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   4 <title>Frequently Asked Questions (FAQ)</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>Frequently Asked Questions (FAQ)</h1>
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  64 <dl>
  65 <dt>Q: Where can I learn more about LuaJIT and Lua?</dt>
  66 <dd>
  67 <ul style="padding: 0;">
  68 <li>The <a href="http://luajit.org/list.html"><span class="ext">&raquo;</span>&nbsp;LuaJIT mailing list</a> focuses on topics
  69 related to LuaJIT.</li>
  70 <li>The <a href="http://wiki.luajit.org/"><span class="ext">&raquo;</span>&nbsp;LuaJIT wiki</a> gathers community
  71 resources about LuaJIT.</li>
  72 <li>News about Lua itself can be found at the
  73 <a href="http://www.lua.org/lua-l.html"><span class="ext">&raquo;</span>&nbsp;Lua mailing list</a>.
  74 The mailing list archives are worth checking out for older postings
  75 about LuaJIT.</li>
  76 <li>The <a href="http://lua.org"><span class="ext">&raquo;</span>&nbsp;main Lua.org site</a> has complete
  77 <a href="http://www.lua.org/docs.html"><span class="ext">&raquo;</span>&nbsp;documentation</a> of the language
  78 and links to books and papers about Lua.</li>
  79 <li>The community-managed <a href="http://lua-users.org/wiki/"><span class="ext">&raquo;</span>&nbsp;Lua Wiki</a>
  80 has information about diverse topics.</li>
  81 </ul>
  82 </dl>
  83
  84 <dl>
  85 <dt>Q: Where can I learn more about the compiler technology used by LuaJIT?</dt>
  86 <dd>
  87 I'm planning to write more documentation about the internals of LuaJIT.
  88 In the meantime, please use the following Google Scholar searches
  89 to find relevant papers:<br>
  90 Search for: <a href="http://scholar.google.com/scholar?q=Trace+Compiler"><span class="ext">&raquo;</span>&nbsp;Trace Compiler</a><br>
  91 Search for: <a href="http://scholar.google.com/scholar?q=JIT+Compiler"><span class="ext">&raquo;</span>&nbsp;JIT Compiler</a><br>
  92 Search for: <a href="http://scholar.google.com/scholar?q=Dynamic+Language+Optimizations"><span class="ext">&raquo;</span>&nbsp;Dynamic Language Optimizations</a><br>
  93 Search for: <a href="http://scholar.google.com/scholar?q=SSA+Form"><span class="ext">&raquo;</span>&nbsp;SSA Form</a><br>
  94 Search for: <a href="http://scholar.google.com/scholar?q=Linear+Scan+Register+Allocation"><span class="ext">&raquo;</span>&nbsp;Linear Scan Register Allocation</a><br>
  95 Here is a list of the <a href="http://article.gmane.org/gmane.comp.lang.lua.general/58908"><span class="ext">&raquo;</span>&nbsp;innovative features in LuaJIT</a>.<br>
  96 And, you know, reading the source is of course the only way to enlightenment. :-)
  97 </dd>
  98 </dl>
  99
 100 <dl>
 101 <dt>Q: Why do I get this error: "attempt to index global 'arg' (a nil value)"?<br>
 102 Q: My vararg functions fail after switching to LuaJIT!</dt>
 103 <dd>LuaJIT is compatible to the Lua 5.1 language standard. It doesn't
 104 support the implicit <tt>arg</tt> parameter for old-style vararg
 105 functions from Lua 5.0.<br>Please convert your code to the
 106 <a href="http://www.lua.org/manual/5.1/manual.html#2.5.9"><span class="ext">&raquo;</span>&nbsp;Lua 5.1
 107 vararg syntax</a>.</dd>
 108 </dl>
 109
 110 <dl>
 111 <dt>Q: Why do I get this error: "bad FPU precision"?<br>
 112 <dt>Q: I get weird behavior after initializing Direct3D.<br>
 113 <dt>Q: Some FPU operations crash after I load a Delphi DLL.<br>
 114 </dt>
 115 <dd>
 116
 117 DirectX/Direct3D (up to version 9) sets the x87 FPU to single-precision
 118 mode by default. This violates the Windows ABI and interferes with the
 119 operation of many programs &mdash; LuaJIT is affected, too. Please make
 120 sure you always use the <tt>D3DCREATE_FPU_PRESERVE</tt> flag when
 121 initializing Direct3D.<br>
 122
 123 Direct3D version 10 or higher do not show this behavior anymore.
 124 Consider testing your application with older versions, too.<br>
 125
 126 Similarly, the Borland/Delphi runtime modifies the FPU control word and
 127 enables FP exceptions. Of course this violates the Windows ABI, too.
 128 Please check the Delphi docs for the Set8087CW method.
 129
 130 </dl>
 131
 132 <dl>
 133 <dt>Q: Sometimes Ctrl-C fails to stop my Lua program. Why?</dt>
 134 <dd>The interrupt signal handler sets a Lua debug hook. But this is
 135 currently ignored by compiled code (this will eventually be fixed). If
 136 your program is running in a tight loop and never falls back to the
 137 interpreter, the debug hook never runs and can't throw the
 138 "interrupted!" error.<br> In the meantime you have to press Ctrl-C
 139 twice to get stop your program. That's similar to when it's stuck
 140 running inside a C function under the Lua interpreter.</dd>
 141 </dl>
 142
 143 <dl>
 144 <dt>Q: Why doesn't my favorite power-patch for Lua apply against LuaJIT?</dt>
 145 <dd>Because it's a completely redesigned VM and has very little code
 146 in common with Lua anymore. Also, if the patch introduces changes to
 147 the Lua semantics, these would need to be reflected everywhere in the
 148 VM, from the interpreter up to all stages of the compiler.<br> Please
 149 use only standard Lua language constructs. For many common needs you
 150 can use source transformations or use wrapper or proxy functions.
 151 The compiler will happily optimize away such indirections.</dd>
 152 </dl>
 153
 154 <dl>
 155 <dt>Q: Lua runs everywhere. Why doesn't LuaJIT support my CPU?</dt>
 156 <dd>Because it's a compiler &mdash; it needs to generate native
 157 machine code. This means the code generator must be ported to each
 158 architecture. And the fast interpreter is written in assembler and
 159 must be ported, too. This is quite an undertaking.<br>
 160 The <a href="install.html">install documentation</a> shows the supported
 161 architectures. Other architectures will follow based on sufficient user
 162 demand and/or sponsoring.</dd>
 163 </dl>
 164
 165 <dl>
 166 <dt>Q: When will feature X be added? When will the next version be released?</dt>
 167 <dd>When it's ready.<br>
 168 C'mon, it's open source &mdash; I'm doing it on my own time and you're
 169 getting it for free. You can either contribute a patch or sponsor
 170 the development of certain features, if they are important to you.
 171 </dd>
 172 </dl>
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