doc/guide/localhost/Redundant

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  22 <a href="./Home">MLton 20180207</a>
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  26 <h1>Redundant</h1>
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  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p><a href="Redundant">Redundant</a> is an optimization pass for the <a href="SSA">SSA</a>
  32 <a href="IntermediateLanguage">IntermediateLanguage</a>, invoked from <a href="SSASimplify">SSASimplify</a>.</p></div>
  33 </div>
  34 </div>
  35 <div class="sect1">
  36 <h2 id="_description">Description</h2>
  37 <div class="sectionbody">
  38 <div class="paragraph"><p>The redundant SSA optimization eliminates redundant function and label
  39 arguments; an argument of a function or label is redundant if it is
  40 always the same as another argument of the same function or label.
  41 The analysis finds an equivalence relation on the arguments of a
  42 function or label, such that all arguments in an equivalence class are
  43 redundant with respect to the other arguments in the equivalence
  44 class; the transformation selects one representative of each
  45 equivalence class and drops the binding occurrence of
  46 non-representative variables and renames use occurrences of the
  47 non-representative variables to the representative variable.  The
  48 analysis finds the equivalence classes via a fixed-point analysis.
  49 Each vector of arguments to a function or label is initialized to
  50 equivalence classes that equate all arguments of the same type; one
  51 could start with an equivalence class that equates all arguments, but
  52 arguments of different type cannot be redundant.  Variables bound in
  53 statements are initialized to singleton equivalence classes.  The
  54 fixed-point analysis repeatedly refines these equivalence classes on
  55 the formals by the equivalence classes of the actuals.</p></div>
  56 </div>
  57 </div>
  58 <div class="sect1">
  59 <h2 id="_implementation">Implementation</h2>
  60 <div class="sectionbody">
  61 <div class="ulist"><ul>
  62 <li>
  63 <p>
  64 <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/redundant.fun"><span class="monospaced">redundant.fun</span></a>
  65 </p>
  66 </li>
  67 </ul></div>
  68 </div>
  69 </div>
  70 <div class="sect1">
  71 <h2 id="_details_and_notes">Details and Notes</h2>
  72 <div class="sectionbody">
  73 <div class="paragraph"><p>The reason <a href="Redundant">Redundant</a> got put in was due to some output of the
  74 <a href="ClosureConvert">ClosureConvert</a> pass converter where the environment record, or
  75 components of it, were passed around in several places.  That may have
  76 been more relevant with polyvariant analyses (which are long gone).
  77 But it still seems possibly relevant, especially with more aggressive
  78 flattening, which should reveal some fields in nested closure records
  79 that are redundant.</p></div>
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