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249 lines
11 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>LLVM Bytecode File Format</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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<style>
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table, tr, td { border: 2px solid gray }
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th { border: 2px sold gray; font-weight: bold; }
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table { border-collapse: collapse; margin-top: 1em margin-bottom: 1em }
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</style>
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</head>
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<body>
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<div class="doc_title"> LLVM Bytecode File Format </div>
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<ol>
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<li><a href="#abstract">Abstract</a></li>
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<li><a href="#general">General Concepts</a></li>
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<ol>
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<li><a href="#blocks">Blocks</a></li>
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<li><a href="#lists">Lists</a></li>
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<li><a href="#fields">Fields</a></li>
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<li><a href="#align">Alignment</a></li>
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</ol>
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<li><a href="#details">Detailed Layout</a>
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<ol>
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<li><a href="#notation">Notation</a></li>
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<li><a href="#blocktypes">Blocks Types</a></li>
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<li><a href="#header">Header Block</a></li>
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<li><a href="#typeool">Global Type Pool</a></li>
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<li><a href="#modinfo">Module Info Block</a></li>
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<li><a href="#constants">Global Constant Pool</a></li>
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<li><a href="#functions">Function Blocks</a><li>
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<li><a href="#symtab">Module Symbol Table</a><li>
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</ol>
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</li>
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</ol>
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<div class="doc_text">
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<p><b>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
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and <a href="mailto:sabre@nondot.org">Chris Lattner</a></b></p>
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<p> </p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"> <a name="abstract">Abstract </a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>This document is an (after the fact) specification of the LLVM bytecode
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file format. It documents the binary encoding rules of the bytecode file format
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so that equivalent systems can encode bytecode files correctly. The LLVM
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bytecode representation is used to store the intermediate representation on
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disk in compacted form.
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"> <a name="general">General Concepts</a> </div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>This section describes the general concepts of the bytecode file format
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without getting into bit and byte level specifics.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="blocks">Blocks</a> </div>
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<div class="doc_text">
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<p>LLVM bytecode files consist simply of a sequence of blocks of bytes.
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Each block begins with an identification value that determines the type of
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the next block. The possible types of blocks are described below in the section
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<a href="#blockstypes">Block Types</a>. The block identifier is used because
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it is possible for entire blocks to be omitted from the file if they are
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empty. The block identifier helps the reader determine which kind of block is
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next in the file.</p>
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<p>
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Except for the <a href="#header">Header Block</a> all blocks are variable
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length. The consume just enough bytes to express their contents.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="lists">Lists</a> </div>
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<div class="doc_text">
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<p>Most blocks are constructed of lists of information. Lists can be constructed
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of other lists, etc. This decomposition of information follows the containment
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hierarchy of the LLVM Intermediate Representation. For example, a function is
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composed of a list of basic blocks. Each basic block is composed of a set of
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instructions. This list of list nesting and hierarchy is maintained in the
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bytecode file.</p>
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<p>A list is encoded into the file simply by encoding the number of entries as
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an integer followed by each of the entries. The reader knows when the list is
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done because it will have filled the list with the required numbe of entries.
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</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="fields">Fields</a> </div>
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<div class="doc_text">
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<p>Fields are units of information that LLVM knows how to write atomically.
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Most fields have a uniform length or some kind of length indication built into
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their encoding. For example, a constant string (array of SByte or UByte) is
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written simply as the length followed by the characters. Although this is
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similar to a list, constant strings are treated atomically and are thus
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fields.</p>
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<p>Fields use a condensed bit format specific to the type of information
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they must contain. As few bits as possible are written for each field. The
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sections that follow will provide the details on how these fields are
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written and how the bits are to be interpreted.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="align">Alignment</a> </div>
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<div class="doc_text">
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<p>To support cross-platform differences, the bytecode file is aligned on
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certain boundaries. This means that a small amount of padding (at most 3 bytes)
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will be added to ensure that the next entry is aligned to a 32-bit boundary.
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"> <a name="details">Detailed Layout</a> </div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>This section provides the detailed layout of the LLVM bytecode file format.
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bit and byte level specifics.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="notation">Notation</a></div>
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<div class="doc_text">
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<p>The descriptions of the bytecode format that follow describe the bit
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fields in detail. These descriptions are provided in tabular form. Each table
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has four columns that specify:</p>
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<ol>
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<li><b>Byte(s)</b>. The offset in bytes of the field from the start of
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its container (block, list, other field).<li>
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<li><b>Bit(s)</b>. The offset in bits of the field from the start of
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the byte field. Bits are always little endian. That is, bit addresses with
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smaller values have smaller address (i.e. 2^0 is at bit 0, 2^1 at 1, etc.)
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</li>
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<li><b>Align?</b> Indicates if this field is aligned to 32 bits or not.
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This indicates where the <em>next</em> field starts, always on a 32 bit
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boundary.</li>
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<li><b>Description</b>. Descripts the contents of the field.</li>
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</ol>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="blocktypes">Block Types</a></div>
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<div class="doc_text">
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<p>The bytecode format encodes the intermediate representation into groups
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of bytes known as blocks. The blocks are written sequentially to the file in
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the following order:</p>
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<ol>
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<li><a href="#header">Header</a>. This block contains the file signature
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(magic number), machine description and file format version (not LLVM
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version).</li>
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<li><a href="#gtypepool">Global Type Pool</a>. This block contains all the
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global (module) level types.</li>
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<li><a href="#modinfo">Module Info</a>. This block contains the types of the
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global variables and functions in the module as well as the constant
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initializers for the global variables</li>
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<li><a href="#constants">Constants</a>. This block contains all the global
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constants except function arguments, global values and constant strings.</li>
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<li><a href="#functions">Functions</a>. One function block is written for
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each function in the module. </li>
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<li><a href="$symtab">Symbol Table</a>. The module level symbol table that
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provides names for the various other entries in the file is the final block
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written.</li>
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</ol>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="header">Header Block</a> </div>
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<div class="doc_text">
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<p>The Header Block occurs in every LLVM bytecode file and is always first. It
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simply provides a few bytes of data to identify the file, its format, and the
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bytecode version. This block is fixed length and always eight bytes, as follows:
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<table class="doc_table" width="90%">
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<tr>
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<th><b>Byte(s)</b></th>
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<th><b>Bit(s)</b></th>
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<th><b>Align?</b></th>
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<th align="left"><b>Field Description</b></th>
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</tr>
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<tr>
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<td>00</td><td>00-07</td><td>No</td>
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<td align="left">Constant "l"</td>
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</tr>
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<tr>
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<td>01</td><td>00-07</td><td>No</td>
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<td align="left">Constant "l"</td>
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</tr>
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<tr>
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<td>02</td><td>00-07</td><td>No</td>
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<td align="left">Constant "v"</td>
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</tr>
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<tr>
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<td>03</td><td>00-07</td><td>No</td>
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<td align="left">Constant "m"</td>
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</tr>
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<tr>
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<td>04-07</td><td>00</td><td>No</td>
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<td align="left">Target is big endian</td>
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</tr>
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<tr>
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<td>04-07</td><td>01</td><td>No</td>
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<td align="left">Target has long pointers</td>
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</tr>
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<tr>
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<td>04-07</td><td>02</td><td>No</td>
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<td align="left">Target has no endianess</td>
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</tr>
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<tr>
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<td>04-07</td><td>03</td><td>No</td>
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<td align="left">Target has no pointer size</td>
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</tr>
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<tr>
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<td>04-07</td><td>04-31</td><td>Yes</td>
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<td align="left">The LLVM bytecode format version number</td>
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</tr>
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</table>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="gtypepool">Global Type Pool</a> </div>
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<div class="doc_text">
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="modinfo">Module Info</a> </div>
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<div class="doc_text">
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="constants">Constants</a> </div>
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<div class="doc_text">
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="functions">Functions</a> </div>
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<div class="doc_text">
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="symtab">Module Symbol Table</a> </div>
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<div class="doc_text">
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="mailto:rspencer@x10sys.com">Reid Spencer</a> and
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a><br>
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Last modified: $Date$
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</address>
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</body>
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</html>
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<!-- vim: sw=2
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-->
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