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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 type="text/css">
TR, TD { border: 2px solid gray; padding: 4pt 4pt 4pt 4pt; }
TH { border: 2px solid gray; font-weight: bold; font-size: 105%; }
TABLE { text-align: center; padding: 4pt 4pt 4pt 4pt; border: 2px solid black;
border-collapse: collapse; margin-top: 1em; margin-left: 1em; margin-right: 1em; margin-bottom: 1em; }
.td_left { border: 2px solid gray; padding: 4pt 4pt 4pt 4pt; text-align: left; }
</style>
</head>
<body>
<div class="doc_title"> LLVM Bytecode File Format </div>
<ol>
<li><a href="#abstract">Abstract</a></li>
<li><a href="#concepts">Concepts</a>
<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>
<li><a href="#encoding">Encoding Primitives</a></li>
<li><a href="#slots">Slots</a></li>
</ol>
</li>
<li><a href="#general">General Layout</a>
<ol>
<li><a href="#structure">Structure</a></li>
</ol>
</li>
<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="#signature">Signature Block</a></li>
<li><a href="#module">Module Block</a></li>
<li><a href="#globaltypes">Global Type Pool</a></li>
<li><a href="#globalinfo">Module Info Block</a></li>
<li><a href="#constantpool">Global Constant Pool</a></li>
<li><a href="#functiondefs">Function Definition</a></li>
<li><a href="#compactiontable">Compaction Table</a></li>
<li><a href="#instructionlist">Instruction List</a></li>
<li><a href="#symtab">Symbol Table</a></li>
</ol>
</li>
<li><a href="#versiondiffs">Version Differences</a>
<ol>
<li><a href="#vers12">Version 1.2 Differences From 1.3</a></li>
<li><a href="#vers11">Version 1.1 Differences From 1.2</a></li>
<li><a href="#vers10">Version 1.0 Differences From 1.1</a></li>
</ol>
</li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
</p>
</div>
<div class="doc_warning">
<p>Warning: This is a work in progress.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="abstract">Abstract </a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This document describes the LLVM bytecode file format as of version 1.3.
It specifies 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="concepts">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. Note that the LLVM bytecode
format may change in the future, but will always be backwards compatible with
older formats. This document only describes the most current version of the
bytecode format.</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 header of two unsigned integers. The first value
identifies the type of block and the second value provides the size of the
block in bytes. 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. Note that blocks
can be nested within other blocks.</p>
<p> All blocks are variable length, and the block header specifies the size of
the block. All blocks begin on a byte index that is aligned to an even 32-bit
boundary. That is, the first block is 32-bit aligned because it starts at offset
0. Each block is padded with zero fill bytes to ensure that the next block also
starts on a 32-bit boundary.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="lists">Lists</a> </div>
<div class="doc_text">
<p>LLVM Bytecode blocks often contain lists of things of a similar type. For
example, a function contains a list of instructions and a function type
contains a list of argument types. There are two basic types of lists:
length lists, and null terminated lists, as described here:</p>
<ul>
<li><b>Length Lists</b>. Length lists are simply preceded by the number
of items in the list. The bytecode reader will read the count first and
then iterate that many times to read in the list contents.</li>
<li><b>Null Terminated Lists</b>. For some lists, the number of elements
in the list is not readily available at the time of writing the bytecode.
In these cases, the list is terminated by some null value. What constitutes
a null value differs, but it almost always boils down to a zero value.</li>
</ul>
</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 bytes) 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>
<!-- _______
|
