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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"><html><head><title>LuaJIT Features</title><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><meta name="Author" content="Mike Pall"><meta name="Copyright" content="Copyright (C) 2005-2008, Mike Pall"><meta name="Language" content="en"><link rel="stylesheet" type="text/css" href="bluequad.css" media="screen"><link rel="stylesheet" type="text/css" href="bluequad-print.css" media="print"></head><body><div id="site"><a href="http://luajit.org/"><span>Lua<span id="logo">JIT</span></span></a></div><div id="head"><h1>LuaJIT Features</h1></div><div id="nav"><ul><li><a href="index.html">Index</a></li><li><a href="luajit.html">LuaJIT</a><ul><li><a class="current" href="luajit_features.html">Features</a></li><li><a href="luajit_install.html">Installation</a></li><li><a href="luajit_run.html">Running</a></li><li><a href="luajit_api.html">API Extensions</a></li><li><a href="luajit_intro.html">Introduction</a></li><li><a href="luajit_performance.html">Performance</a></li><li><a href="luajit_debug.html">Debugging</a></li><li><a href="luajit_changes.html">Changes</a></li></ul></li><li><a href="coco.html">Coco</a><ul><li><a href="coco_portability.html">Portability</a></li><li><a href="coco_api.html">API Extensions</a></li><li><a href="coco_changes.html">Changes</a></li></ul></li><li><a href="dynasm.html">DynASM</a><ul><li><a href="dynasm_features.html">Features</a></li><li><a href="dynasm_examples.html">Examples</a></li></ul></li><li><a href="http://luajit.org/download.html">Download <span class="ext">»</span></a></li></ul></div><div id="main"><p>LuaJIT tries to keep the spirit of Lua — it's <em>light-weight</em>,<em>efficient</em> and <em>extensible</em>.</p><h2>Features</h2><p>All functions are by default compiled Just-In-Time (JIT) tomachine code:</p><ul><li>Functions that are unused are not compiled at all.</li><li>Compilation can be enabled/disabled selectively for individualfunctions and subfunctions or even whole modules.</li><li>Interpreted and compiled functions can be freely mixed.</li></ul><p>Ahead-Of-Time (AOT) compilation (at runtime) is supported, too:</p><ul><li>A number of API functions and command line options allowsfull user control over the compilation process.</li></ul><p>The JIT compiler is extensible:</p><ul><li>The optimizer is an extra module that attaches to the compilerpipeline.</li><li>Various modules provide trace and debug information aboutthe compilation process.</li><li>All of these features can be activated with command line options.</li></ul><h2>Performance</h2><p>The compiled machine code is <em>very efficient</em>:</p><ul><li>Have a look at some<a href="luajit_performance.html">Performance Measurements</a>.</li><li>Aggressive optimizations (specialization, inlining) are enabledwherever possible. Inlined contracts catch wrong optimizer predictionsat runtime (undetected polymorphism).</li><li>Adaptive deoptimization is used to recompile individual bytecodeinstructions with broken contracts. This avoids generating code for thegeneric fallback cases most of the time (faster compilation, reducedI-cache contention).</li><li>Special CPU features (such as conditional moves or SSE2)are automatically used when detected.</li></ul><p>The JIT compiler is <em>very fast</em>:</p><ul><li>Compilation times vary a great deal (depending on the nature ofthe function to be compiled) but are generally in the<em>microsecond</em> range.</li><li>Even compiling large functions (hundreds of lines) with themaximum optimization level takes only a few milliseconds in theworst case.</li></ul><p>LuaJIT is <em>very small</em>:</p><ul><li>The whole JIT compiler engine adds only around <strong>32K</strong>of code to the Lua core (if compiled with <tt>-Os</tt>).</li><li>The optimizer is split into several optional modules thatcan be loaded at runtime if requested.</li><li>LuaJIT adds around 6.000 lines of C and assembler code and2.000 lines of Lua code to the Lua 5.1 core (17.000 lines of C).</li><li>Required build tools (<a href="dynasm.html">DynASM</a>)take another 2.500 lines of Lua code.</li></ul><h2>Compatibility</h2><p>LuaJIT is designed to be <em>fully compatible</em> with Lua 5.1.It accepts the same source code and/or precompiled bytecode.It supports all standard language semantics. In particular:</p><ul><li>All standard types, operators and metamethods are supported.</li><li>Implicit type coercions (number/string) work as expected.</li><li>Full IEEE-754 semantics for floating point arithmetics(NaN, +-Inf, +-0, ...).</li><li>Full support for lexical closures.Proper tail calls do not consume a call frame.</li><li>Exceptions are precise. Backtraces work fine.</li><li>Coroutines are supported with the help of <a href="coco.html">Coco</a>.</li><li>No changes to the Lua 5.1 incremental garbage collector.</li><li>No changes to the standard Lua/C API.</li><li>Dynamically loaded C modules are link compatible with Lua 5.1(same ABI).</li><li>LuaJIT can be <a href="luajit_install.html#embedding">embedded</a>into an application just like Lua.</li></ul><p>Some minor differences are related to debugging:</p><ul><li>Debug hooks are only called if debug code generation is enabled.</li><li>There is no support for tailcall counting in JIT compiled code.HOOKTAILRET is not called, too. Note: this won't affect you unlessyou are writing a Lua debugger. <sup>*</sup></li></ul><p style="font-size: 80%;"><sup>*</sup> There is not much I can do to improve this situation without unduecomplications. A suggestion to modify the behaviour of standard Luahas been made on the mailing list (it would be beneficial there, too).</p><h2>Restrictions</h2><ul><li>Only x86 (i386+) CPUs are supported right now (but see below).</li><li>Only the default type for <tt>lua_Number</tt> is supported(<tt>double</tt>).</li><li>The interrupt signal (Ctrl-C) is ignored unless you enabledebug hooks (with <tt>-j debug</tt>). But this will seriouslyslow down your application. I'm looking for better ways to handlethis. In the meantime you have to press Ctrl-C twice to interrupta currently running JIT compiled function (just like C functions).</li><li>GDB, Valgrind and other debugging tools can't report symbolsor stack frames for JIT compiled code. This is rather difficult to solve.Have a look at <a href="luajit_debug.html">Debugging LuaJIT</a>, too.</li></ul><h2>Caveats</h2><ul><li>LuaJIT allocates executable memory for the generated machine codeif your OS has support for it: either <tt>HeapCreate()</tt> for Windows or<tt>mmap()</tt> on POSIX systems.<br>The fallback is the standard Lua allocator (i.e. malloc()).But this usually means the allocated memory is not marked executable.Running compiled code will trap on CPUs/OS with the NX (No eXecute)extension <em>if you can only use the fallback</em>.</li><li><a href="dynasm.html">DynASM</a> is needed to regenerate the<tt>ljit_x86.h</tt> file. But only in case you want to <em>modify</em>the <tt>*.dasc</tt>/<tt>*.dash</tt> files. A pre-processed <tt>*.h</tt>file is supplied with LuaJIT.<br>DynASM is written in Lua and needs a plain copy of Lua 5.1(installed as <tt>lua</tt>). Or you can run it with LuaJIT built fromthe <tt>*.h</tt> file supplied with the distribution (modify<tt>DASM=</tt> in <tt>src/Makefile</tt>). It's a good idea to installa known good copy of LuaJIT under a different name for this.</li><li>LuaJIT ships with <tt>LUA_COMPAT_VARARG</tt> turned off.I.e. the implicit <tt>arg</tt> parameter is not created anymore.Please have a look at the comments in <tt>luaconf.h</tt> forthis configuration option. You can turn it on, if you really need it.Or better yet, convert your code to the new Lua 5.1 vararg syntax.</li></ul><h2>TODOs</h2><ul><li>Work on LuaJIT 2.x is in progress. For more details please checkthe <a href="http://lua-users.org/lists/lua-l/2008-02/msg00051.html"><span class="ext">»</span> LuaJIT roadmap 2008</a>.</li></ul><br class="flush"></div><div id="foot"><hr class="hide">Copyright © 2005-2008 Mike Pall<span class="noprint">·<a href="contact.html">Contact</a></span></div></body></html>⌨️ 快捷键说明
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