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440 lines
16 KiB
HTML
<!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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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>LLVM Bitcode File Format</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title"> LLVM Bitcode 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="#overview">Overview</a></li>
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<li><a href="#bitstream">Bitstream Format</a>
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<ol>
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<li><a href="#magic">Magic Numbers</a></li>
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<li><a href="#primitives">Primitives</a></li>
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<li><a href="#abbrevid">Abbreviation IDs</a></li>
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<li><a href="#blocks">Blocks</a></li>
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<li><a href="#datarecord">Data Records</a></li>
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<li><a href="#abbreviations">Abbreviations</a></li>
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</ol>
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</li>
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<li><a href="#llvmir">LLVM IR Encoding</a></li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>.
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</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 describes the LLVM bitstream file format and the encoding of
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the LLVM IR into it.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"> <a name="overview">Overview</a></div>
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<div class="doc_text">
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<p>
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What is commonly known as the LLVM bitcode file format (also, sometimes
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anachronistically known as bytecode) is actually two things: a <a
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href="#bitstream">bitstream container format</a>
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and an <a href="#llvmir">encoding of LLVM IR</a> into the container format.</p>
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<p>
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The bitstream format is an abstract encoding of structured data, like very
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similar to XML in some ways. Like XML, bitstream files contain tags, and nested
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structures, and you can parse the file without having to understand the tags.
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Unlike XML, the bitstream format is a binary encoding, and unlike XML it
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provides a mechanism for the file to self-describe "abbreviations", which are
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effectively size optimizations for the content.</p>
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<p>This document first describes the LLVM bitstream format, then describes the
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record structure used by LLVM IR files.
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"> <a name="bitstream">Bitstream Format</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>
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The bitstream format is literally a stream of bits, with a very simple
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structure. This structure consists of the following concepts:
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</p>
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<ul>
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<li>A "<a href="#magic">magic number</a>" that identifies the contents of
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the stream.</li>
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<li>Encoding <a href="#primitives">primitives</a> like variable bit-rate
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integers.</li>
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<li><a href="#blocks">Blocks</a>, which define nested content.</li>
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<li><a href="#datarecord">Data Records</a>, which describe entities within the
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file.</li>
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<li>Abbreviations, which specify compression optimizations for the file.</li>
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</ul>
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<p>Note that the <a
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href="CommandGuide/html/llvm-bcanalyzer.html">llvm-bcanalyzer</a> tool can be
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used to dump and inspect arbitrary bitstreams, which is very useful for
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understanding the encoding.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="magic">Magic Numbers</a>
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</div>
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<div class="doc_text">
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<p>The first four bytes of the stream identify the encoding of the file. This
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is used by a reader to know what is contained in the file.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="primitives">Primitives</a>
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</div>
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<div class="doc_text">
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<p>
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A bitstream literally consists of a stream of bits. This stream is made up of a
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number of primitive values that encode a stream of integer values. These
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integers are are encoded in two ways: either as <a href="#fixedwidth">Fixed
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Width Integers</a> or as <a href="#variablewidth">Variable Width
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Integers</a>.
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</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection"> <a name="fixedwidth">Fixed Width Integers</a>
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</div>
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<div class="doc_text">
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<p>Fixed-width integer values have their low bits emitted directly to the file.
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For example, a 3-bit integer value encodes 1 as 001. Fixed width integers
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are used when there are a well-known number of options for a field. For
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example, boolean values are usually encoded with a 1-bit wide integer.
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</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection"> <a name="variablewidth">Variable Width
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Integers</a></div>
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<div class="doc_text">
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<p>Variable-width integer (VBR) values encode values of arbitrary size,
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optimizing for the case where the values are small. Given a 4-bit VBR field,
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any 3-bit value (0 through 7) is encoded directly, with the high bit set to
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zero. Values larger than N-1 bits emit their bits in a series of N-1 bit
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chunks, where all but the last set the high bit.</p>
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<p>For example, the value 27 (0x1B) is encoded as 1011 0011 when emitted as a
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vbr4 value. The first set of four bits indicates the value 3 (011) with a
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continuation piece (indicated by a high bit of 1). The next word indicates a
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value of 24 (011 << 3) with no continuation. The sum (3+24) yields the value
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27.
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</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection"> <a name="char6">6-bit characters</a></div>
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<div class="doc_text">
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<p>6-bit characters encode common characters into a fixed 6-bit field. They
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represent the following characters with the following 6-bit values:</p>
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<ul>
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<li>'a' .. 'z' - 0 .. 25</li>
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<li>'A' .. 'Z' - 26 .. 52</li>
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<li>'0' .. '9' - 53 .. 61</li>
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<li>'.' - 62</li>
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<li>'_' - 63</li>
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</ul>
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<p>This encoding is only suitable for encoding characters and strings that
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consist only of the above characters. It is completely incapable of encoding
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characters not in the set.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection"> <a name="wordalign">Word Alignment</a></div>
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<div class="doc_text">
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<p>Occasionally, it is useful to emit zero bits until the bitstream is a
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multiple of 32 bits. This ensures that the bit position in the stream can be
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represented as a multiple of 32-bit words.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="abbrevid">Abbreviation IDs</a>
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</div>
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<div class="doc_text">
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<p>
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A bitstream is a sequential series of <a href="#blocks">Blocks</a> and
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<a href="#datarecord">Data Records</a>. Both of these start with an
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abbreviation ID encoded as a fixed-bitwidth field. The width is specified by
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the current block, as described below. The value of the abbreviation ID
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specifies either a builtin ID (which have special meanings, defined below) or
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one of the abbreviation IDs defined by the stream itself.
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</p>
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<p>
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The set of builtin abbrev IDs is:
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</p>
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<ul>
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<li>0 - <a href="#END_BLOCK">END_BLOCK</a> - This abbrev ID marks the end of the
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current block.</li>
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<li>1 - <a href="#ENTER_SUBBLOCK">ENTER_SUBBLOCK</a> - This abbrev ID marks the
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beginning of a new block.</li>
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<li>2 - <a href="#DEFINE_ABBREV">DEFINE_ABBREV</a> - This defines a new
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abbreviation.</li>
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<li>3 - <a href="#UNABBREV_RECORD">UNABBREV_RECORD</a> - This ID specifies the
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definition of an unabbreviated record.</li>
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</ul>
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<p>Abbreviation IDs 4 and above are defined by the stream itself, and specify
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an <a href="#abbrev_records">abbreviated record encoding</a>.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="blocks">Blocks</a>
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</div>
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<div class="doc_text">
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<p>
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Blocks in a bitstream denote nested regions of the stream, and are identified by
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a content-specific id number (for example, LLVM IR uses an ID of 12 to represent
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function bodies). Nested blocks capture the hierachical structure of the data
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encoded in it, and various properties are associated with blocks as the file is
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parsed. Block definitions allow the reader to efficiently skip blocks
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in constant time if the reader wants a summary of blocks, or if it wants to
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efficiently skip data they do not understand. The LLVM IR reader uses this
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mechanism to skip function bodies, lazily reading them on demand.
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</p>
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<p>
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When reading and encoding the stream, several properties are maintained for the
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block. In particular, each block maintains:
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</p>
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<ol>
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<li>A current abbrev id width. This value starts at 2, and is set every time a
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block record is entered. The block entry specifies the abbrev id width for
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the body of the block.</li>
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<li>A set of abbreviations. Abbreviations may be defined within a block, or
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they may be associated with all blocks of a particular ID.
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</li>
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</ol>
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<p>As sub blocks are entered, these properties are saved and the new sub-block
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has its own set of abbreviations, and its own abbrev id width. When a sub-block
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is popped, the saved values are restored.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection"> <a name="ENTER_SUBBLOCK">ENTER_SUBBLOCK
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Encoding</a></div>
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<div class="doc_text">
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<p><tt>[ENTER_SUBBLOCK, blockid<sub>vbr8</sub>, newabbrevlen<sub>vbr4</sub>,
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<align32bits>, blocklen<sub>32</sub>]</tt></p>
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<p>
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The ENTER_SUBBLOCK abbreviation ID specifies the start of a new block record.
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The <tt>blockid</tt> value is encoded as a 8-bit VBR identifier, and indicates
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the type of block being entered (which is application specific). The
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<tt>newabbrevlen</tt> value is a 4-bit VBR which specifies the
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abbrev id width for the sub-block. The <tt>blocklen</tt> is a 32-bit aligned
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value that specifies the size of the subblock, in 32-bit words. This value
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allows the reader to skip over the entire block in one jump.
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</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection"> <a name="END_BLOCK">END_BLOCK
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Encoding</a></div>
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<div class="doc_text">
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<p><tt>[END_BLOCK, <align32bits>]</tt></p>
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<p>
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The END_BLOCK abbreviation ID specifies the end of the current block record.
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Its end is aligned to 32-bits to ensure that the size of the block is an even
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multiple of 32-bits.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="datarecord">Data Records</a>
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</div>
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<div class="doc_text">
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<p>
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Data records consist of a record code and a number of (up to) 64-bit integer
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values. The interpretation of the code and values is application specific and
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there are multiple different ways to encode a record (with an unabbrev record
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or with an abbreviation). In the LLVM IR format, for example, there is a record
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which encodes the target triple of a module. The code is MODULE_CODE_TRIPLE,
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and the values of the record are the ascii codes for the characters in the
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string.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection"> <a name="UNABBREV_RECORD">UNABBREV_RECORD
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Encoding</a></div>
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<div class="doc_text">
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<p><tt>[UNABBREV_RECORD, code<sub>vbr6</sub>, numops<sub>vbr6</sub>,
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op0<sub>vbr6</sub>, op1<sub>vbr6</sub>, ...]</tt></p>
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<p>An UNABBREV_RECORD provides a default fallback encoding, which is both
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completely general and also extremely inefficient. It can describe an arbitrary
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record, by emitting the code and operands as vbrs.</p>
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<p>For example, emitting an LLVM IR target triple as an unabbreviated record
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requires emitting the UNABBREV_RECORD abbrevid, a vbr6 for the
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MODULE_CODE_TRIPLE code, a vbr6 for the length of the string (which is equal to
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the number of operands), and a vbr6 for each character. Since there are no
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letters with value less than 32, each letter would need to be emitted as at
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least a two-part VBR, which means that each letter would require at least 12
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bits. This is not an efficient encoding, but it is fully general.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection"> <a name="abbrev_records">Abbreviated Record
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Encoding</a></div>
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<div class="doc_text">
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<p><tt>[<abbrevid>, fields...]</tt></p>
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<p>An abbreviated record is a abbreviation id followed by a set of fields that
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are encoded according to the <a href="#abbreviations">abbreviation
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definition</a>. This allows records to be encoded significantly more densely
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than records encoded with the <a href="#UNABBREV_RECORD">UNABBREV_RECORD</a>
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type, and allows the abbreviation types to be specified in the stream itself,
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which allows the files to be completely self describing. The actual encoding
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of abbreviations is defined below.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="abbreviations">Abbreviations</a>
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</div>
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<div class="doc_text">
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<p>
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Abbreviations are an important form of compression for bitstreams. The idea is
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to specify a dense encoding for a class of records once, then use that encoding
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to emit many records. It takes space to emit the encoding into the file, but
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the space is recouped (hopefully plus some) when the records that use it are
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emitted.
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</p>
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<p>
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Abbreviations can be determined dynamically per client, per file. Since the
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abbreviations are stored in the bitstream itself, different streams of the same
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format can contain different sets of abbreviations if the specific stream does
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not need it. As a concrete example, LLVM IR files usually emit an abbreviation
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for binary operators. If a specific LLVM module contained no or few binary
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operators, the abbreviation does not need to be emitted.
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</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection"><a name="DEFINE_ABBREV">DEFINE_ABBREV
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Encoding</a></div>
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<div class="doc_text">
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<p><tt>[DEFINE_ABBREV, numabbrevops<sub>vbr5</sub>, abbrevop0, abbrevop1,
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...]</tt></p>
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<p>An abbreviation definition consists of the DEFINE_ABBREV abbrevid followed
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by a VBR that specifies the number of abbrev operands, then the abbrev
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operands themselves. Abbreviation operands come in three forms. They all start
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with a single bit that indicates whether the abbrev operand is a literal operand
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(when the bit is 1) or an encoding operand (when the bit is 0).</p>
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<ol>
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<li>Literal operands - <tt>[1<sub>1</sub>, litvalue<sub>vbr8</sub>]</tt> -
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Literal operands specify that the value in the result
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is always a single specific value. This specific value is emitted as a vbr8
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after the bit indicating that it is a literal operand.</li>
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<li>Encoding info without data - <tt>[0<sub>1</sub>, encoding<sub>3</sub>]</tt>
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- blah
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</li>
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<li>Encoding info with data - <tt>[0<sub>1</sub>, encoding<sub>3</sub>,
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value<sub>vbr5</sub>]</tt> -
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</li>
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</ol>
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</div>
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<!-- *********************************************************************** -->
|
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<div class="doc_section"> <a name="llvmir">LLVM IR Encoding</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p></p>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address> <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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<a href="http://validator.w3.org/check/referer"><img
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src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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<a href="http://llvm.org">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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