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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="css">
table, tr, td { border: 2px solid gray }
th { border: 2px solid 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>
<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="#slots">Slots</a></li>
<li><a href="#encoding">Encoding Rules</a></li>
<li><a href="#align">Alignment</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="#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_author">
<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
</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. 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="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. 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 identification value that determines the type of
the next block. The possible types of blocks are described below in the section
<a href="#blocktypes">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>The following block identifiers are currently in use
(from llvm/Bytecode/Format.h):</p>
<ol>
<li><b>Module (0x01)</b>.</li>
<li><b>Function (0x11)</b>.</li>
<li><b>ConstantPool (0x12)</b>.</li>
<li><b>SymbolTable (0x13)</b>.</li>
<li><b>ModuleGlobalInfo (0x14)</b>.</li>
<li><b>GlobalTypePlane (0x15)</b>.</li>
<li><b>BasicBlock (0x31)</b>.</li>
<li><b>InstructionList (0x32)</b>.</li>
<li><b>CompactionTable (0x33)</b>.</li>
</ol>
<p> All blocks are variable length, and the block header specifies the size of
the block. All blocks are rounded aligned to even 32-bit boundaries, so they
always start and end of this boundary. Each block begins with an integer
identifier and the length of the block, which does not include the padding
bytes needed for alignment.</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
contains a list of instructions (the terminator instructions implicitly define
the end of the basic blocks).</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 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>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="slots">Slots</a> </div>
<div class="doc_text">
<p>The bytecode format uses the notion of a "slot" to reference Types and
Values. Since the bytecode file is a <em>direct</em> representation of LLVM's
intermediate representation, there is a need to represent pointers in the file.
Slots are used for this purpose. For example, if one has the following assembly:
</p>
<div class="doc_code">
%MyType = type { int, sbyte }<br>
%MyVar = external global %MyType
</div>
<p>there are two definitions. The definition of <tt>%MyVar</tt> uses
<tt>%MyType</tt>. In the C++ IR this linkage between <tt>%MyVar</tt> and
<tt>%MyType</tt> is
explicit through the use of C++ pointers. In bytecode, however, there's no
ability to store memory addresses. Instead, we compute and write out slot
numbers for every type and Value written to the file.</p>
<p>A slot number is simply an unsigned 32-bit integer encoded in the variable
bit rate scheme (see <a href="#encoding">encoding</a> below). This ensures that
low slot numbers are encoded in one byte. Through various bits of magic LLVM
attempts to always keep the slot numbers low. The first attempt is to associate
slot numbers with their "type plane". That is, Values of the same type are
written to the bytecode file in a list (sequentially). Their order in that list
determines their slot number. This means that slot #1 doesn't mean anything
unless you also specify for which type you want slot #1. Types are handled
specially and are always written to the file first (in the Global Type Pool) and
in such a way that both forward and backward references of the types can often be
resolved with a single pass through the type pool. </p>
<p>Slot numbers are also kept small by rearranging their order. Because of the
structure of LLVM, certain values are much more likely to be used frequently
in the body of a function. For this reason, a compaction table is provided in
the body of a function if its use would make the function body smaller.
Suppose you have a function body that uses just the types "int*" and "{double}"
but uses them thousands of time. Its worthwhile to ensure that the slot number
for these types are low so they can be encoded in a single byte (via vbr).
This is exactly what the compaction table does.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="encoding">Encoding Primitives</a> </div>
<div class="doc_text">
<p>Each field that can be put out is encoded into the file using a small set
of primitives. The rules for these primitives are described below.</p>
<h3>Variable Bit Rate Encoding</h3>
<p>Most of the values written to LLVM bytecode files are small integers. To
minimize the number of bytes written for these quantities, an encoding
scheme similar to UTF-8 is used to write integer data. The scheme is known as
variable bit rate (vbr) encoding. In this encoding, the high bit of each
byte is used to indicate if more bytes follow. If (byte & 0x80) is non-zero
in any given byte, it means there is another byte immediately following that
also contributes to the value. For the final byte (byte & 0x80) is false
(the high bit is not set). In each byte only the low seven bits contribute to
the value. Consequently 32-bit quantities can take from one to <em>five</em>
bytes to encode. In general, smaller quantities will encode in fewer bytes,
as follows:</p>
<table class="doc_table_nw">
<tr>
<th>Byte #</th>
<th>Significant Bits</th>
<th>Maximum Value</th>
</tr>
<tr><td>1</td><td>0-6</td><td>127</td></tr>
<tr><td>2</td><td>7-13</td><td>16,383</td></tr>
<tr><td>3</td><td>14-20</td><td>2,097,151</td></tr>
<tr><td>4</td><td>21-27</td><td>268,435,455</td></tr>
<tr><td>5</td><td>28-34</td><td>34,359,738,367</td></tr>
<tr><td>6</td><td>35-41</td><td>4,398,046,511,103</td></tr>
<tr><td>7</td><td>42-48</td><td>562,949,953,421,311</td></tr>
<tr><td>8</td><td>49-55</td><td>72,057,594,037,927,935</td></tr>
<tr><td>9</td><td>56-62</td><td>9,223,372,036,854,775,807</td></tr>
<tr><td>10</td><td>63-69</td><td>1,180,591,620,717,411,303,423</td></tr>
</table>
<p
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