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Shrink
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Shrink is a rewrite pass for the <a href="SSA">SSA</a> and <a href="SSA2">SSA2</a> <a href="IntermediateLanguage">IntermediateLanguage</a>s, invoked from every optimization pass (see <a href="SSASimplify">SSASimplify</a> and <a href="SSA2Simplify">SSA2Simplify</a>). <h2 id="head-55f8ebc805e65b5b71ddafdae390e3be2bcd69af">Description</h2>
<p>
This pass implements a whole family of compile-time reductions, like:
</p>
<ul>
<li>
<p>
<tt>#1(a, b) --> a</tt>
</p>
</li>
<li>
<p>
<tt>case C x of C y => e --> let y = x in e</tt>
</p>
</li>
<li>
<p>
constant folding, copy propagation
</p>
</li>
<li>
<p>
eta blocks
</p>
</li>
<li>
<p>
tuple reconstruction elimination
</p>
</li>
</ul>
<h2 id="head-8781d615fd77be9578225c40ac67b9471394cced">Implementation</h2>
<p>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/shrink.sig?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">shrink.sig</a> <a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/shrink.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">shrink.fun</a><br>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/shrink2.sig?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">shrink2.sig</a> <a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/shrink2.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">shrink2.fun</a>
</p>
<h2 id="head-35ec00231a68203708e39f0e2cc10b50c6bf62de">Details and Notes</h2>
<p>
The Shrink pass is run after every <a href="SSA">SSA</a> and <a href="SSA2">SSA2</a> optimization pass.
</p>
<p>
The Shrink implementation also includes functions to eliminate unreachable blocks from a <a href="SSA">SSA</a> or <a href="SSA2">SSA2</a> program or function. The Shrink pass does not guarantee to eliminate all unreachable blocks. Doing so would unduly complicate the implementation, and it is almost always the case that all unreachable blocks are eliminated. However, a small number of optimization passes require that the input have no unreachable blocks (essentially, when the analysis works on the control flow graph and the rewrite iterates on the vector of blocks). These passes explicitly call <tt>eliminateDeadBlocks</tt>.
</p>
<p>
The Shrink pass has a special case to turn a non-tail call where the continuation and handler only do <tt>Profile</tt> statements into a tail call where the <tt>Profile</tt> statements precede the tail call.
</p>
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<p>
<hr>
Last edited on 2006-11-02 17:52:51 by <span title="76.16.241.4"><a href="MatthewFluet">MatthewFluet</a></span>.
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