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LuaJIT
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<title>Status &amp; Roadmap</title>
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<a href="http://luajit.org"><span>Lua<span id="logo">JIT</span></span></a>
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<h1>Status &amp; Roadmap</h1>
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<p>
The <span style="color: #0000c0;">LuaJIT 1.x</span> series represents
the current <span style="color: #0000c0;">stable branch</span>. As of
this writing there have been no open bugs since about a year. So, if
you need a rock-solid VM, you are encouraged to fetch the latest
release of LuaJIT 1.x from the <a href="http://luajit.org/download.html"><span class="ext">&raquo;</span>&nbsp;Download</a>
page.
</p>
<p>
<span style="color: #c00000;">LuaJIT 2.0</span> is the currently active
<span style="color: #c00000;">development branch</span>.
It has <b>Beta Test</b> status and is still undergoing
substantial changes. It's expected to quickly mature within the next
months. You should definitely start to evaluate it for new projects
right now. But deploying it in production environments is not yet
recommended.
</p>

<h2>Current Status</h2>
<p>
This is a list of the things you should know about the LuaJIT 2.0 beta test:
</p>
<ul>
<li>
The JIT compiler can only generate code for CPUs with <b>SSE2</b> at the
moment. I.e. you need at least a P4, Core 2/i5/i7 or K8/K10 to use it. I
plan to fix this during the beta phase and add support for emitting x87
instructions to the backend.
</li>
<li>
Obviously there will be many <b>bugs</b> in a VM which has been
rewritten from the ground up. Please report your findings together with
the circumstances needed to reproduce the bug. If possible reduce the
problem down to a simple test cases.<br>
There is no formal bug tracker at the moment. The best place for
discussion is the
<a href="http://www.lua.org/lua-l.html"><span class="ext">&raquo;</span>&nbsp;Lua mailing list</a>. Of course
you may also send your bug report directly to me, especially when they
contains lengthy debug output. Please check the
<a href="contact.html">Contact</a> page for details.
</li>
<li>
The VM is complete in the sense that it <b>should</b> run all Lua code
just fine. It's considered a serious bug if the VM crashes or produces
unexpected results &mdash; please report it. There are only very few
known incompatibilities with standard Lua:
<ul>
<li>
The Lua <b>debug API</b> is missing a couple of features (call/return
hooks) and shows slightly different behavior (no per-coroutine hooks).
</li>
<li>
Most other issues you're likely to find (e.g. with the existing test
suites) are differences in the <b>implementation-defined</b> behavior.
These either have a good reason (like early tail call resolving which
may cause differences in error reporting), are arbitrary design choices
or are due to quirks in the VM. The latter cases may get fixed if a
demonstrable need is shown.
</li>
</ul>
</li>
<li>
The <b>JIT compiler</b> is not complete (yet) and falls back to the
interpreter in some cases. All of this works transparently, so unless
you use -jv, you'll probably never notice (the interpreter is quite
fast, too). Here are the known issues:
<ul>
<li>
Many known issues cause a <b>NYI</b> (not yet implemented) trace abort
message. E.g. for calls to vararg functions or many string library
functions. Reporting these is only mildly useful, except if you have good
example code that shows the problem. Obviously, reports accompanied with
a patch to fix the issue are more than welcome. But please check back
with me, before writing major improvements, to avoid duplication of
effort.
</li>
<li>
<b>Recursion</b> is not traced yet. Often no trace will be generated at
all or some unroll limit will catch it and aborts the trace.
</li>
<li>
The trace compiler currently does not back off specialization for
function call dispatch. It should really fall back to specializing on
the prototype, not the closure identity. This can lead to the so-called
"trace explosion" problem with <b>closure-heavy programming</b>. The
trace linking heuristics prevent this, but in the worst case this
means the code always falls back to the interpreter.
</li>
<li>
<b>Trace management</b> needs more tuning: better blacklisting of aborted
traces, less drastic countermeasures against trace explosion and better
heuristics in general.
</li>
<li>
Some checks are missing in the JIT-compiled code for obscure situations
with <b>open upvalues aliasing</b> one of the SSA slots later on (or
vice versa). Bonus points, if you can find a real world test case for
this.
</li>
</ul>
</li>
</ul>

<h2>Roadmap</h2>
<p>
Rather than stating exact release dates (I'm well known for making
spectacularly wrong guesses), this roadmap lists the general project
plan, sorted by priority, as well as ideas for the future:
</p>
<ul>
<li>
The main goal right now is to stabilize LuaJIT 2.0 and get it out of
beta test. <b>Correctness</b> has priority over completeness. This
implies the first stable release will certainly NOT compile every
library function call and will fall back to the interpreter from time
to time. This is perfectly ok, since it still executes all Lua code,
just not at the highest possible speed.
</li>
<li>
The next step is to get it to compile more library functions and handle
more cases where the compiler currently bails out. This doesn't mean it
will compile every corner case. It's much more important that it
performs well in a majority of use cases. Every compiler has to make
these trade-offs &mdash; <b>completeness</b> just cannot be the
overriding goal for a low-footprint, low-overhead JIT compiler.
</li>
<li>
More <b>optimizations</b> will be added in parallel to the last step on
an as-needed basis. Array-bounds-check (ABC) removal, sinking of stores
to aggregates and sinking of allocations are high on the list. Faster
handling of NEWREF and better alias analysis are desirable, too. More
complex optimizations with less pay-off, such as value-range-propagation
(VRP) will have to wait.
</li>
<li>
LuaJIT 2.0 has been designed with <b>portability</b> in mind.
Nonetheless, it compiles to native code and needs to be adapted to each
architecture. Porting the compiler backend is probably the easier task,
but a key element of its design is the fast interpreter, written in
machine-specific assembler.<br>
The code base and the internal structures are already prepared for
easier porting to 64 bit architectures. The most likely next target is a
port to <b>x64</b>, but this will have to wait until the x86 port
stabilizes. Other ports will follow &mdash; companies which are
interested in sponsoring a port to a particular architecture, please
<a href="contact.html">contact me</a>.
</li>
<li>
There are some planned <b>structural improvements</b> to the compiler,
like compressed snapshot maps or generic handling of calls to helper
methods. These are of lesser importance, unless other developments
elevate their priority.
</li>
<li>
<b>Documentation</b> about the <b>internals</b> of LuaJIT is still sorely
missing. Although the source code is included and is IMHO well
commented, many basic design decisions are in need of an explanation.
The rather un-traditional compiler architecture and the many highly
optimized data structures are a barrier for outside participation in
the development. Alas, as I've repeatedly stated, I'm better at
writing code than papers and I'm not in need of any academical merits.
Someday I will find the time for it. :-)
</li>
<li>
Producing good code for unbiased branches is a key problem for trace
compilers. This is the main cause for "trace explosion".
<b>Hyperblock scheduling</b> promises to solve this nicely at the
price of a major redesign of the compiler. This would also pave the
way for emitting predicated instructions, which is a prerequisite
for efficient <b>vectorization</b>.
</li>
<li>
Currently Lua is missing a standard library for access to <b>structured
binary data</b> and <b>arrays/buffers</b> holding low-level data types.
Allowing calls to arbitrary C functions (<b>FFI</b>) would obviate the
need to write manual bindings. A variety of extension modules is floating
around, with different scope and capabilities. Alas, none of them has been
designed with a JIT compiler in mind.
</li>
</ul>
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