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<h1>API Extensions</h1>
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<p>
LuaJIT is fully upwards-compatible with Lua 5.1. It supports all
<a href="http://www.lua.org/manual/5.1/manual.html#5"><span class="ext">»</span> standard Lua
library functions</a> and the full set of
<a href="http://www.lua.org/manual/5.1/manual.html#3"><span class="ext">»</span> Lua/C API
functions</a>.
</p>
<p>
LuaJIT is also fully ABI-compatible to Lua 5.1 at the linker/dynamic
loader level. This means you can compile a C module against the
standard Lua headers and load the same shared library from either Lua
or LuaJIT.
</p>
<h2 id="bit"><tt>bit.*</tt> — Bitwise Operations</h2>
<p>
LuaJIT supports all bitwise operations as defined by
<a href="http://bitop.luajit.org"><span class="ext">»</span> Lua BitOp</a>:
</p>
<pre class="code">
bit.tobit bit.tohex bit.bnot bit.band bit.bor bit.bxor
bit.lshift bit.rshift bit.arshift bit.rol bit.ror bit.bswap
</pre>
<p>
This module is a LuaJIT built-in — you don't need to download or
install Lua BitOp. The Lua BitOp site has full documentation for all
<a href="http://bitop.luajit.org/api.html"><span class="ext">»</span> Lua BitOp API functions</a>.
</p>
<p>
Please make sure to <tt>require</tt> the module before using any of
its functions:
</p>
<pre class="code">
local bit = require("bit")
</pre>
<p>
An already installed Lua BitOp module is ignored by LuaJIT.
This way you can use bit operations from both Lua and LuaJIT on a
shared installation.
</p>
<h2 id="jit"><tt>jit.*</tt> — JIT compiler control</h2>
<p>
The functions in this built-in module control the behavior
of the JIT compiler engine.
</p>
<h3 id="jit_onoff"><tt>jit.on()<br>
jit.off()</tt></h3>
<p>
Turns the whole JIT compiler on (default) or off.
</p>
<p>
These functions are typically used with the command line options
<tt>-j on</tt> or <tt>-j off</tt>.
</p>
<h3 id="jit_flush"><tt>jit.flush()</tt></h3>
<p>
Flushes the whole cache of compiled code.
</p>
<h3 id="jit_onoff_func"><tt>jit.on(func|true [,true|false])<br>
jit.off(func|true [,true|false])<br>
jit.flush(func|true [,true|false])</tt></h3>
<p>
<tt>jit.on</tt> enables JIT compilation for a Lua function (this is
the default).
</p>
<p>
<tt>jit.off</tt> disables JIT compilation for a Lua function and
flushes any already compiled code from the code cache.
</p>
<p>
<tt>jit.flush</tt> flushes the code, but doesn't affect the
enable/disable status.
</p>
<p>
The current function, i.e. the Lua function calling this library
function, can also be specified by passing <tt>true</tt> as the first
argument.
</p>
<p>
If the second argument is <tt>true</tt>, JIT compilation is also
enabled, disabled or flushed recursively for all sub-functions of a
function. With <tt>false</tt> only the sub-functions are affected.
</p>
<p>
The <tt>jit.on</tt> and <tt>jit.off</tt> functions only set a flag
which is checked when the function is about to be compiled. They do
not trigger immediate compilation.
</p>
<p>
Typical usage is <tt>jit.off(true, true)</tt> in the main chunk
of a module to turn off JIT compilation for the whole module for
debugging purposes.
</p>
<h3 id="jit_flush_tr"><tt>jit.flush(tr)</tt></h3>
<p>
Flushes the specified root trace and all of its side traces from the cache.
The code for the trace will be retained as long as there are any other
traces which link to it.
</p>
<h3 id="jit_status"><tt>status, ... = jit.status()</tt></h3>
<p>
Returns the current status of the JIT compiler. The first result is
either <tt>true</tt> or <tt>false</tt> if the JIT compiler is turned
on or off. The remaining results are strings for CPU-specific features
and enabled optimizations.
</p>
<h3 id="jit_version"><tt>jit.version</tt></h3>
<p>
Contains the LuaJIT version string.
</p>
<h3 id="jit_version_num"><tt>jit.version_num</tt></h3>
<p>
Contains the version number of the LuaJIT core. Version xx.yy.zz
is represented by the decimal number xxyyzz.
</p>
<h3 id="jit_arch"><tt>jit.arch</tt></h3>
<p>
Contains the target architecture name (CPU and optional ABI).
</p>
<h2 id="jit_opt"><tt>jit.opt.*</tt> — JIT compiler optimization control</h2>
<p>
This module provides the backend for the <tt>-O</tt> command line
option.
</p>
<p>
You can also use it programmatically, e.g.:
</p>
<pre class="code">
jit.opt.start(2) -- same as -O2
jit.opt.start("-dce")
jit.opt.start("hotloop=10", "hotexit=2")
</pre>
<p>
Unlike in LuaJIT 1.x, the module is built-in and
<b>optimization is turned on by default!</b>
It's no longer necessary to run <tt>require("jit.opt").start()</tt>,
which was one of the ways to enable optimization.
</p>
<h2 id="jit_util"><tt>jit.util.*</tt> — JIT compiler introspection</h2>
<p>
This module holds functions to introspect the bytecode, generated
traces, the IR and the generated machine code. The functionality
provided by this module is still in flux and therefore undocumented.
</p>
<p>
The debug modules <tt>-jbc</tt>, <tt>-jv</tt> and <tt>-jdump</tt> make
extensive use of these functions. Please check out their source code,
if you want to know more.
</p>
<h2 id="c_api">C API extensions</h2>
<p>
LuaJIT adds some extensions to the Lua/C API. The LuaJIT include
directory must be in the compiler search path (<tt>-I<i>path</i></tt>)
to be able to include the required header for C code:
</p>
<pre class="code">
#include "luajit.h"
</pre>
<p>
Or for C++ code:
</p>
<pre class="code">
#include "lua.hpp"
</pre>
<h2 id="luaJIT_setmode"><tt>luaJIT_setmode(L, idx, mode)</tt>
— Control VM</h2>
<p>
This is a C API extension to allow control of the VM from C code. The
full prototype of <tt>LuaJIT_setmode</tt> is:
</p>
<pre class="code">
LUA_API int luaJIT_setmode(lua_State *L, int idx, int mode);
</pre>
<p>
The returned status is either success (<tt>1</tt>) or failure (<tt>0</tt>).
The second argument is either <tt>0</tt> or a stack index (similar to the
other Lua/C API functions).
</p>
<p>
The third argument specifies the mode, which is 'or'ed with a flag.
The flag can be <tt>LUAJIT_MODE_OFF</tt> to turn a feature on,
<tt>LUAJIT_MODE_ON</tt> to turn a feature off, or
<tt>LUAJIT_MODE_FLUSH</tt> to flush cached code.
</p>
<p>
The following modes are defined:
</p>
<h3 id="mode_engine"><tt>luaJIT_setmode(L, 0, LUAJIT_MODE_ENGINE|flag)</tt></h3>
<p>
Turn the whole JIT compiler on or off or flush the whole cache of compiled code.
</p>
<h3 id="mode_func"><tt>luaJIT_setmode(L, idx, LUAJIT_MODE_FUNC|flag)</tt><br>
<tt>luaJIT_setmode(L, idx, LUAJIT_MODE_ALLFUNC|flag)</tt><br>
<tt>luaJIT_setmode(L, idx, LUAJIT_MODE_ALLSUBFUNC|flag)</tt></h3>
<p>
This sets the mode for the function at the stack index <tt>idx</tt> or
the parent of the calling function (<tt>idx = 0</tt>). It either
enables JIT compilation for a function, disables it and flushes any
already compiled code or only flushes already compiled code. This
applies recursively to all sub-functions of the function with
<tt>LUAJIT_MODE_ALLFUNC</tt> or only to the sub-functions with
<tt>LUAJIT_MODE_ALLSUBFUNC</tt>.
</p>
<h3 id="mode_engine"><tt>luaJIT_setmode(L, trace,<br>
LUAJIT_MODE_TRACE|LUAJIT_MODE_FLUSH)</tt></h3>
<p>
Flushes the specified root trace and all of its side traces from the cache.
The code for the trace will be retained as long as there are any other
traces which link to it.
</p>
<h3 id="mode_engine"><tt>luaJIT_setmode(L, idx, LUAJIT_MODE_WRAPCFUNC|flag)</tt></h3>
<p>
This mode defines a wrapper function for calls to C functions. If
called with <tt>LUAJIT_MODE_ON</tt>, the stack index at <tt>idx</tt>
must be a <tt>lightuserdata</tt> object holding a pointer to the wrapper
function. From now on all C functions are called through the wrapper
function. If called with <tt>LUAJIT_MODE_OFF</tt> this mode is turned
off and all C functions are directly called.
</p>
<p>
The wrapper function can be used for debugging purposes or to catch
and convert foreign exceptions. Recommended usage can be seen in this
C++ code excerpt:
</p>
<pre class="code">
#include <exception>
#include "lua.hpp"
// Catch C++ exceptions and convert them to Lua error messages.
// Customize as needed for your own exception classes.
static int wrap_exceptions(lua_State *L, lua_CFunction f)
{
try {
return f(L); // Call wrapped function and return result.
} catch (const char *s) { // Catch and convert exceptions.
lua_pushstring(L, s);
} catch (std::exception& e) {
lua_pushstring(L, e.what());
} catch (...) {
lua_pushliteral(L, "caught (...)");
}
return lua_error(L); // Rethrow as a Lua error.
}
static int myinit(lua_State *L)
{
...
// Define wrapper function and enable it.
lua_pushlightuserdata(L, (void *)wrap_exceptions);
luaJIT_setmode(L, -1, LUAJIT_MODE_WRAPCFUNC|LUAJIT_MODE_ON);
lua_pop(L, 1);
...
}
</pre>
<p>
Note that you can only define <b>a single global wrapper function</b>,
so be careful when using this mechanism from multiple C++ modules.
Also note that this mechanism is not without overhead.
</p>
<p>
LuaJIT already intercepts exception handling for systems using DWARF2
stack unwinding (e.g. Linux or OSX) and for Windows/x64 (but <b>not</b>
for Windows/x86). This is a zero-cost mechanism and always enabled.
You don't need to use any wrapper functions, except when you want to get
a more specific error message than <tt>"C++ exception"</tt>.
</p>
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