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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>LLVM Bitcode File Format</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<div class="doc_title"> LLVM Bitcode File Format </div>
<ol>
<li><a href="#abstract">Abstract</a></li>
<li><a href="#overview">Overview</a></li>
<li><a href="#bitstream">Bitstream Format</a>
<ol>
<li><a href="#magic">Magic Numbers</a></li>
<li><a href="#primitives">Primitives</a></li>
<li><a href="#abbrevid">Abbreviation IDs</a></li>
<li><a href="#blocks">Blocks</a></li>
<li><a href="#datarecord">Data Records</a></li>
<li><a href="#abbreviations">Abbreviations</a></li>
<li><a href="#stdblocks">Standard Blocks</a></li>
</ol>
</li>
<li><a href="#llvmir">LLVM IR Encoding</a>
<ol>
<li><a href="#basics">Basics</a></li>
</ol>
</li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
and <a href="http://www.reverberate.org">Joshua Haberman</a>.
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="abstract">Abstract</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This document describes the LLVM bitstream file format and the encoding of
the LLVM IR into it.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="overview">Overview</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>
What is commonly known as the LLVM bitcode file format (also, sometimes
anachronistically known as bytecode) is actually two things: a <a
href="#bitstream">bitstream container format</a>
and an <a href="#llvmir">encoding of LLVM IR</a> into the container format.</p>
<p>
The bitstream format is an abstract encoding of structured data, very
similar to XML in some ways. Like XML, bitstream files contain tags, and nested
structures, and you can parse the file without having to understand the tags.
Unlike XML, the bitstream format is a binary encoding, and unlike XML it
provides a mechanism for the file to self-describe "abbreviations", which are
effectively size optimizations for the content.</p>
<p>This document first describes the LLVM bitstream format, then describes the
record structure used by LLVM IR files.
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="bitstream">Bitstream Format</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>
The bitstream format is literally a stream of bits, with a very simple
structure. This structure consists of the following concepts:
</p>
<ul>
<li>A "<a href="#magic">magic number</a>" that identifies the contents of
the stream.</li>
<li>Encoding <a href="#primitives">primitives</a> like variable bit-rate
integers.</li>
<li><a href="#blocks">Blocks</a>, which define nested content.</li>
<li><a href="#datarecord">Data Records</a>, which describe entities within the
file.</li>
<li>Abbreviations, which specify compression optimizations for the file.</li>
</ul>
<p>Note that the <a
href="CommandGuide/html/llvm-bcanalyzer.html">llvm-bcanalyzer</a> tool can be
used to dump and inspect arbitrary bitstreams, which is very useful for
understanding the encoding.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="magic">Magic Numbers</a>
</div>
<div class="doc_text">
<p>The first two bytes of a bitcode file are 'BC' (0x42, 0x43).
The second two bytes are an application-specific magic number. Generic
bitcode tools can look at only the first two bytes to verify the file is
bitcode, while application-specific programs will want to look at all four.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="primitives">Primitives</a>
</div>
<div class="doc_text">
<p>
A bitstream literally consists of a stream of bits, which are read in order
starting with the least significant bit of each byte. The stream is made up of a
number of primitive values that encode a stream of unsigned integer values.
These
integers are are encoded in two ways: either as <a href="#fixedwidth">Fixed
Width Integers</a> or as <a href="#variablewidth">Variable Width
Integers</a>.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="fixedwidth">Fixed Width Integers</a>
</div>
<div class="doc_text">
<p>Fixed-width integer values have their low bits emitted directly to the file.
For example, a 3-bit integer value encodes 1 as 001. Fixed width integers
are used when there are a well-known number of options for a field. For
example, boolean values are usually encoded with a 1-bit wide integer.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="variablewidth">Variable Width
Integers</a></div>
<div class="doc_text">
<p>Variable-width integer (VBR) values encode values of arbitrary size,
optimizing for the case where the values are small. Given a 4-bit VBR field,
any 3-bit value (0 through 7) is encoded directly, with the high bit set to
zero. Values larger than N-1 bits emit their bits in a series of N-1 bit
chunks, where all but the last set the high bit.</p>
<p>For example, the value 27 (0x1B) is encoded as 1011 0011 when emitted as a
vbr4 value. The first set of four bits indicates the value 3 (011) with a
continuation piece (indicated by a high bit of 1). The next word indicates a
value of 24 (011 << 3) with no continuation. The sum (3+24) yields the value
27.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="char6">6-bit characters</a></div>
<div class="doc_text">
<p>6-bit characters encode common characters into a fixed 6-bit field. They
represent the following characters with the following 6-bit values:</p>
<ul>
<li>'a' .. 'z' - 0 .. 25</li>
<li>'A' .. 'Z' - 26 .. 51</li>
<li>'0' .. '9' - 52 .. 61</li>
<li>'.' - 62</li>
<li>'_' - 63</li>
</ul>
<p>This encoding is only suitable for encoding characters and strings that
consist only of the above characters. It is completely incapable of encoding
characters not in the set.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="wordalign">Word Alignment</a></div>
<div class="doc_text">
<p>Occasionally, it is useful to emit zero bits until the bitstream is a
multiple of 32 bits. This ensures that the bit position in the stream can be
represented as a multiple of 32-bit words.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="abbrevid">Abbreviation IDs</a>
</div>
<div class="doc_text">
<p>
A bitstream is a sequential series of <a href="#blocks">Blocks</a> and
<a href="#datarecord">Data Records</a>. Both of these start with an
abbreviation ID encoded as a fixed-bitwidth field. The width is specified by
the current block, as described below. The value of the abbreviation ID
specifies either a builtin ID (which have special meanings, defined below) or
one of the abbreviation IDs defined by the stream itself.
</p>
<p>
The set of builtin abbrev IDs is:
</p>
<ul>
<li>0 - <a href="#END_BLOCK">END_BLOCK</a> - This abbrev ID marks the end of the
current block.</li>
<li>1 - <a href="#ENTER_SUBBLOCK">ENTER_SUBBLOCK</a> - This abbrev ID marks the
beginning of a new block.</li>
<li>2 - <a href="#DEFINE_ABBREV">DEFINE_ABBREV</a> - This defines a new
abbreviation.</li>
<li>3 - <a href="#UNABBREV_RECORD">UNABBREV_RECORD</a> - This ID specifies the
definition of an unabbreviated record.</li>
</ul>
<p>Abbreviation IDs 4 and above are defined by the stream itself, and specify
an <a href="#abbrev_records">abbreviated record encoding</a>.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="blocks">Blocks</a>
</div>
<div class="doc_text">
<p>
Blocks in a bitstream denote nested regions of the stream, and are identified by
a content-specific id number (for example, LLVM IR uses an ID of 12 to represent
function bodies). Block IDs 0-7 are reserved for <a href="#stdblocks">standard blocks</a>
whose meaning is defined by Bitcode; block IDs 8 and greater are
application specific. Nested blocks capture the hierachical structure of the data
encoded in it, and various properties are associated with blocks as the file is
parsed. Block definitions allow the reader to efficiently skip blocks
in constant time if the reader wants a summary of blocks, or if it wants to
efficiently skip data they do not understand. The LLVM IR reader uses this
mechanism to skip function bodies, lazily reading them on demand.
</p>
<
|
