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<a href="./Home">MLton 20201002</a>
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<h1>SSA</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p><a href="SSA">SSA</a> is an <a href="IntermediateLanguage">IntermediateLanguage</a>, translated from <a href="SXML">SXML</a> by
<a href="ClosureConvert">ClosureConvert</a>, optimized by <a href="SSASimplify">SSASimplify</a>, and translated by
<a href="ToSSA2">ToSSA2</a> to <a href="SSA2">SSA2</a>.</p></div>
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<div class="sect1">
<h2 id="_description">Description</h2>
<div class="sectionbody">
<div class="paragraph"><p><a href="SSA">SSA</a> is a <a href="FirstOrder">FirstOrder</a>, <a href="SimplyTyped">SimplyTyped</a> <a href="IntermediateLanguage">IntermediateLanguage</a>.
It is the main <a href="IntermediateLanguage">IntermediateLanguage</a> used for optimizations.</p></div>
<div class="paragraph"><p>An <a href="SSA">SSA</a> program consists of a collection of datatype declarations,
a sequence of global statements, and a collection of functions, along
with a distinguished "main" function.  Each function consists of a
collection of basic blocks, where each basic block is a sequence of
statements ending with some control transfer.</p></div>
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</div>
<div class="sect1">
<h2 id="_implementation">Implementation</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/ssa.sig"><span class="monospaced">ssa.sig</span></a>
</p>
</li>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/ssa.fun"><span class="monospaced">ssa.fun</span></a>
</p>
</li>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/ssa-tree.sig"><span class="monospaced">ssa-tree.sig</span></a>
</p>
</li>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/ssa-tree.fun"><span class="monospaced">ssa-tree.fun</span></a>
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_type_checking">Type Checking</h2>
<div class="sectionbody">
<div class="paragraph"><p>Type checking (<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/type-check.sig"><span class="monospaced">type-check.sig</span></a>,
<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/type-check.fun"><span class="monospaced">type-check.fun</span></a>) of a <a href="SSA">SSA</a> program
verifies the following:</p></div>
<div class="ulist"><ul>
<li>
<p>
no duplicate definitions (tycons, cons, vars, labels, funcs)
</p>
</li>
<li>
<p>
no out of scope references (tycons, cons, vars, labels, funcs)
</p>
</li>
<li>
<p>
variable definitions dominate variable uses
</p>
</li>
<li>
<p>
case transfers are exhaustive and irredundant
</p>
</li>
<li>
<p>
<span class="monospaced">Enter</span>/<span class="monospaced">Leave</span> profile statements match
</p>
</li>
<li>
<p>
"traditional" well-typedness
</p>
</li>
</ul></div>
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<div class="sect1">
<h2 id="_details_and_notes">Details and Notes</h2>
<div class="sectionbody">
<div class="paragraph"><p>SSA is an abbreviation for Static Single Assignment.</p></div>
<div class="paragraph"><p>For some initial design discussion, see the thread at:</p></div>
<div class="ulist"><ul>
<li>
<p>
<a href="http://mlton.org/pipermail/mlton/2001-August/019689.html">http://mlton.org/pipermail/mlton/2001-August/019689.html</a>
</p>
</li>
</ul></div>
<div class="paragraph"><p>For some retrospectives, see the threads at:</p></div>
<div class="ulist"><ul>
<li>
<p>
<a href="http://mlton.org/pipermail/mlton/2003-January/023054.html">http://mlton.org/pipermail/mlton/2003-January/023054.html</a>
</p>
</li>
<li>
<p>
<a href="http://mlton.org/pipermail/mlton/2007-February/029597.html">http://mlton.org/pipermail/mlton/2007-February/029597.html</a>
</p>
</li>
</ul></div>
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