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SimplifyTypes
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SimplifyTypes 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 computes a "cardinality" of each datatype, which is an abstraction of the number of values of the datatype.
</p>
<ul>
<li>
<p>
<tt>Zero</tt> means the datatype has no values (except for bottom).
</p>
</li>
<li>
<p>
<tt>One</tt> means the datatype has one value (except for bottom).
</p>
</li>
<li>
<p>
<tt>Many</tt> means the datatype has many values.
</p>
</li>
</ul>
<p>
This pass removes all datatypes whose cardinality is <tt>Zero</tt> or <tt>One</tt> and removes:
</p>
<ul>
<li>
<p>
components of tuples
</p>
</li>
<li>
<p>
function args
</p>
</li>
<li>
<p>
constructor args
</p>
</li>
</ul>
<p>
which are such datatypes.
</p>
<p>
This pass marks constructors as one of:
</p>
<ul>
<li>
<p>
<tt>Useless</tt>: it never appears in a <tt>ConApp</tt>.
</p>
</li>
<li>
<p>
<tt>Transparent</tt>: it is the only variant in its datatype and its argument type does not contain any uses of <tt>array</tt> or <tt>vector</tt>.
</p>
</li>
<li>
<p>
<tt>Useful</tt>: otherwise
</p>
</li>
</ul>
<p>
This pass also removes <tt>Useless</tt> and <tt>Transparent</tt> constructors.
</p>
<h2 id="head-8781d615fd77be9578225c40ac67b9471394cced">Implementation</h2>
<p>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/simplify-types.sig?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">simplify-types.sig</a> <a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/simplify-types.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">simplify-types.fun</a>
</p>
<h2 id="head-35ec00231a68203708e39f0e2cc10b50c6bf62de">Details and Notes</h2>
<p>
This pass must happen before polymorphic equality is implemented because
</p>
<ol type="1">
<li>
<p>
it will make polymorphic equality faster because some types are simpler
</p>
</li>
<li>
<p>
it removes uses of polymorphic equality that must return true
</p>
</li>
</ol>
<p>
We must keep track of <tt>Transparent</tt> constructors whose argument type uses <tt>array</tt> because of datatypes like the following:
<pre class=code>
<B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">T</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> t array
</FONT></B></PRE>
Such a datatype has <tt>Cardinality.Many</tt>, but we cannot eliminate the datatype and replace the lhs by the rhs, i.e. we must keep the circularity around.
</p>
<p>
Must do similar things for <tt>vectors</tt>.
</p>
<p>
Also, to eliminate as many <tt>Transparent</tt> constructors as possible, for something like the following,
<pre class=code>
<B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">T</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> u array
</FONT></B><B><FONT COLOR="#A020F0">and</FONT></B><B><FONT COLOR="#228B22"> u </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">U</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> t vector
</FONT></B></PRE>
we (arbitrarily) expand one of the datatypes first. The result will be something like
<pre class=code>
<B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> u </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">U</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> u array array
</FONT></B></PRE>
where all uses of <tt>t</tt> are replaced by <tt>u array</tt>.
</p>
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<p>
<hr>
Last edited on 2009-12-11 14:54:29 by <span title="fenrir.cs.rit.edu"><a href="MatthewFluet">MatthewFluet</a></span>.
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