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    <p>Not quite what you're looking for, but I've managed some reverse-engineering of the jit.* tracing facilities. What follows is a bit rough, inaccurate, subject to change and very incomplete. I'll start making use of it in <a href="https://github.com/geoffleyland/luatrace" rel="noreferrer">luatrace</a> soon. These functions are used in several of the -j library files. <code>dump.lua</code> is probably a good place to start.</p> <p><strong>jit.attach</strong></p> <p>You can attach callbacks to a number of compiler events with <code>jit.attach</code>. The callback can be called:</p> <ul> <li>when a function has been compiled to bytecode ("bc");</li> <li>when trace recording starts or stops ("trace");</li> <li>as a trace is being recorded ("record");</li> <li>or when a trace exits through a side exit ("texit").</li> </ul> <p>Set a callback with <code>jit.attach(callback, "event")</code> and clear the same callback with <code>jit.attach(callback)</code></p> <p>The arguments passed to the callback depend on the event being reported:</p> <ul> <li>"bc": <code>callback(func)</code>. <code>func</code> is the function that's just been recorded.</li> <li>"trace": <code>callback(what, tr, func, pc, otr, oex)</code> <ul> <li><code>what</code> is a description of the trace event: "flush", "start", "stop", "abort". Available for all events.</li> <li><code>tr</code> is the trace number. Not available for flush.</li> <li><code>func</code> is the function being traced. Available for start and abort.</li> <li><code>pc</code> is the program counter - the bytecode number of the function being recorded (if this a Lua function). Available for start and abort.</li> <li><code>otr</code> start: the parent trace number if this is a side trace, abort: abort code (integer)?</li> <li><code>oex</code> start: the exit number for the parent trace, abort: abort reason (string) </li> </ul></li> <li>"record": <code>callback(tr, func, pc, depth)</code>. The first arguments are the same as for trace start. <code>depth</code> is the depth of the inlining of the current bytecode.</li> <li>"texit": <code>callback(tr, ex, ngpr, nfpr)</code>. <ul> <li><code>tr</code> is the trace number as before</li> <li><code>ex</code> is the exit number</li> <li><code>ngpr</code> and <code>nfpr</code> are the number of general-purpose and floating point registers that are active at the exit.</li> </ul></li> </ul> <p><strong>jit.util.funcinfo(func, pc)</strong></p> <p>When passed <code>func</code> and <code>pc</code> from a <code>jit.attach</code> callback, <code>jit.util.funcinfo</code> returns a table of information about the function, much like <code>debug.getinfo</code>.</p> <p>The fields of the table are:</p> <ul> <li><code>linedefined</code>: as for <code>debug.getinfo</code></li> <li><code>lastlinedefined</code>: as for <code>debug.getinfo</code></li> <li><code>params</code>: the number of parameters the function takes</li> <li><code>stackslots</code>: the number of stack slots the function's local variable use</li> <li><code>upvalues</code>: the number of upvalues the function uses</li> <li><code>bytecodes</code>: the number of bytecodes it the compiled function</li> <li><code>gcconsts</code>: ??</li> <li><code>nconsts</code>: ??</li> <li><code>currentline</code>: as for <code>debug.getinfo</code></li> <li><code>isvararg</code>: if the function is a vararg function`</li> <li><code>source</code>: as for <code>debug.getinfo</code></li> <li><code>loc</code>: a string describing the source and currentline, like "&lt;source&gt;:&lt;line&gt;"</li> <li><code>ffid</code>: the fast function id of the function (if it is one). In this case only <code>upvalues</code> above and <code>addr</code> below are valid</li> <li><code>addr</code>: the address of the function (if it is not a Lua function). If it's a C function rather than a fast function, only <code>upvalues</code> above is valid</li> </ul>
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