LuaJIT

# Frequently Asked Questions (FAQ)

Q: Where can I learn more about LuaJIT and Lua?
  - The LuaJIT mailing list focuses on topics related to LuaJIT.
  - The » LuaJIT wiki gathers community resources about LuaJIT.
  - News about Lua itself can be found at the Lua mailing list. The mailing list archives are worth checking out for older postings about LuaJIT.
  - The main Lua.org site has complete documentation of the language and links to books and papers about Lua.
  - The community-managed » Lua Wiki has information about diverse topics.

Q: Where can I learn more about the compiler technology used by LuaJIT?
  I'm planning to write more documentation about the internals of LuaJIT. In the meantime, please use the following Google Scholar searches to find relevant papers:
  Search for: Trace Compiler
  Search for: JIT Compiler
  Search for: Dynamic Language Optimizations
  Search for: SSA Form
  Search for: Linear Scan Register Allocation
  Here is a list of the » innovative features in LuaJIT.
  And, you know, reading the source is of course the only way to enlightenment. :-)

Q: Why do I get this error: "attempt to index global 'arg' (a nil value)"?
Q: My vararg functions fail after switching to LuaJIT!
  LuaJIT is compatible to the Lua 5.1 language standard. It doesn't support the implicit `arg` parameter for old-style vararg functions from Lua 5.0.
  Please convert your code to the Lua 5.1 vararg syntax.

Q: Why do I get this error: "bad FPU precision"?
Q: I get weird behavior after initializing Direct3D.
Q: Some FPU operations crash after I load a Delphi DLL.
  DirectX/Direct3D (up to version 9) sets the x87 FPU to single-precision mode by default. This violates the Windows ABI and interferes with the operation of many programs — LuaJIT is affected, too. Please make sure you always use the `D3DCREATE_FPU_PRESERVE` flag when initializing Direct3D.
  Direct3D version 10 or higher do not show this behavior anymore. Consider testing your application with older versions, too.
  Similarly, the Borland/Delphi runtime modifies the FPU control word and enables FP exceptions. Of course this violates the Windows ABI, too. Please check the Delphi docs for the Set8087CW method.

Q: Sometimes Ctrl-C fails to stop my Lua program. Why?
  The interrupt signal handler sets a Lua debug hook. But this is currently ignored by compiled code (this will eventually be fixed). If your program is running in a tight loop and never falls back to the interpreter, the debug hook never runs and can't throw the "interrupted!" error.
  In the meantime you have to press Ctrl-C twice to get stop your program. That's similar to when it's stuck running inside a C function under the Lua interpreter.

Q: Why doesn't my favorite power-patch for Lua apply against LuaJIT?
  Because it's a completely redesigned VM and has very little code in common with Lua anymore. Also, if the patch introduces changes to the Lua semantics, these would need to be reflected everywhere in the VM, from the interpreter up to all stages of the compiler.
  Please use only standard Lua language constructs. For many common needs you can use source transformations or use wrapper or proxy functions. The compiler will happily optimize away such indirections.

Q: Lua runs everywhere. Why doesn't LuaJIT support my CPU?
  Because it's a compiler — it needs to generate native machine code. This means the code generator must be ported to each architecture. And the fast interpreter is written in assembler and must be ported, too. This is quite an undertaking.
  The install documentation shows the supported architectures. Other architectures will follow based on sufficient user demand and/or sponsoring.

Q: When will feature X be added? When will the next version be released?
  When it's ready.
  C'mon, it's open source — I'm doing it on my own time and you're getting it for free. You can either contribute a patch or sponsor the development of certain features, if they are important to you.

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