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Added an initial template/overview for documentation of the LLVM Bytecode

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file format. This format is being extracted from the source code and the
small recesses of Chris' brain.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13625 91177308-0d34-0410-b5e6-96231b3b80d8
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>LLVM Bytecode File Format</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
<style>
table, tr, td { border: 2px solid gray }
th { border: 2px sold gray; font-weight: bold; }
table { border-collapse: collapse; margin-top: 1em margin-bottom: 1em }
</style>
</head>
<body>
<div class="doc_title"> LLVM Bytecode File Format </div>
<ol>
<li><a href="#abstract">Abstract</a></li>
<li><a href="#general">General Concepts</a></li>
<ol>
<li><a href="#blocks">Blocks</a></li>
<li><a href="#lists">Lists</a></li>
<li><a href="#fields">Fields</a></li>
<li><a href="#align">Alignment</a></li>
</ol>
<li><a href="#details">Detailed Layout</a>
<ol>
<li><a href="#notation">Notation</a></li>
<li><a href="#blocktypes">Blocks Types</a></li>
<li><a href="#header">Header Block</a></li>
<li><a href="#typeool">Global Type Pool</a></li>
<li><a href="#modinfo">Module Info Block</a></li>
<li><a href="#constants">Global Constant Pool</a></li>
<li><a href="#functions">Function Blocks</a><li>
<li><a href="#symtab">Module Symbol Table</a><li>
</ol>
</li>
</ol>
<div class="doc_text">
<p><b>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
and <a href="mailto:sabre@nondot.org">Chris Lattner</a></b></p>
<p> </p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="abstract">Abstract </a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This document is an (after the fact) specification of the LLVM bytecode
file format. It documents the binary encoding rules of the bytecode file format
so that equivalent systems can encode bytecode files correctly. The LLVM
bytecode representation is used to store the intermediate representation on
disk in compacted form.
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="general">General Concepts</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section describes the general concepts of the bytecode file format
without getting into bit and byte level specifics.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="blocks">Blocks</a> </div>
<div class="doc_text">
<p>LLVM bytecode files consist simply of a sequence of blocks of bytes.
Each block begins with an identification value that determines the type of
the next block. The possible types of blocks are described below in the section
<a href="#blockstypes">Block Types</a>. The block identifier is used because
it is possible for entire blocks to be omitted from the file if they are
empty. The block identifier helps the reader determine which kind of block is
next in the file.</p>
<p>
Except for the <a href="#header">Header Block</a> all blocks are variable
length. The consume just enough bytes to express their contents.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="lists">Lists</a> </div>
<div class="doc_text">
<p>Most blocks are constructed of lists of information. Lists can be constructed
of other lists, etc. This decomposition of information follows the containment
hierarchy of the LLVM Intermediate Representation. For example, a function is
composed of a list of basic blocks. Each basic block is composed of a set of
instructions. This list of list nesting and hierarchy is maintained in the
bytecode file.</p>
<p>A list is encoded into the file simply by encoding the number of entries as
an integer followed by each of the entries. The reader knows when the list is
done because it will have filled the list with the required numbe of entries.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="fields">Fields</a> </div>
<div class="doc_text">
<p>Fields are units of information that LLVM knows how to write atomically.
Most fields have a uniform length or some kind of length indication built into
their encoding. For example, a constant string (array of SByte or UByte) is
written simply as the length followed by the characters. Although this is
similar to a list, constant strings are treated atomically and are thus
fields.</p>
<p>Fields use a condensed bit format specific to the type of information
they must contain. As few bits as possible are written for each field. The
sections that follow will provide the details on how these fields are
written and how the bits are to be interpreted.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="align">Alignment</a> </div>
<div class="doc_text">
<p>To support cross-platform differences, the bytecode file is aligned on
certain boundaries. This means that a small amount of padding (at most 3 bytes)
will be added to ensure that the next entry is aligned to a 32-bit boundary.
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="details">Detailed Layout</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section provides the detailed layout of the LLVM bytecode file format.
bit and byte level specifics.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="notation">Notation</a></div>
<div class="doc_text">
<p>The descriptions of the bytecode format that follow describe the bit
fields in detail. These descriptions are provided in tabular form. Each table
has four columns that specify:</p>
<ol>
<li><b>Byte(s)</b>. The offset in bytes of the field from the start of
its container (block, list, other field).<li>
<li><b>Bit(s)</b>. The offset in bits of the field from the start of
the byte field. Bits are always little endian. That is, bit addresses with
smaller values have smaller address (i.e. 2^0 is at bit 0, 2^1 at 1, etc.)
</li>
<li><b>Align?</b> Indicates if this field is aligned to 32 bits or not.
This indicates where the <em>next</em> field starts, always on a 32 bit
boundary.</li>
<li><b>Description</b>. Descripts the contents of the field.</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="blocktypes">Block Types</a></div>
<div class="doc_text">
<p>The bytecode format encodes the intermediate representation into groups
of bytes known as blocks. The blocks are written sequentially to the file in
the following order:</p>
<ol>
<li><a href="#header">Header</a>. This block contains the file signature
(magic number), machine description and file format version (not LLVM
version).</li>
<li><a href="#gtypepool">Global Type Pool</a>. This block contains all the
global (module) level types.</li>
<li><a href="#modinfo">Module Info</a>. This block contains the types of the
global variables and functions in the module as well as the constant
initializers for the global variables</li>
<li><a href="#constants">Constants</a>. This block contains all the global
constants except function arguments, global values and constant strings.</li>
<li><a href="#functions">Functions</a>. One function block is written for
each function in the module. </li>
<li><a href="$symtab">Symbol Table</a>. The module level symbol table that
provides names for the various other entries in the file is the final block
written.</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="header">Header Block</a> </div>
<div class="doc_text">
<p>The Header Block occurs in every LLVM bytecode file and is always first. It
simply provides a few bytes of data to identify the file, its format, and the
bytecode version. This block is fixed length and always eight bytes, as follows:
<table class="doc_table" width="90%">
<tr>
<th><b>Byte(s)</b></th>
<th><b>Bit(s)</b></th>
<th><b>Align?</b></th>
<th align="left"><b>Field Description</b></th>
</tr>
<tr>
<td>00</td><td>00-07</td><td>No</td>
<td align="left">Constant "l"</td>
</tr>
<tr>
<td>01</td><td>00-07</td><td>No</td>
<td align="left">Constant "l"</td>
</tr>
<tr>
<td>02</td><td>00-07</td><td>No</td>
<td align="left">Constant "v"</td>
</tr>
<tr>
<td>03</td><td>00-07</td><td>No</td>
<td align="left">Constant "m"</td>
</tr>
<tr>
<td>04-07</td><td>00</td><td>No</td>
<td align="left">Target is big endian</td>
</tr>
<tr>
<td>04-07</td><td>01</td><td>No</td>
<td align="left">Target has long pointers</td>
</tr>
<tr>
<td>04-07</td><td>02</td><td>No</td>
<td align="left">Target has no endianess</td>
</tr>
<tr>
<td>04-07</td><td>03</td><td>No</td>
<td align="left">Target has no pointer size</td>
</tr>
<tr>
<td>04-07</td><td>04-31</td><td>Yes</td>
<td align="left">The LLVM bytecode format version number</td>
</tr>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="gtypepool">Global Type Pool</a> </div>
<div class="doc_text">
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="modinfo">Module Info</a> </div>
<div class="doc_text">
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="constants">Constants</a> </div>
<div class="doc_text">
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="functions">Functions</a> </div>
<div class="doc_text">
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="symtab">Module Symbol Table</a> </div>
<div class="doc_text">
</div>
<!-- *********************************************************************** -->
<hr>
<address>
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<a href="mailto:rspencer@x10sys.com">Reid Spencer</a> and
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
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</body>
</html>
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<li><a href="CommandLine.html">Command Line Library</a></li>
<li><a href="ExtendingLLVM.html">Extending LLVM</a></li>
<li><a href="CodingStandards.html">Coding Standards</a></li>
<li><a href="BytecodeFormat.html">LLVM Bytecode File Format</a></li>
</ul>
</div>