openresty/doc/LuaJIT-2.1/faq.pod

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LuaJIT

=head1 Frequently Asked Questions (FAQ)

=over

=item * LuaJIT

=over

=item * Download E<rchevron>

=item * Installation

=item * Running

=back

=item * Extensions

=over

=item * FFI Library

=over

=item * FFI Tutorial

=item * ffi.* API

=item * FFI Semantics

=back

=item * jit.* Library

=item * Lua/C API

=item * Profiler

=back

=item * Status

=over

=item * Changes

=back

=item * FAQ

=item * Performance E<rchevron>

=item * Wiki E<rchevron>

=item * Mailing List E<rchevron>

=back

=over

=item Q: Where can I learn more about LuaJIT and Lua?

=over

=item * The E<rchevron> LuaJIT mailing list focuses on topics related
to LuaJIT.

=item * The E<rchevron> LuaJIT wiki gathers community resources about
LuaJIT.

=item * News about Lua itself can be found at the E<rchevron> Lua
mailing list. The mailing list archives are worth checking out for
older postings about LuaJIT.

=item * The E<rchevron> main Lua.org site has complete E<rchevron>
documentation of the language and links to books and papers about Lua.

=item * The community-managed E<rchevron> Lua Wiki has information
about diverse topics.

=back

=back

=over

=item 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: E<rchevron> Trace Compiler

Search for: E<rchevron> JIT Compiler

Search for: E<rchevron> Dynamic Language Optimizations

Search for: E<rchevron> SSA Form

Search for: E<rchevron> Linear Scan Register Allocation

Here is a list of the E<rchevron> innovative features in LuaJIT.

And, you know, reading the source is of course the only way to
enlightenment. :-)

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=over

=item 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 C<arg> parameter for old-style vararg functions
from Lua 5.0.

Please convert your code to the E<rchevron> Lua 5.1 vararg syntax.

=back

=over

=item Q: Why do I get this error: "bad FPU precision"?

=item Q: I get weird behavior after initializing Direct3D.

=item 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 E<mdash> LuaJIT is affected, too. Please
make sure you always use the C<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.

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=over

=item 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.

=back

=over

=item 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.

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=over

=item Q: Lua runs everywhere. Why doesn't LuaJIT support my CPU?

Because it's a compiler E<mdash> 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.

=back

=over

=item Q: When will feature X be added? When will the next version be
released?

When it's ready.

C'mon, it's open source E<mdash> 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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----

Copyright E<copy> 2005-2017 Mike Pall E<middot> Contact

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