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4 <title>Status &amp; Roadmap</title>
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17 <a href="http://luajit.org"><span>Lua<span id="logo">JIT</span></span></a>
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20 <h1>Status &amp; Roadmap</h1>
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60 <p>
61 The <span style="color: #0000c0;">LuaJIT 1.x</span> series represents
62 the current <span style="color: #0000c0;">stable branch</span>.
63 Only a single bug has been discovered in the last two years. So, if
64 you need a rock-solid VM, you are encouraged to fetch the latest
65 release of LuaJIT 1.x from the <a href="http://luajit.org/download.html"><span class="ext">&raquo;</span>&nbsp;Download</a>
66 page.
67 </p>
68 <p>
69 <span style="color: #c00000;">LuaJIT 2.0</span> is the currently active
70 <span style="color: #c00000;">development branch</span>.
71 It has <b>Beta Test</b> status and is still undergoing
72 substantial changes.
73 It has <a href="http://luajit.org/performance.html"><span class="ext">&raquo;</span>&nbsp;much better performance</a> than LuaJIT 1.x.
74 It's maturing quickly, so you should definitely
75 start to evaluate it for new projects right now.
76 </p>
78 <h2>Current Status</h2>
79 <p>
80 This is a list of the things you should know about the LuaJIT 2.0 beta test:
81 </p>
82 <ul>
83 <li>
84 Obviously there will be some <b>bugs</b> in a VM which has been
85 rewritten from the ground up. Please report your findings together with
86 the circumstances needed to reproduce the bug. If possible, reduce the
87 problem down to a simple test case.<br>
88 There is no formal bug tracker at the moment. The best place for
89 discussion is the
90 <a href="http://www.lua.org/lua-l.html"><span class="ext">&raquo;</span>&nbsp;Lua mailing list</a>. Of course
91 you may also send your bug reports <a href="contact.html">directly to me</a>,
92 especially when they contain lengthy debug output or if you require
93 confidentiality.
94 </li>
95 <li>
96 The x86 JIT compiler only generates code for CPUs with support for
97 <b>SSE2</b> instructions. I.e. you need at least a P4, Core 2/i3/i5/i7,
98 Atom or K8/K10 to get the full benefit.<br>
99 If you run LuaJIT on older CPUs without SSE2 support, the JIT compiler
100 is disabled and the VM falls back to the LuaJIT interpreter. This is faster
101 than the Lua interpreter, but not nearly as fast as the JIT compiler of course.
102 Run the command line executable without arguments to show the current status
103 (<tt>JIT: ON</tt> or <tt>JIT: OFF</tt>).
104 </li>
105 <li>
106 The VM is complete in the sense that it <b>should</b> run all Lua code
107 just fine. It's considered a serious bug if the VM crashes or produces
108 unexpected results &mdash; please report this. There are only very few
109 known incompatibilities with standard Lua:
110 <ul>
111 <li>
112 The Lua <b>debug API</b> is missing a couple of features (return
113 hooks for non-Lua functions) and shows slightly different behavior
114 (no per-coroutine hooks, no tail call counting).
115 </li>
116 <li>
117 Some of the <b>configuration options</b> of Lua&nbsp;5.1 are not supported:
118 <ul>
119 <li>The <b>number type</b> cannot be changed (it's always a <tt>double</tt>).</li>
120 <li>The stand-alone executable cannot be linked with <b>readline</b>
121 to enable line editing. It's planned to add support for loading it
122 on-demand.</li>
123 </ul>
124 </li>
125 <li>
126 Most other issues you're likely to find (e.g. with the existing test
127 suites) are differences in the <b>implementation-defined</b> behavior.
128 These either have a good reason (like early tail call resolving which
129 may cause differences in error reporting), are arbitrary design choices
130 or are due to quirks in the VM. The latter cases may get fixed if a
131 demonstrable need is shown.
132 </li>
133 </ul>
134 </li>
135 <li>
136 The <b>JIT compiler</b> falls back to the
137 interpreter in some cases. All of this works transparently, so unless
138 you use <tt>-jv</tt>, you'll probably never notice (the interpreter is
139 <a href="http://luajit.org/performance.html"><span class="ext">&raquo;</span>&nbsp;quite fast</a>, too). Here are the known issues:
140 <ul>
141 <li>
142 Most known issues cause a <b>NYI</b> (not yet implemented) trace abort
143 message. E.g. for calls to some internal library
144 functions. Reporting these is only mildly useful, except if you have good
145 example code that shows the problem. Obviously, reports accompanied with
146 a patch to fix the issue are more than welcome. But please check back
147 with me, before writing major improvements, to avoid duplication of
148 effort.
149 </li>
150 <li>
151 Some checks are missing in the JIT-compiled code for obscure situations
152 with <b>open upvalues aliasing</b> one of the SSA slots later on (or
153 vice versa). Bonus points, if you can find a real world test case for
154 this.
155 </li>
156 <li>
157 Currently some <b>out-of-memory</b> errors from <b>on-trace code</b> are not
158 handled correctly. The error may fall through an on-trace
159 <tt>pcall</tt> (x86) or it may be passed on to the function set with
160 <tt>lua_atpanic</tt> (x64).
161 </li>
162 </ul>
163 </li>
164 </ul>
166 <h2>Roadmap</h2>
168 Please refer to the
169 <a href="http://lua-users.org/lists/lua-l/2011-01/msg01238.html"><span class="ext">&raquo;</span>&nbsp;LuaJIT
170 Roadmap 2011</a> for the latest release plan. Here's the general
171 project plan for LuaJIT 2.0:
172 </p>
173 <ul>
174 <li>
175 The main goal right now is to stabilize LuaJIT 2.0 and get it out of
176 beta test. <b>Correctness</b> has priority over completeness. This
177 implies the first stable release will certainly NOT compile every
178 library function call and will fall back to the interpreter from time
179 to time. This is perfectly ok, since it still executes all Lua code,
180 just not at the highest possible speed.
181 </li>
182 <li>
183 The next step is to get it to compile more library functions and handle
184 more cases where the compiler currently bails out. This doesn't mean it
185 will compile every corner case. It's much more important that it
186 performs well in a majority of use cases. Every compiler has to make
187 these trade-offs &mdash; <b>completeness</b> just cannot be the
188 overriding goal for a low-footprint, low-overhead JIT compiler.
189 </li>
190 <li>
191 More <b>optimizations</b> will be added in parallel to the last step on
192 an as-needed basis. Sinking of stores
193 to aggregates and sinking of allocations are high on the list.
194 More complex optimizations with less pay-off, such as value-range-propagation
195 (VRP) will have to wait.
196 </li>
197 <li>
198 LuaJIT 2.0 has been designed with <b>portability</b> in mind.
199 Nonetheless, it compiles to native code and needs to be adapted to each
200 architecture. The two major work items are porting the the fast interpreter,
201 which is written in assembler, and porting the compiler backend.
202 Most other portability issues like endianess or 32 vs. 64&nbsp;bit CPUs
203 have already been taken care of.<br>
204 Several ports are already available, thanks to the
205 <a href="http://luajit.org/sponsors.html"><span class="ext">&raquo;</span>&nbsp;LuaJIT sponsorship program</a>.
206 More ports will follow in the future &mdash; companies which are
207 interested in sponsoring a port to a particular architecture, please
208 use the given contact address.
209 </li>
210 <li>
211 <b>Documentation</b> about the <b>internals</b> of LuaJIT is still sorely
212 missing. Although the source code is included and is IMHO well
213 commented, many basic design decisions are in need of an explanation.
214 The rather un-traditional compiler architecture and the many highly
215 optimized data structures are a barrier for outside participation in
216 the development. Alas, as I've repeatedly stated, I'm better at
217 writing code than papers and I'm not in need of any academic merits.
218 Someday I will find the time for it. :-)
219 </li>
220 <li>
221 Producing good code for unbiased branches is a key problem for trace
222 compilers. This is the main cause for "trace explosion".
223 <b>Hyperblock scheduling</b> promises to solve this nicely at the
224 price of a major redesign of the compiler. This would also pave the
225 way for emitting predicated instructions, which is a prerequisite
226 for efficient <b>vectorization</b>.
227 </li>
228 </ul>
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