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BLOCKNAME and SETRECORDNAME. This allows a bitcode file to be self describing with pretty names for records and blocks in addition to numbers. This enhances llvm-bcanalyzer to use this to print prettily. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@70165 91177308-0d34-0410-b5e6-96231b3b80d8
762 lines
27 KiB
HTML
762 lines
27 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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<li><a href="#stdblocks">Standard Blocks</a></li>
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</ol>
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</li>
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<li><a href="#wrapper">Bitcode Wrapper Format</a>
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</li>
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<li><a href="#llvmir">LLVM IR Encoding</a>
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<ol>
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<li><a href="#basics">Basics</a></li>
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</ol>
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</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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and <a href="http://www.reverberate.org">Joshua Haberman</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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<!-- *********************************************************************** -->
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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, 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>LLVM IR files may be optionally embedded into a <a
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href="#wrapper">wrapper</a> structure that makes it easy to embed extra data
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along with LLVM IR files.</p>
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<p>This document first describes the LLVM bitstream format, describes the
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wrapper format, then describes the 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 two bytes of a bitcode file are 'BC' (0x42, 0x43).
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The second two bytes are an application-specific magic number. Generic
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bitcode tools can look at only the first two bytes to verify the file is
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bitcode, while application-specific programs will want to look at all four.</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, which are read in order
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starting with the least significant bit of each byte. The stream is made up of a
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number of primitive values that encode a stream of unsigned integer values.
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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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<div class="doc_code">
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<pre>
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'a' .. 'z' — 0 .. 25
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'A' .. 'Z' — 26 .. 51
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'0' .. '9' — 52 .. 61
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'.' — 62
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'_' — 63
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</pre>
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</div>
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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><tt>0 - <a href="#END_BLOCK">END_BLOCK</a></tt> — This abbrev ID marks
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the end of the current block.</li>
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<li><tt>1 - <a href="#ENTER_SUBBLOCK">ENTER_SUBBLOCK</a></tt> — This
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abbrev ID marks the beginning of a new block.</li>
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<li><tt>2 - <a href="#DEFINE_ABBREV">DEFINE_ABBREV</a></tt> — This defines
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a new abbreviation.</li>
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<li><tt>3 - <a href="#UNABBREV_RECORD">UNABBREV_RECORD</a></tt> — This ID
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specifies the 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). Block IDs 0-7 are reserved for <a href="#stdblocks">standard blocks</a>
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whose meaning is defined by Bitcode; block IDs 8 and greater are
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application specific. 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, in
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which case they are only defined in that block (neither subblocks nor
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enclosing blocks see the abbreviation). Abbreviations can also be defined
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inside a <tt><a href="#BLOCKINFO">BLOCKINFO</a></tt> block, in which case
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they are defined in all blocks that match the ID that the BLOCKINFO block is
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describing.
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</li>
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</ol>
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<p>
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As sub blocks are entered, these properties are saved and the new sub-block has
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its own set of abbreviations, and its own abbrev id width. When a sub-block is
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popped, the saved values are restored.
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</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 <tt>ENTER_SUBBLOCK</tt> abbreviation ID specifies the start of a new block
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record. The <tt>blockid</tt> value is encoded as an 8-bit VBR identifier, and
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indicates the type of block being entered, which can be
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a <a href="#stdblocks">standard block</a> or an application-specific block.
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The <tt>newabbrevlen</tt> value is a 4-bit VBR, which specifies the abbrev id
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width for the sub-block. The <tt>blocklen</tt> value is a 32-bit aligned value
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that specifies the size of the subblock in 32-bit words. This value allows the
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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 <tt>END_BLOCK</tt> abbreviation ID specifies the end of the current block
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record. Its end is aligned to 32-bits to ensure that the size of the block is
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an even multiple of 32-bits.
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</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 or
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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
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<tt>MODULE_CODE_TRIPLE</tt>, and the values of the record are the ASCII codes
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for the characters in the string.
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</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>
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An <tt>UNABBREV_RECORD</tt> provides a default fallback encoding, which is both
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completely general and extremely inefficient. It can describe an arbitrary
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record by emitting the code and operands as vbrs.
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</p>
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<p>
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For example, emitting an LLVM IR target triple as an unabbreviated record
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requires emitting the <tt>UNABBREV_RECORD</tt> abbrevid, a vbr6 for the
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<tt>MODULE_CODE_TRIPLE</tt> code, a vbr6 for the length of the string, which is
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equal to the number of operands, and a vbr6 for each character. Because there
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are no letters with values less than 32, each letter would need to be emitted as
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at 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.
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</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>
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An abbreviated record is a abbreviation id followed by a set of fields that are
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encoded according to the <a href="#abbreviations">abbreviation definition</a>.
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This allows records to be encoded significantly more densely than records
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encoded with the <tt><a href="#UNABBREV_RECORD">UNABBREV_RECORD</a></tt> type,
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and allows the abbreviation types to be specified in the stream itself, which
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allows the files to be completely self describing. The actual encoding of
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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. Because 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>
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A <tt>DEFINE_ABBREV</tt> record adds an abbreviation to the list of currently
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defined abbreviations in the scope of this block. This definition only exists
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inside this immediate block — it is not visible in subblocks or enclosing
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blocks. Abbreviations are implicitly assigned IDs sequentially starting from 4
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(the first application-defined abbreviation ID). Any abbreviations defined in a
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<tt>BLOCKINFO</tt> record receive IDs first, in order, followed by any
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abbreviations defined within the block itself. Abbreviated data records
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reference this ID to indicate what abbreviation they are invoking.
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</p>
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<p>
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An abbreviation definition consists of the <tt>DEFINE_ABBREV</tt> abbrevid
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followed 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).
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</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 is always a single
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specific value. This specific value is emitted as a vbr8 after the bit
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indicating that it is a literal operand.</li>
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<li>Encoding info without data — <tt>[0<sub>1</sub>,
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encoding<sub>3</sub>]</tt> — Operand encodings that do not have extra
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data are just emitted as their code.
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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> — Operand encodings that do have extra data are
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emitted as their code, followed by the extra data.
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</li>
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</ol>
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<p>The possible operand encodings are:</p>
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<ol>
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<li value="1">Fixed: The field should be emitted as
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a <a href="#fixedwidth">fixed-width value</a>, whose width is specified by
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the operand's extra data.</li>
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|
<li value="2">VBR: The field should be emitted as
|
|
a <a href="#variablewidth">variable-width value</a>, whose width is
|
|
specified by the operand's extra data.</li>
|
|
<li value="3">Array: This field is an array of values. The array operand
|
|
has no extra data, but expects another operand to follow it which indicates
|
|
the element type of the array. When reading an array in an abbreviated
|
|
record, the first integer is a vbr6 that indicates the array length,
|
|
followed by the encoded elements of the array. An array may only occur as
|
|
the last operand of an abbreviation (except for the one final operand that
|
|
gives the array's type).</li>
|
|
<li value="4">Char6: This field should be emitted as
|
|
a <a href="#char6">char6-encoded value</a>. This operand type takes no
|
|
extra data.</li>
|
|
<li value="5">Blob: This field is emitted as a vbr6, followed by padding to a
|
|
32-bit boundary (for alignment) and an array of 8-bit objects. The array of
|
|
bytes is further followed by tail padding to ensure that its total length is
|
|
a multiple of 4 bytes. This makes it very efficient for the reader to
|
|
decode the data without having to make a copy of it: it can use a pointer to
|
|
the data in the mapped in file and poke directly at it. A blob may only
|
|
occur as the last operand of an abbreviation.</li>
|
|
</ol>
|
|
|
|
<p>
|
|
For example, target triples in LLVM modules are encoded as a record of the
|
|
form <tt>[TRIPLE, 'a', 'b', 'c', 'd']</tt>. Consider if the bitstream emitted
|
|
the following abbrev entry:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
[0, Fixed, 4]
|
|
[0, Array]
|
|
[0, Char6]
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
When emitting a record with this abbreviation, the above entry would be emitted
|
|
as:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<p>
|
|
<tt>[4<sub>abbrevwidth</sub>, 2<sub>4</sub>, 4<sub>vbr6</sub>, 0<sub>6</sub>,
|
|
1<sub>6</sub>, 2<sub>6</sub>, 3<sub>6</sub>]</tt>
|
|
</p>
|
|
</div>
|
|
|
|
<p>These values are:</p>
|
|
|
|
<ol>
|
|
<li>The first value, 4, is the abbreviation ID for this abbreviation.</li>
|
|
<li>The second value, 2, is the code for <tt>TRIPLE</tt> in LLVM IR files.</li>
|
|
<li>The third value, 4, is the length of the array.</li>
|
|
<li>The rest of the values are the char6 encoded values
|
|
for <tt>"abcd"</tt>.</li>
|
|
</ol>
|
|
|
|
<p>
|
|
With this abbreviation, the triple is emitted with only 37 bits (assuming a
|
|
abbrev id width of 3). Without the abbreviation, significantly more space would
|
|
be required to emit the target triple. Also, because the <tt>TRIPLE</tt> value
|
|
is not emitted as a literal in the abbreviation, the abbreviation can also be
|
|
used for any other string value.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="stdblocks">Standard Blocks</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
In addition to the basic block structure and record encodings, the bitstream
|
|
also defines specific builtin block types. These block types specify how the
|
|
stream is to be decoded or other metadata. In the future, new standard blocks
|
|
may be added. Block IDs 0-7 are reserved for standard blocks.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection"><a name="BLOCKINFO">#0 - BLOCKINFO
|
|
Block</a></div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The <tt>BLOCKINFO</tt> block allows the description of metadata for other
|
|
blocks. The currently specified records are:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
[SETBID (#1), blockid]
|
|
[DEFINE_ABBREV, ...]
|
|
[BLOCKNAME, ...name...]
|
|
[SETRECORDNAME, RecordID, ...name...]
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
The <tt>SETBID</tt> record indicates which block ID is being
|
|
described. <tt>SETBID</tt> records can occur multiple times throughout the
|
|
block to change which block ID is being described. There must be
|
|
a <tt>SETBID</tt> record prior to any other records.
|
|
</p>
|
|
|
|
<p>
|
|
Standard <tt>DEFINE_ABBREV</tt> records can occur inside <tt>BLOCKINFO</tt>
|
|
blocks, but unlike their occurrence in normal blocks, the abbreviation is
|
|
defined for blocks matching the block ID we are describing, <i>not</i> the
|
|
<tt>BLOCKINFO</tt> block itself. The abbreviations defined
|
|
in <tt>BLOCKINFO</tt> blocks receive abbreviation IDs as described
|
|
in <tt><a href="#DEFINE_ABBREV">DEFINE_ABBREV</a></tt>.
|
|
</p>
|
|
|
|
<p>The <tt>BLOCKNAME</tt> can optionally occur in this block. The elements of
|
|
the record are the bytes for the string name of the block. llvm-bcanalyzer uses
|
|
this to dump out bitcode files symbolically.</p>
|
|
|
|
<p>The <tt>SETRECORDNAME</tt> record can optionally occur in this block. The
|
|
first entry is a record ID number and the rest of the elements of the record are
|
|
the bytes for the string name of the record. llvm-bcanalyzer uses
|
|
this to dump out bitcode files symbolically.</p>
|
|
|
|
<p>
|
|
Note that although the data in <tt>BLOCKINFO</tt> blocks is described as
|
|
"metadata," the abbreviations they contain are essential for parsing records
|
|
from the corresponding blocks. It is not safe to skip them.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"> <a name="wrapper">Bitcode Wrapper Format</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
Bitcode files for LLVM IR may optionally be wrapped in a simple wrapper
|
|
structure. This structure contains a simple header that indicates the offset
|
|
and size of the embedded BC file. This allows additional information to be
|
|
stored alongside the BC file. The structure of this file header is:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<p>
|
|
<tt>[Magic<sub>32</sub>, Version<sub>32</sub>, Offset<sub>32</sub>,
|
|
Size<sub>32</sub>, CPUType<sub>32</sub>]</tt>
|
|
</p>
|
|
</div>
|
|
|
|
<p>
|
|
Each of the fields are 32-bit fields stored in little endian form (as with
|
|
the rest of the bitcode file fields). The Magic number is always
|
|
<tt>0x0B17C0DE</tt> and the version is currently always <tt>0</tt>. The Offset
|
|
field is the offset in bytes to the start of the bitcode stream in the file, and
|
|
the Size field is a size in bytes of the stream. CPUType is a target-specific
|
|
value that can be used to encode the CPU of the target.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"> <a name="llvmir">LLVM IR Encoding</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
LLVM IR is encoded into a bitstream by defining blocks and records. It uses
|
|
blocks for things like constant pools, functions, symbol tables, etc. It uses
|
|
records for things like instructions, global variable descriptors, type
|
|
descriptions, etc. This document does not describe the set of abbreviations
|
|
that the writer uses, as these are fully self-described in the file, and the
|
|
reader is not allowed to build in any knowledge of this.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="basics">Basics</a>
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection"><a name="ir_magic">LLVM IR Magic Number</a></div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The magic number for LLVM IR files is:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<p>
|
|
<tt>[0x0<sub>4</sub>, 0xC<sub>4</sub>, 0xE<sub>4</sub>, 0xD<sub>4</sub>]</tt>
|
|
</p>
|
|
</div>
|
|
|
|
<p>
|
|
When combined with the bitcode magic number and viewed as bytes, this is
|
|
<tt>"BC 0xC0DE"</tt>.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection"><a name="ir_signed_vbr">Signed VBRs</a></div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
<a href="#variablewidth">Variable Width Integers</a> are an efficient way to
|
|
encode arbitrary sized unsigned values, but is an extremely inefficient way to
|
|
encode signed values (as signed values are otherwise treated as maximally large
|
|
unsigned values).
|
|
</p>
|
|
|
|
<p>
|
|
As such, signed vbr values of a specific width are emitted as follows:
|
|
</p>
|
|
|
|
<ul>
|
|
<li>Positive values are emitted as vbrs of the specified width, but with their
|
|
value shifted left by one.</li>
|
|
<li>Negative values are emitted as vbrs of the specified width, but the negated
|
|
value is shifted left by one, and the low bit is set.</li>
|
|
</ul>
|
|
|
|
<p>
|
|
With this encoding, small positive and small negative values can both be emitted
|
|
efficiently.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection"><a name="ir_blocks">LLVM IR Blocks</a></div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
LLVM IR is defined with the following blocks:
|
|
</p>
|
|
|
|
<ul>
|
|
<li>8 — <tt>MODULE_BLOCK</tt> — This is the top-level block that
|
|
contains the entire module, and describes a variety of per-module
|
|
information.</li>
|
|
<li>9 — <tt>PARAMATTR_BLOCK</tt> — This enumerates the parameter
|
|
attributes.</li>
|
|
<li>10 — <tt>TYPE_BLOCK</tt> — This describes all of the types in
|
|
the module.</li>
|
|
<li>11 — <tt>CONSTANTS_BLOCK</tt> — This describes constants for a
|
|
module or function.</li>
|
|
<li>12 — <tt>FUNCTION_BLOCK</tt> — This describes a function
|
|
body.</li>
|
|
<li>13 — <tt>TYPE_SYMTAB_BLOCK</tt> — This describes the type symbol
|
|
table.</li>
|
|
<li>14 — <tt>VALUE_SYMTAB_BLOCK</tt> — This describes a value symbol
|
|
table.</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="MODULE_BLOCK">MODULE_BLOCK Contents</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
</p>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<hr>
|
|
<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-blue" alt="Valid CSS"></a>
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src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
|
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
|
</body>
|
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</html>
|