html>
3<head>
4<title>LuaJIT Features</title>
5<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
6<meta name="Author" content="Mike Pall">
7<meta name="Copyright" content="Copyright (C) 2005-2011, Mike Pall">
8<meta name="Language" content="en">
9<link rel="stylesheet" type="text/css" href="bluequad.css" media="screen">
10<link rel="stylesheet" type="text/css" href="bluequad-print.css" media="print">
11</head>
12<body>
13<div id="site">
14<a href="http://luajit.org/"><span>Lua<span id="logo">JIT</span></span></a>
15</div>
16<div id="head">
17<h1>LuaJIT Features</h1>
18</div>
19<div id="nav">
20<ul><li>
21<a href="index.html">Index</a>
22</li><li>
23<a href="luajit.html">LuaJIT</a>
24<ul><li>
25<a class="current" href="luajit_features.html">Features</a>
26</li><li>
27<a href="luajit_install.html">Installation</a>
28</li><li>
29<a href="luajit_run.html">Running</a>
30</li><li>
31<a href="luajit_api.html">API Extensions</a>
32</li><li>
33<a href="luajit_intro.html">Introduction</a>
34</li><li>
35<a href="luajit_performance.html">Performance</a>
36</li><li>
37<a href="luajit_debug.html">Debugging</a>
38</li><li>
39<a href="luajit_changes.html">Changes</a>
40</li></ul>
41</li><li>
42<a href="coco.html">Coco</a>
43<ul><li>
44<a href="coco_portability.html">Portability</a>
45</li><li>
46<a href="coco_api.html">API Extensions</a>
47</li><li>
48<a href="coco_changes.html">Changes</a>
49</li></ul>
50</li><li>
51<a href="dynasm.html">DynASM</a>
52<ul><li>
53<a href="dynasm_features.html">Features</a>
54</li><li>
55<a href="dynasm_examples.html">Examples</a>
56</li></ul>
57</li><li>
58<a href="http://luajit.org/download.html">Download <span class="ext">&raquo;</span></a>
59</li></ul>
60</div>
61<div id="main">
62<p>
63LuaJIT tries to keep the spirit of Lua &mdash; it's <em>light-weight</em>,
64<em>efficient</em> and <em>extensible</em>.
65</p>
66
67<h2>Features</h2>
68<p>
69All functions are by default compiled Just-In-Time (JIT) to
70machine code:
71</p>
72<ul>
73<li>Functions that are unused are not compiled at all.</li>
74<li>Compilation can be enabled/disabled selectively for individual
75functions and subfunctions or even whole modules.</li>
76<li>Interpreted and compiled functions can be freely mixed.</li>
77</ul>
78<p>
79Ahead-Of-Time (AOT) compilation (at runtime) is supported, too:
80</p>
81<ul>
82<li>A number of API functions and command line options allows
83full user control over the compilation process.</li>
84</ul>
85<p>
86The JIT compiler is extensible:
87</p>
88<ul>
89<li>The optimizer is an extra module that attaches to the compiler
90pipeline.</li>
91<li>Various modules provide trace and debug information about
92the compilation process.</li>
93<li>All of these features can be activated with command line options.</li>
94</ul>
95
96<h2>Performance</h2>
97<p>
98The compiled machine code is <em>very efficient</em>:
99</p>
100<ul>
101<li>Have a look at some
102<a href="luajit_performance.html">Performance Measurements</a>.</li>
103<li>Aggressive optimizations (specialization, inlining) are enabled
104wherever possible. Inlined contracts catch wrong optimizer predictions
105at runtime (undetected polymorphism).</li>
106<li>Adaptive deoptimization is used to recompile individual bytecode
107instructions with broken contracts. This avoids generating code for the
108generic fallback cases most of the time (faster compilation, reduced
109I-cache contention).</li>
110<li>Special CPU features (such as conditional moves or SSE2)
111are automatically used when detected.</li>
112</ul>
113<p>
114The JIT compiler is <em>very fast</em>:
115</p>
116<ul>
117<li>Compilation times vary a great deal (depending on the nature of
118the function to be compiled) but are generally in the
119<em>microsecond</em> range.</li>
120<li>Even compiling large functions (hundreds of lines) with the
121maximum optimization level takes only a few milliseconds in the
122worst case.</li>
123</ul>
124<p>
125LuaJIT is <em>very small</em>:
126</p>
127<ul>
128<li>The whole JIT compiler engine adds only around <strong>32K</strong>
129of code to the Lua core (if compiled with <tt>-Os</tt>).</li>
130<li>The optimizer is split into several optional modules that
131can be loaded at runtime if requested.</li>
132<li>LuaJIT adds around 6.000 lines of C and assembler code and
1332.000 lines of Lua code to the Lua 5.1 core (17.000 lines of C).</li>
134<li>Required build tools (<a href="dynasm.html">DynASM</a>)
135take another 2.500 lines of Lua code.</li>
136</ul>
137
138<h2>Compatibility</h2>
139<p>
140LuaJIT is designed to be <em>fully compatible</em> with Lua 5.1.
141It accepts the same source code and/or precompiled bytecode.
142It supports all standard language semantics. In particular:
143</p>
144<ul>
145<li>All standard types, operators and metamethods are supported.</li>
146<li>Implicit type coercions (number/string) work as expected.</li>
147<li>Full IEEE-754 semantics for floating point arithmetics
148(NaN, +-Inf, +-0, ...).</li>
149<li>Full support for lexical closures.
150Proper tail calls do not consume a call frame.</li>
151<li>Exceptions are precise. Backtraces work fine.</li>
152<li>Coroutines are supported with the help of <a href="coco.html">Coco</a>.</li>
153<li>No changes to the Lua 5.1 incremental garbage collector.</li>
154<li>No changes to the standard Lua/C API.</li>
155<li>Dynamically loaded C modules are link compatible with Lua 5.1
156(same ABI).</li>
157<li>LuaJIT can be <a href="luajit_install.html#embedding">embedded</a>
158into an application just like Lua.</li>
159</ul>
160<p>
161Some minor differences are related to debugging:
162</p>
163<ul>
164<li>Debug hooks are only called if debug code generation is enabled.</li>
165<li>There is no support for tailcall counting in JIT compiled code.
166HOOKTAILRET is not called, too. Note: this won't affect you unless
167you are writing a Lua debugger. <sup>*</sup></li>
168</ul>
169<p style="font-size: 80%;">
170<sup>*</sup> There is not much I can do to improve this situation without undue
171complications. A suggestion to modify the behaviour of standard Lua
172has been made on the mailing list (it would be beneficial there, too).
173</p>
174
175<h2>Restrictions</h2>
176<ul>
177<li>Only x86 (i386+) CPUs are supported right now (but see below).</li>
178<li>Only the default type for <tt>lua_Number</tt> is supported
179(<tt>double</tt>).</li>
180<li>The interrupt signal (Ctrl-C) is ignored unless you enable
181debug hooks (with <tt>-j debug</tt>). But this will seriously
182slow down your application. I'm looking for better ways to handle
183this. In the meantime you have to press Ctrl-C twice to interrupt
184a currently running JIT compiled function (just like C functions).</li>
185<li>GDB, Valgrind and other debugging tools can't report symbols
186or stack frames for JIT compiled code. This is rather difficult to solve.
187Have a look at <a href="luajit_debug.html">Debugging LuaJIT</a>, too.</li>
188</ul>
189
190<h2>Caveats</h2>
191<ul>
192<li>LuaJIT allocates executable memory for the generated machine code
193if your OS has support for it: either <tt>HeapCreate()</tt> for Windows or
194<tt>mmap()</tt> on POSIX systems.<br>
195The fallback is the standard Lua allocator (i.e. malloc()).
196But this usually means the allocated memory is not marked executable.
197Running compiled code will trap on CPUs/OS with the NX (No eXecute)
198extension <em>if you can only use the fallback</em>.</li>
199<li><a href="dynasm.html">DynASM</a> is needed to regenerate the
200<tt>ljit_x86.h</tt> file. But only in case you want to <em>modify</em>
201the <tt>*.dasc</tt>/<tt>*.dash</tt> files. A pre-processed <tt>*.h</tt>
202file is supplied with LuaJIT.<br>
203DynASM is written in Lua and needs a plain copy of Lua 5.1
204(installed as <tt>lua</tt>). Or you can run it with LuaJIT built from
205the <tt>*.h</tt> file supplied with the distribution (modify
206<tt>DASM=</tt> in <tt>src/Makefile</tt>). It's a good idea to install
207a known good copy of LuaJIT under a different name for this.</li>
208<li>LuaJIT ships with <tt>LUA_COMPAT_VARARG</tt> turned off.
209I.e. the implicit <tt>arg</tt> parameter is not created anymore.
210Please have a look at the comments in <tt>luaconf.h</tt> for
211this configuration option. You can turn it on, if you really need it.
212Or better yet, convert your code to the new Lua 5.1 vararg syntax.</li>
213</ul>
214
215<br class="flush">
216</div>
217<div id="foot">
218<hr class="hide">
219Copyright &copy; 2005-2011 Mike Pall
220<span class="noprint">
221&middot;
222<a href="contact.html">Contact</a>
223</span>
224</div>
225</body>
226</html>
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