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KnownCase
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KnownCase is an optimization pass for the <a href="SSA">SSA</a> <a href="IntermediateLanguage">IntermediateLanguage</a>, invoked from <a href="SSASimplify">SSASimplify</a>. <h2 id="head-55f8ebc805e65b5b71ddafdae390e3be2bcd69af">Description</h2>
<p>
This pass duplicates and simplifies <tt>Case</tt> transfers when the constructor of the scrutinee is known.
</p>
<p>
Uses <a href="Restore">Restore</a>.
</p>
<p>
For example, the program
<pre class=code>
<B><FONT COLOR="#A020F0">val</FONT></B> <B><FONT COLOR="#A020F0">rec</FONT></B> last =
<B><FONT COLOR="#A020F0">fn</FONT></B> [] => <B><FONT COLOR="#5F9EA0">0</FONT></B>
| [x] => x
| _ :: l => last l
<B><FONT COLOR="#A020F0">val</FONT></B> _ = <B><FONT COLOR="#5F9EA0">1</FONT></B> + last [<B><FONT COLOR="#5F9EA0">2</FONT></B>, <B><FONT COLOR="#5F9EA0">3</FONT></B>, <B><FONT COLOR="#5F9EA0">4</FONT></B>, <B><FONT COLOR="#5F9EA0">5</FONT></B>, <B><FONT COLOR="#5F9EA0">6</FONT></B>, <B><FONT COLOR="#5F9EA0">7</FONT></B>]
</PRE>
gives rise to the <a href="SSA">SSA</a> function
<pre>fun last_0 (x_142) = loopS_1 ()
loopS_1 ()
loop_11 (x_142)
loop_11 (x_143)
case x_143 of
nil_1 => L_73 | ::_0 => L_74
L_73 ()
return global_5
L_74 (x_145, x_144)
case x_145 of
nil_1 => L_75 | _ => L_76
L_75 ()
return x_144
L_76 ()
loop_11 (x_145)
</pre>which is simplified to
<pre>fun last_0 (x_142) = loopS_1 ()
loopS_1 ()
case x_142 of
nil_1 => L_73 | ::_0 => L_118
L_73 ()
return global_5
L_118 (x_230, x_229)
L_74 (x_230, x_229, x_142)
L_74 (x_145, x_144, x_232)
case x_145 of
nil_1 => L_75 | ::_0 => L_114
L_75 ()
return x_144
L_114 (x_227, x_226)
L_74 (x_227, x_226, x_145)
</pre>
</p>
<h2 id="head-8781d615fd77be9578225c40ac67b9471394cced">Implementation</h2>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/known-case.sig?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">known-case.sig</a> <a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/known-case.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">known-case.fun</a> <h2 id="head-35ec00231a68203708e39f0e2cc10b50c6bf62de">Details and Notes</h2>
<p>
One interesting aspect of KnownCase, is that it often has the effect of unrolling list traversals by one iteration, moving the <tt>nil</tt>/<tt>::</tt> check to the end of the loop, rather than the beginning.
</p>
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<p>
<hr>
Last edited on 2006-11-02 17:54:28 by <span title="76.16.241.4"><a href="MatthewFluet">MatthewFluet</a></span>.
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