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<title>The LLVM Lexicon</title>
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content="A glossary of terms used with the LLVM project.">
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<h1>The LLVM Lexicon</h1>
<p class="doc_warning">NOTE: This document is a work in progress!</p>
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<h2>Table Of Contents</h2>
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<div>
<table>
<tr><th colspan="8"><b>- <a href="#A">A</a> -</b></th></tr>
<tr>
<td><a href="#ADCE">ADCE</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#B">B</a> -</b></th></tr>
<tr>
<td><a href="#BURS">BURS</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#C">C</a> -</b></th></tr>
<tr>
<td><a href="#CSE">CSE</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#D">D</a> -</b></th></tr>
<tr>
<td><a href="#DAG">DAG</a></td>
<td><a href="#Derived_Pointer">Derived Pointer</a></td>
<td><a href="#DSA">DSA</a></td>
<td><a href="#DSE">DSE</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#F">F</a> -</b></th></tr>
<tr>
<td><a href="#FCA">FCA</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#G">G</a> -</b></th></tr>
<tr>
<td><a href="#GC">GC</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#I">I</a> -</b></th></tr>
<tr>
<td><a href="#IPA">IPA</a></td>
<td><a href="#IPO">IPO</a></td>
<td><a href="#ISel">ISel</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#L">L</a> -</b></th></tr>
<tr>
<td><a href="#LCSSA">LCSSA</a></td>
<td><a href="#LICM">LICM</a></td>
<td><a href="#Load-VN">Load-VN</a></td>
<td><a href="#LTO">LTO</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#M">M</a> -</b></th></tr>
<tr>
<td><a href="#MC">MC</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#O">O</a> -</b></th></tr>
<tr>
<td><a href="#Object_Pointer">Object Pointer</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#P">P</a> -</b></th></tr>
<tr>
<td><a href="#PRE">PRE</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#R">R</a> -</b></th></tr>
<tr>
<td><a href="#RAUW">RAUW</a></td>
<td><a href="#Reassociation">Reassociation</a></td>
<td><a href="#Root">Root</a></td>
</tr>
<tr><th colspan="8"><b>- <a href="#S">S</a> -</b></th></tr>
<tr>
<td><a href="#Safe_Point">Safe Point</a></td>
<td><a href="#SCC">SCC</a></td>
<td><a href="#SCCP">SCCP</a></td>
<td><a href="#SDISel">SDISel</a></td>
<td><a href="#SRoA">SRoA</a></td>
<td><a href="#Stack_Map">Stack Map</a></td>
</tr>
</table>
</div>
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<h2>Definitions</h2>
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<div>
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<h3><a name="A">- A -</a></h3>
<div>
<dl>
<dt><a name="ADCE"><b>ADCE</b></a></dt>
<dd>Aggressive Dead Code Elimination</dd>
</dl>
</div>
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<h3><a name="B">- B -</a></h3>
<div>
<dl>
<dt><a name="BURS"><b>BURS</b></a></dt>
<dd>Bottom Up Rewriting System—A method of instruction selection for
code generation. An example is the <a
href="http://www.program-transformation.org/Transform/BURG">BURG</a> tool.</dd>
</dl>
</div>
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<h3><a name="C">- C -</a></h3>
<div>
<dl>
<dt><a name="CSE"><b>CSE</b></a></dt>
<dd>Common Subexpression Elimination. An optimization that removes common
subexpression compuation. For example <tt>(a+b)*(a+b)</tt> has two
subexpressions that are the same: <tt>(a+b)</tt>. This optimization would
perform the addition only once and then perform the multiply (but only if
it's compulationally correct/safe).
</dl>
</div>
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<h3><a name="D">- D -</a></h3>
<div>
<dl>
<dt><a name="DAG"><b>DAG</b></a></dt>
<dd>Directed Acyclic Graph</dd>
<dt><a name="Derived_Pointer"><b>Derived Pointer</b></a></dt>
<dd>A pointer to the interior of an object, such that a garbage collector
is unable to use the pointer for reachability analysis. While a derived
pointer is live, the corresponding object pointer must be kept in a root,
otherwise the collector might free the referenced object. With copying
collectors, derived pointers pose an additional hazard that they may be
invalidated at any <a href="Safe_Point">safe point</a>. This term is used in
opposition to <a href="#Object_Pointer">object pointer</a>.</dd>
<dt><a name="DSA"><b>DSA</b></a></dt>
<dd>Data Structure Analysis</dd>
<dt><a name="DSE"><b>DSE</b></a></dt>
<dd>Dead Store Elimination</dd>
</dl>
</div>
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<h3><a name="F">- F -</a></h3>
<div>
<dl>
<dt><a name="FCA"><b>FCA</b></a></dt>
<dd>First Class Aggregate</dd>
</dl>
</div>
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<h3><a name="G">- G -</a></h3>
<div>
<dl>
<dt><a name="GC"><b>GC</b></a></dt>
<dd>Garbage Collection. The practice of using reachability analysis instead
of explicit memory management to reclaim unused memory.</dd>
</dl>
</div>
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<h3><a name="H">- H -</a></h3>
<div>
<dl>
<dt><a name="Heap"><b>Heap</b></a></dt>
<dd>In garbage collection, the region of memory which is managed using
reachability analysis.</dd>
</dl>
</div>
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<h3><a name="I">- I -</a></h3>
<div>
<dl>
<dt><a name="IPA"><b>IPA</b></a></dt>
<dd>Inter-Procedural Analysis. Refers to any variety of code analysis that
occurs between procedures, functions or compilation units (modules).</dd>
<dt><a name="IPO"><b>IPO</b></a></dt>
<dd>Inter-Procedural Optimization. Refers to any variety of code
optimization that occurs between procedures, functions or compilation units
(modules).</dd>
<dt><a name="ISel"><b>ISel</b></a></dt>
<dd>Instruction Selection.</dd>
</dl>
</div>
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<h3><a name="L">- L -</a></h3>
<div>
<dl>
<dt><a name="LCSSA"><b>LCSSA</b></a></dt>
<dd>Loop-Closed Static Single Assignment Form</dd>
<dt><a name="LICM"><b>LICM</b></a></dt>
<dd>Loop Invariant Code Motion</dd>
<dt><a name="Load-VN"><b>Load-VN</b></a></dt>
<dd>Load Value Numbering</dd>
<dt><a name="LTO"><b>LTO</b></a></dt>
<dd>Link-Time Optimization</dd>
</dl>
</div>
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<h3><a name="M">- M -</a></h3>
<div>
<dl>
<dt><a name="MC"><b>MC</b></a></dt>
<dd>Machine Code</dd>
</dl>
</div>
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<h3><a name="O">- O -</a></h3>
<div>
<dl>
<dt><a name="Object_Pointer"><b>Object Pointer</b></a></dt>
<dd>A pointer to an object such that the garbage collector is able to trace
references contained within the object. This term is used in opposition to
<a href="#Derived_Pointer">derived pointer</a>.</dd>
</dl>
</div>
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<h3><a name="P">- P -</a></h3>
<div>
<dl>
<dt><a name="PRE"><b>PRE</b></a></dt>
<dd>Partial Redundancy Elimination</dd>
</dl>
</div>
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<h3><a name="R">- R -</a></h3>
<div>
<dl>
<dt><a name="RAUW"><b>RAUW</b></a></dt> <dd>An abbreviation for Replace
All Uses With. The functions User::replaceUsesOfWith(),
Value::replaceAllUsesWith(), and Constant::replaceUsesOfWithOnConstant()
implement the replacement of one Value with another by iterating over its
def/use chain and fixing up all of the pointers to point to the new value.
See also <a href="ProgrammersManual.html#iterate_chains">def/use chains</a>.
</dd>
<dt><a name="Reassociation"><b>Reassociation</b></a></dt> <dd>Rearranging
associative expressions to promote better redundancy elimination and other
optimization. For example, changing (A+B-A) into (B+A-A), permitting it to
be optimized into (B+0) then (B).</dd>
<dt><a name="Root"><b>Root</b></a></dt> <dd>In garbage collection, a
pointer variable lying outside of the <a href="#Heap">heap</a> from which
the collector begins its reachability analysis. In the context of code
generation, "root" almost always refers to a "stack root" -- a local or
temporary variable within an executing function.</dd>
</dl>
</div>
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<h3><a name="S">- S -</a></h3>
<div>
<dl>
<dt><a name="Safe_Point"><b>Safe Point</b></a></dt>
<dd>In garbage collection, it is necessary to identify <a href="#Root">stack
roots</a> so that reachability analysis may proceed. It may be infeasible to
provide this information for every instruction, so instead the information
may is calculated only at designated safe points. With a copying collector,
<a href="#Derived_Pointers">derived pointers</a> must not be retained across
safe points and <a href="#Object_Pointers">object pointers</a> must be
reloaded from stack roots.</dd>
<dt><a name="SDISel"><b>SDISel</b></a></dt>
<dd>Selection DAG Instruction Selection.</dd>
<dt><a name="SCC"><b>SCC</b></a></dt>
<dd>Strongly Connected Component</dd>
<dt><a name="SCCP"><b>SCCP</b></a></dt>
<dd>Sparse Conditional Constant Propagation</dd>
<dt><a name="SRoA"><b>SRoA</b></a></dt>
<dd>Scalar Replacement of Aggregates</dd>
<dt><a name="SSA"><b>SSA</b></a></dt>
<dd>Static Single Assignment</dd>
<dt><a name="Stack_Map"><b>Stack Map</b></a></dt>
<dd>In garbage collection, metadata emitted by the code generator which
identifies <a href="#Root">roots</a> within the stack frame of an executing
function.</dd>
</dl>
</div>
</div>
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