LuaJIT has only a single stand-alone executable, called luajit on POSIX systems or luajit.exe on Windows. It can be used to run simple Lua statements or whole Lua applications from the command line. It has an interactive mode, too.
Note: the beta test releases only install under the versioned name on POSIX systems (to avoid overwriting a previous version). You either need to type luajit-2.0.0-beta4 to start it or create a symlink with a command like this:
sudo ln -sf luajit-2.0.0-beta4 /usr/local/bin/luajit
Unlike previous versions optimization is turned on by default in
LuaJIT 2.0!
It's no longer necessary to use luajit -O.
Command Line Options
The luajit stand-alone executable is just a slightly modified version of the regular lua stand-alone executable. It supports the same basic options, too. luajit -h prints a short list of the available options. Please have a look at the » Lua manual for details.
Two additional options control the behavior of LuaJIT:
-j cmd[=arg[,arg...]]
This option performs a LuaJIT control command or activates one of the loadable extension modules. The command is first looked up in the jit.* library. If no matching function is found, a module named jit.<cmd> is loaded and the start() function of the module is called with the specified arguments (if any). The space between -j and cmd is optional.
Here are the available LuaJIT control commands:
- -jon — Turns the JIT compiler on (default).
- -joff — Turns the JIT compiler off (only use the interpreter).
- -jflush — Flushes the whole cache of compiled code.
- -jv — Shows verbose information about the progress of the JIT compiler.
- -jdump — Dumps the code and structures used in various compiler stages.
The -jv and -jdump commands are extension modules written in Lua. They are mainly used for debugging the JIT compiler itself. For a description of their options and output format, please read the comment block at the start of their source. They can be found in the lib directory of the source distribution or installed under the jit directory. By default this is /usr/local/share/luajit-2.0.0-beta4/jit on POSIX systems.
-O[level]
-O[+]flag -O-flag
-Oparam=value
This options allows fine-tuned control of the optimizations used by the JIT compiler. This is mainly intended for debugging LuaJIT itself. Please note that the JIT compiler is extremely fast (we are talking about the microsecond to millisecond range). Disabling optimizations doesn't have any visible impact on its overhead, but usually generates code that runs slower.
The first form sets an optimization level — this enables a specific mix of optimization flags. -O0 turns off all optimizations and higher numbers enable more optimizations. Omitting the level (i.e. just -O) sets the default optimization level, which is -O3 in the current version.
The second form adds or removes individual optimization flags. The third form sets a parameter for the VM or the JIT compiler to a specific value.
You can either use this option multiple times (like -Ocse -O-dce -Ohotloop=10) or separate several settings with a comma (like -O+cse,-dce,hotloop=10). The settings are applied from left to right and later settings override earlier ones. You can freely mix the three forms, but note that setting an optimization level overrides all earlier flags.
Here are the available flags and at what optimization levels they are enabled:
| Flag | -O1 | -O2 | -O3 | |
| fold | • | • | • | Constant Folding, Simplifications and Reassociation |
| cse | • | • | • | Common-Subexpression Elimination |
| dce | • | • | • | Dead-Code Elimination |
| narrow | • | • | Narrowing of numbers to integers | |
| loop | • | • | Loop Optimizations (code hoisting) | |
| fwd | • | Load Forwarding (L2L) and Store Forwarding (S2L) | ||
| dse | • | Dead-Store Elimination | ||
| abc | • | Array Bounds Check Elimination | ||
| fuse | • | Fusion of operands into instructions |
Here are the parameters and their default settings:
| Parameter | Default | |
| maxtrace | 1000 | Max. number of traces in the cache |
| maxrecord | 2000 | Max. number of recorded IR instructions |
| maxirconst | 500 | Max. number of IR constants of a trace |
| maxside | 100 | Max. number of side traces of a root trace |
| maxsnap | 100 | Max. number of snapshots for a trace |
| hotloop | 56 | Number of iterations to detect a hot loop or hot call |
| hotexit | 10 | Number of taken exits to start a side trace |
| tryside | 4 | Number of attempts to compile a side trace |
| instunroll | 4 | Max. unroll factor for instable loops |
| loopunroll | 7 | Max. unroll factor for loop ops in side traces |
| callunroll | 3 | Max. unroll factor for pseudo-recursive calls |
| recunroll | 2 | Min. unroll factor for true recursion |
| sizemcode | 32 | Size of each machine code area in KBytes (Windows: 64K) |
| maxmcode | 512 | Max. total size of all machine code areas in KBytes |