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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> [] =&gt; <B><FONT COLOR="#5F9EA0">0</FONT></B>
    | [x] =&gt; x
    | _ :: l =&gt; 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 =&gt; L_73 | ::_0 =&gt; L_74
  L_73 ()
    return global_5
  L_74 (x_145, x_144)
    case x_145 of
      nil_1 =&gt; L_75 | _ =&gt; 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 =&gt; L_73 | ::_0 =&gt; 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 =&gt; L_75 | ::_0 =&gt; 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-20051202-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-20051202-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 2005-12-02 04:21:19 by <span title="ppp-71-139-183-221.dsl.snfc21.pacbell.net"><a href="StephenWeeks">StephenWeeks</a></span>.
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