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Here's how <a href="Profiling">Profiling</a> works. If profiling is on, the front end (elaborator) inserts <tt>Enter</tt> and <tt>Leave</tt> statements into the source program for function entry and exit. For example,
<pre class=code>
<B><FONT COLOR="#A020F0">fun </FONT></B><B><FONT COLOR="#0000FF"><B><I><FONT COLOR="#000000">f</FONT></I></B></FONT></B> n <B><FONT COLOR="#5F9EA0">=</FONT></B> <B><FONT COLOR="#A020F0">if</FONT></B> n <B><FONT COLOR="#5F9EA0">=</FONT></B> 0 <B><FONT COLOR="#A020F0">then</FONT></B> 0 <B><FONT COLOR="#A020F0">else</FONT></B> 1 <B><FONT COLOR="#5F9EA0">+</FONT></B> f (n <B><FONT COLOR="#5F9EA0">-</FONT></B> 1)
</PRE>
<p>
becomes
<pre class=code>
<B><FONT COLOR="#A020F0">fun </FONT></B><B><FONT COLOR="#0000FF"><B><I><FONT COLOR="#000000">f</FONT></I></B></FONT></B> n <B><FONT COLOR="#5F9EA0">=</FONT></B>
<B><FONT COLOR="#A020F0">let</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> () <B><FONT COLOR="#5F9EA0">=</FONT></B> Enter <B><FONT COLOR="#BC8F8F">"f"</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> res <B><FONT COLOR="#5F9EA0">=</FONT></B> (<B><FONT COLOR="#A020F0">if</FONT></B> n <B><FONT COLOR="#5F9EA0">=</FONT></B> 0 <B><FONT COLOR="#A020F0">then</FONT></B> 0 <B><FONT COLOR="#A020F0">else</FONT></B> 1 <B><FONT COLOR="#5F9EA0">+</FONT></B> f (n <B><FONT COLOR="#5F9EA0">-</FONT></B> 1))
<B><FONT COLOR="#A020F0">handle</FONT></B> e <B><FONT COLOR="#5F9EA0">=</FONT></B><B><FONT COLOR="#5F9EA0">></FONT></B> (Leave <B><FONT COLOR="#BC8F8F">"f"</FONT></B>; <B><FONT COLOR="#A020F0">raise</FONT></B> e)
<B><FONT COLOR="#A020F0">val</FONT></B> () <B><FONT COLOR="#5F9EA0">=</FONT></B> Leave <B><FONT COLOR="#BC8F8F">"f"</FONT></B>
<B><FONT COLOR="#A020F0">in</FONT></B>
res
<B><FONT COLOR="#A020F0">end</FONT></B>
</PRE>
Actually there is a bit more information than just the source function name; there is also lexical nesting and file position.
</p>
<p>
Most of the middle of the compiler ignores, but preserves, <tt>Enter</tt> and <tt>Leave</tt>. However, so that profiling preserves tail calls, the <a href="Shrink">Ssa shrinker</a> has an optimization that notices when the only operations that cause a call to be a nontail call are profiling operations, and if so, moves them before the call, turning it into a tail call. If you observe a program that has a tail call that appears to be turned into a nontail when compiled with profiling, please <a href="Bug">report a bug</a>.
</p>
<p>
There is the <tt>checkProf</tt> function in
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-20061025-release/mlton/ssa/type-check.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">type-check.fun</a>
, which checks that the <tt>Enter</tt>/<tt>Leave</tt> statements match up.
</p>
<p>
In the backend, just before translating to the <a href="Machine">Machine</a> IL, the profiler uses the <tt>Enter</tt>/<tt>Leave</tt> statements to infer the "local" portion of the control stack at each program point. The profiler then removes the <tt>Enter</tt>s/<tt>Leave</tt>s and inserts different information depending on which kind of profiling is happening. For time profiling, the profiler inserts labels that cover the code (i.e. each statement has a unique label in its basic block that prefixes it) and associates each label with the local control stack. For allocation profiling, the profiler inserts calls to a C function that will maintain byte counts. With stack profiling, the profiler also inserts a call to a C function at each nontail call in order to maintain information at runtime about what SML functions are on the stack.
</p>
<p>
At run time, the profiler associates counters (either clock ticks or byte counts) with source functions. When the program finishes, the profiler writes the counts out to the <tt>mlmon.out</tt> file. Then, <tt>mlprof</tt> uses source information stored in the executable to associate the counts in the <tt>mlmon.out</tt> file with source functions.
</p>
<p>
For time profiling, the profiler catches the <tt>SIGPROF</tt> signal 100 times per second and increments the appropriate counter, determined by looking at the label prefixing the current program counter and mapping that to the current source function.
</p>
<h2 id="head-bcaa33a7ae44bd5042c37a9cdbea7f843b1cf7c8">Caveats</h2>
<p>
There may be a few missed clock ticks or bytes allocated at the very end of the program after the data is written.
</p>
<p>
Profiling has not been tested with signals or threads. In particular, stack profiling may behave strangely.
</p>
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<p>
<hr>
Last edited on 2005-12-01 04:35:20 by <span title="ppp-71-139-183-221.dsl.snfc21.pacbell.net"><a href="StephenWeeks">StephenWeeks</a></span>.
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