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CombineConversions
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Combine conversions 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 looks for and simplifies nested calls to (signed) extension/truncation.
</p>
<h2 id="head-8781d615fd77be9578225c40ac67b9471394cced">Implementation</h2>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/combine-conversions.sig?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">combine-conversions.sig</a> <a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/combine-conversions.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">combine-conversions.fun</a> <h2 id="head-35ec00231a68203708e39f0e2cc10b50c6bf62de">Details and Notes</h2>
<p>
It processes each block in dfs order (visiting definitions before uses):
</p>
<ul>
<li>
<p>
If the statement is not a <tt>PrimApp</tt> <tt>Word_extdToWord</tt>, skip it.
</p>
</li>
<li>
<p>
After processing a conversion, it tags the <tt>Var</tt> for subsequent use.
</p>
</li>
<li>
<p>
When inspecting a conversion, check if the <tt>Var</tt> operand on is also the result of a conversion. If it is, try to combine the two operations. Repeatedly simplify until hitting either a non-conversion <tt>Var</tt> or a case where the conversion cannot be simplified.
</p>
</li>
</ul>
<p>
The optimization rules are very simple:
</p>
<ul>
<pre>x1 = ...
x2 = Word_extdToWord (W1, W2, {signed=s1}) x1
x3 = Word_extdToWord (W2, W3, {signed=s2}) x2
</pre> <li class="gap">
<p>
If <tt>W1 = W2</tt>, then there is no conversions before <tt>x_1</tt>.
</p>
</li>
<ul>
This is guaranteed because <tt>W2 = W3</tt> will always trigger optimization.
</ul>
<li class="gap">
<p>
Case <tt>W1 <= W3 <= W2</tt>:
</p>
</li>
<ul>
<pre>x3 = Word_extdToWord (W1, W3, {signed=s1}) x1
</pre>
</ul>
<li>
<p>
Case <tt>W1 < W2 < W3 AND (NOT s1 OR s2)</tt>:
</p>
</li>
<ul>
<pre>x3 = Word_extdToWord (W1, W3, {signed=s1}) x1
</pre>
</ul>
<li>
<p>
Case <tt>W1 = W2 < W3</tt>:
</p>
</li>
<ul>
unoptimized because there are no conversions past <tt>W1</tt> and <tt>x2 = x1</tt>
</ul>
<li class="gap">
<p>
Case <tt>W3 <= W2 <= W1</tt>:
</p>
</li>
<li>
<p>
Case <tt>W3 <= W1 <= W2</tt>:
</p>
</li>
<ul>
<pre>x_3 = Word_extdToWord (W1, W3, {signed=_}) x1
</pre> because <tt>W3 <= W1 && W3 <= W2</tt>, just clip <tt>x1</tt>
</ul>
<li class="gap">
<p>
Case <tt>W2 < W1 <= W3</tt>:
</p>
</li>
<li>
<p>
Case <tt>W2 < W3 <= W1</tt>:
</p>
</li>
<ul>
unoptimized because <tt>W2 < W1 && W2 < W3</tt>, has truncation effect
</ul>
<li class="gap">
<p>
Case <tt>W1 < W2 < W3 AND s1 AND (NOT s2)</tt>:
</p>
</li>
<ul>
unoptimized because each conversion affects the result separately </ul>
</ul>
</div>
<p>
<hr>
Last edited on 2009-08-02 22:19:10 by <span title="173.86.192.189"><a href="MatthewFluet">MatthewFluet</a></span>.
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