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<head>
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<title>LLVM Bytecode File Format</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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table { border-collapse: collapse; margin-top: 1em margin-bottom: 1em }
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</style>
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</head>
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<body>
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<div class="doc_title"> LLVM Bytecode 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="#general">General Concepts</a>
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<ol>
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<li><a href="#blocks">Blocks</a></li>
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<li><a href="#lists">Lists</a></li>
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<li><a href="#fields">Fields</a></li>
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<li><a href="#encoding">Encoding Rules</a></li>
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<li><a href="#align">Alignment</a></li>
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</ol>
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</li>
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<li><a href="#details">Detailed Layout</a>
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<ol>
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<li><a href="#notation">Notation</a></li>
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<li><a href="#blocktypes">Blocks Types</a></li>
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<li><a href="#signature">Signature Block</a></li>
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<li><a href="#module">Module Block</a></li>
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<li><a href="#typeool">Global Type Pool</a></li>
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<li><a href="#modinfo">Module Info Block</a></li>
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<li><a href="#constants">Global Constant Pool</a></li>
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<li><a href="#functions">Function Blocks</a></li>
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<li><a href="#symtab">Module Symbol Table</a></li>
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</ol>
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</li>
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</ol>
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<div class="doc_text">
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<p><b>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
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and <a href="mailto:sabre@nondot.org">Chris Lattner</a></b></p>
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<p> </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 is an (after the fact) specification of the LLVM bytecode
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file format. It documents the binary encoding rules of the bytecode file format
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so that equivalent systems can encode bytecode files correctly. The LLVM
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bytecode representation is used to store the intermediate representation on
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disk in compacted form.
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"> <a name="general">General Concepts</a> </div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>This section describes the general concepts of the bytecode file format
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without getting into bit and byte level specifics.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="blocks">Blocks</a> </div>
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<div class="doc_text">
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<p>LLVM bytecode files consist simply of a sequence of blocks of bytes.
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Each block begins with an identification value that determines the type of
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the next block. The possible types of blocks are described below in the section
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<a href="#blocktypes">Block Types</a>. The block identifier is used because
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it is possible for entire blocks to be omitted from the file if they are
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empty. The block identifier helps the reader determine which kind of block is
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next in the file.</p>
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<p>The following block identifiers are currently in use
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(from llvm/Bytecode/Format.h):</p>
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<ol>
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<li><b>Module (0x01)</b>.</li>
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<li><b>Function (0x11)</b>.</li>
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<li><b>ConstantPool (0x12)</b>.</li>
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<li><b>SymbolTable (0x13)</b>.</li>
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<li><b>ModuleGlobalInfo (0x14)</b>.</li>
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<li><b>GlobalTypePlane (0x15)</b>.</li>
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<li><b>BasicBlock (0x31)</b>.</li>
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<li><b>InstructionList (0x32)</b>.</li>
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<li><b>CompactionTable (0x33)</b>.</li>
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</ol>
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<p> All blocks are variable length. They consume just enough bytes to express
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their contents. Each block begins with an integer identifier and the length
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of the block.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="lists">Lists</a> </div>
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<div class="doc_text">
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<p>Most blocks are constructed of lists of information. Lists can be constructed
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of other lists, etc. This decomposition of information follows the containment
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hierarchy of the LLVM Intermediate Representation. For example, a function is
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composed of a list of basic blocks. Each basic block is composed of a set of
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instructions. This list of list nesting and hierarchy is maintained in the
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bytecode file.</p>
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<p>A list is encoded into the file simply by encoding the number of entries as
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an integer followed by each of the entries. The reader knows when the list is
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done because it will have filled the list with the required numbe of entries.
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</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="fields">Fields</a> </div>
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<div class="doc_text">
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<p>Fields are units of information that LLVM knows how to write atomically.
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Most fields have a uniform length or some kind of length indication built into
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their encoding. For example, a constant string (array of SByte or UByte) is
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written simply as the length followed by the characters. Although this is
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similar to a list, constant strings are treated atomically and are thus
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fields.</p>
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<p>Fields use a condensed bit format specific to the type of information
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they must contain. As few bits as possible are written for each field. The
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sections that follow will provide the details on how these fields are
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written and how the bits are to be interpreted.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="encoding">Encoding Primitives</a> </div>
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<div class="doc_text">
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<p>Each field that can be put out is encoded into the file using a small set
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of primitives. The rules for these primitives are described below.</p>
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<h3>Variable Bit Rate Encoding</h3>
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<p>To minimize the number of bytes written for small quantities, an encoding
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scheme similar to UTF-8 is used to write integer data. The scheme is known as
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variable bit rate (vbr) encoding. In this encoding, the high bit of each
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byte is used to indicate if more bytes follow. If (byte & 0x80) is non-zero
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in any given byte, it means there is another byte immediately following that
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also contributes to the value. For the final byte (byte & 0x80) is false
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(the high bit is not set). In each byte only the low seven bits contribute to
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the value. Consequently 32-bit quantities can take from one to <em>five</em>
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bytes to encode. In general, smaller quantities will encode in fewer bytes,
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as follows:</p>
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<table class="doc_table_nw">
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<tr>
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<th>Byte #</th>
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<th>Significant Bits</th>
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<th>Maximum Value</th>
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</tr>
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<tr><td>1</td><td>0-6</td><td>127</td></tr>
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<tr><td>2</td><td>7-13</td><td>16,383</td></tr>
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<tr><td>3</td><td>14-20</td><td>2,097,151</td></tr>
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<tr><td>4</td><td>21-27</td><td>268,435,455</td></tr>
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<tr><td>5</td><td>28-34</td><td>34,359,738,367</td></tr>
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<tr><td>6</td><td>35-41</td><td>4,398,046,511,103</td></tr>
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<tr><td>7</td><td>42-48</td><td>562,949,953,421,311</td></tr>
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<tr><td>8</td><td>49-55</td><td>72,057,594,037,927,935</td></tr>
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<tr><td>9</td><td>56-62</td><td>9,223,372,036,854,775,807</td></tr>
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<tr><td>10</td><td>63-69</td><td>1,180,591,620,717,411,303,423</td></tr>
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</table>
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<p>Note that in practice, the tenth byte could only encode bits 63 and 64
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since the maximum quantity to use this encoding is a 64-bit integer.</p>
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<p>The table below defines the encoding rules for type names used in the
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descriptions of blocks and fields in the next section. Any type name with
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the suffix <em>_vbr</em> indicate a quantity that is encoded using
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variable bit rate encoding as described above.</p>
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<table class="doc_table" >
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<tr>
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<th><b>Type</b></th>
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<th align="left"><b>Rule</b></th>
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</tr>
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<tr>
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<td>unsigned</td>
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<td align="left">A 32-bit unsigned integer that always occupies four
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consecutive bytes. The unsigned integer is encoded using LSB first
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ordering. That is bits 2<sup>0</sup> through 2<sup>7</sup> are in the
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byte with the lowest file offset (little endian).</td>
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</tr><tr>
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<td>uint_vbr</td>
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<td align="left">A 32-bit unsigned integer that occupies from one to five
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bytes using variable bit rate encoding.</td>
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</tr><tr>
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<td>uint64_vbr</td>
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<td align="left">A 64-bit unsigned integer that occupies from one to ten
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bytes using variable bit rate encoding.</td>
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</tr><tr>
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<td>int64_vbr</td>
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<td align="left">A 64-bit signed integer that occupies from one to ten
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bytes using variable bit rate encoding.</td>
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</tr><tr>
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<td>char</td>
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<td align="left">A single unsigned character encoded into one byte</td>
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</tr><tr>
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<td>bit</td>
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<td align="left">A single bit within a byte.</td>
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</tr><tr>
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<td>string</td>
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<td align="left">A uint_vbr indicating the length of the character string
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immediately followed by the characters of the string. There is no
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terminating null byte in the string. Characters are interpreted as unsigned
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char and are generally US-ASCII encoded.</td>
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</tr><tr>
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<td>data</td>
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<td align="left">An arbitrarily long segment of data to which no
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interpretation is implied. This is used for float, double, and constant
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initializers.</td>
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</tr>
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</table>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="align">Alignment</a> </div>
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<div class="doc_text">
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<p>To support cross-platform differences, the bytecode file is aligned on
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certain boundaries. This means that a small amount of padding (at most 3 bytes)
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will be added to ensure that the next entry is aligned to a 32-bit boundary.
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"> <a name="details">Detailed Layout</a> </div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>This section provides the detailed layout of the LLVM bytecode file format.
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bit and byte level specifics.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="notation">Notation</a></div>
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<div class="doc_text">
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<p>The descriptions of the bytecode format that follow describe the bit
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fields in detail. These descriptions are provided in tabular form. Each table
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has four columns that specify:</p>
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<ol>
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<li><b>Byte(s)</b>. The offset in bytes of the field from the start of
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its container (block, list, other field).</li>
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<li><b>Bit(s)</b>. The offset in bits of the field from the start of
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the byte field. Bits are always little endian. That is, bit addresses with
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smaller values have smaller address (i.e. 2<sup>0</sup> is at bit 0,
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2<sup>1</sup> at 1, etc.)
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</li>
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<li><b>Align?</b> Indicates if this field is aligned to 32 bits or not.
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This indicates where the <em>next</em> field starts, always on a 32 bit
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boundary.</li>
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<li><b>Type</b>. The basic type of information contained in the field.</li>
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<li><b>Description</b>. Descripts the contents of the field.</li>
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</ol>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="blocktypes">Block Types</a></div>
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<div class="doc_text">
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<p>The bytecode format encodes the intermediate representation into groups
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of bytes known as blocks. The blocks are written sequentially to the file in
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the following order:</p>
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<ol>
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<li><a href="#signature">Signature</a>. This block contains the file signature
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(magic number) that identifies the file as LLVM bytecode.</li>
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<li><a href="#module">Module Block</a>. This is the top level block in a
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bytecode file. It contains all the other blocks.</li>
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<li><a href="#gtypepool">Global Type Pool</a>. This block contains all the
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global (module) level types.</li>
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<li><a href="#modinfo">Module Info</a>. This block contains the types of the
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global variables and functions in the module as well as the constant
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initializers for the global variables</li>
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<li><a href="#constants">Constants</a>. This block contains all the global
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constants except function arguments, global values and constant strings.</li>
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<li><a href="#functions">Functions</a>. One function block is written for
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each function in the module. </li>
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<li><a href="$symtab">Symbol Table</a>. The module level symbol table that
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provides names for the various other entries in the file is the final block
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written.</li>
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</ol>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="signature">Signature Block</a> </div>
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2004-05-22 02:28:36 +00:00
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<div class="doc_text">
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2004-05-23 17:05:09 +00:00
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<p>The signature block occurs in every LLVM bytecode file and is always first.
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It simply provides a few bytes of data to identify the file as being an LLVM
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bytecode file. This block is always four bytes in length and differs from the
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other blocks because there is no identifier and no block length at the start
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of the block. Essentially, this block is just the "magic number" for the file.
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<table class="doc_table_nw" >
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<tr>
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<th><b>Byte(s)</b></th>
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<th><b>Bit(s)</b></th>
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<th><b>Align?</b></th>
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2004-05-22 05:56:41 +00:00
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<th><b>Type</b></th>
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<th align="left"><b>Field Description</b></th>
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</tr><tr>
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<td>00</td><td>-</td><td>No</td><td>char</td>
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2004-05-22 05:56:41 +00:00
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<td align="left">Constant "l" (0x6C)</td>
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</tr><tr>
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<td>01</td><td>-</td><td>No</td><td>char</td>
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<td align="left">Constant "l" (0x6C)</td>
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2004-05-23 17:05:09 +00:00
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</tr><tr>
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<td>02</td><td>-</td><td>No</td><td>char</td>
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<td align="left">Constant "v" (0x76)</td>
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</tr><tr>
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<td>03</td><td>-</td><td>No</td><td>char</td>
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2004-05-22 05:56:41 +00:00
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<td align="left">Constant "m" (0x6D)</td>
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2004-05-22 02:28:36 +00:00
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</tr>
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2004-05-23 17:05:09 +00:00
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</table>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="module">Module Block</a> </div>
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<div class="doc_text">
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<p>The module block contains a small pre-amble and all the other blocks in
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the file. Of particular note, the bytecode format number is simply a 28-bit
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monotonically increase integer that identifiers the version of the bytecode
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format. While the bytecode format version is not related to the LLVM release
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(it doesn't automatically get increased with each new LLVM release), there is
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a definite correspondence between the bytecode format version and the LLVM
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release.</p>
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<p>The table below shows the format of the module block header. The blocks it
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contains are detailed in other sections.</p>
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<table class="doc_table_nw" >
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<tr>
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2004-05-23 17:05:09 +00:00
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<th><b>Byte(s)</b></th>
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<th><b>Bit(s)</b></th>
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<th><b>Align?</b></th>
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<th><b>Type</b></th>
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<th align="left"><b>Field Description</b></th>
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</tr><tr>
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<td>04-07</td><td>-</td><td>No</td><td>unsigned</td>
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<td align="left">Module Identifier (0x01)</td>
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</tr><tr>
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<td>08-11</td><td>-</td><td>No</td><td>unsigned</td>
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<td align="left">Size of the module block in bytes</td>
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</tr><tr>
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<td>12-15</td><td>00</td><td>Yes</td><td>uint32_vbr</td>
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<td align="left">Format Information</td>
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</tr><tr>
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<td>''</td><td>0</td><td>-</td><td>bit</td>
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<td align="left">Big Endian?</td>
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</tr><tr>
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<td>''</td><td>1</td><td>-</td><td>bit</td>
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<td align="left">Pointers Are 64-bit?</td>
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</tr><tr>
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<td>''</td><td>2</td><td>-</td><td>bit</td>
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<td align="left">Has No Endianess?</td>
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</tr><tr>
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<td>''</td><td>3</td><td>-</td><td>bit</td>
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<td align="left">Has No Pointer Size?</td>
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</tr><tr>
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<td>''</td><td>4-31</td><td>-</td><td>bit</td>
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<td align="left">Bytecode Format Version</td>
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</tr><tr>
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<td>16-end</td><td>-</td><td>-</td><td>blocks</td>
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<td align="left">The remaining bytes in the block consist
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solely of other block types in sequence.</td>
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2004-05-22 02:28:36 +00:00
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</tr>
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</table>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="gtypepool">Global Type Pool</a> </div>
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<div class="doc_text">
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2004-05-23 17:05:09 +00:00
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<p>The global type pool consists of type definitions. Their order of appearnce
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in the file determines their slot number (0 based). Slot numbers are used to
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replace pointers in the intermediate representation. Each slot number uniquely
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identifies one entry in a type plane (a collection of values of the same type).
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Since all values have types and are associated with the order in which the type
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pool is written, the global type pool <em>must</em> be written as the first
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block of a module. If it is not, attempts to read the file will fail because
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both forward and backward type resolution will not be possible.</p>
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<p>The type pool is simply a list of types definitions, as shown in the table
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below.</p>
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<table class="doc_table_nw" >
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<tr>
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<th><b>Byte(s)</b></th>
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<th><b>Bit(s)</b></th>
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<th><b>Align?</b></th>
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<th><b>Type</b></th>
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<th align="left"><b>Field Description</b></th>
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</tr><tr>
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<td>00-03</td><td>-</td><td>No</td><td>unsigned</td>
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<td align="left">Type Pool Identifier (0x13)</td>
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</tr><tr>
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<td>04-07</td><td>-</td><td>No</td><td>unsigned</td>
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<td align="left">Size in bytes of the symbol table block.</td>
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</tr><tr>
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<td>08-11<sup>1</sup></td><td>-</td><td>No</td><td>uint32_vbr</td>
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<td align="left">Number of entries in type plane</td>
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</tr><tr>
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<td>12-15<sup>1</sup></td><td>-</td><td>No</td><td>uint32_vbr</td>
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<td align="left">Type plane index for following entries</td>
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</tr><tr>
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<td>16-end<sup>1,2</sup></td><td>-</td><td>No</td><td>type</td>
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<td align="left">Each of the type definitions.</td>
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</tr><tr>
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<td align="left" colspan="5"><sup>1</sup>Maximum length shown,
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may be smaller<br><sup>2</sup>Repeated field.
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</tr>
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</table>
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2004-05-22 02:28:36 +00:00
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="modinfo">Module Info</a> </div>
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<div class="doc_text">
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2004-05-23 17:05:09 +00:00
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<p>To be determined.</p>
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2004-05-22 02:28:36 +00:00
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="constants">Constants</a> </div>
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<div class="doc_text">
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2004-05-23 17:05:09 +00:00
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<p>To be determined.</p>
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2004-05-22 02:28:36 +00:00
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="functions">Functions</a> </div>
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<div class="doc_text">
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2004-05-23 17:05:09 +00:00
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<p>To be determined.</p>
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2004-05-22 02:28:36 +00:00
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</div>
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<!-- _______________________________________________________________________ -->
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2004-05-23 17:05:09 +00:00
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<div class="doc_subsection"><a name="symtab">Symbol Table</a> </div>
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2004-05-22 02:28:36 +00:00
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<div class="doc_text">
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2004-05-23 17:05:09 +00:00
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<p>A symbol table can be put out in conjunction with a module or a function.
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A symbol table is a list of type planes. Each type plane starts with the number
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of entries in the plane and the type plane's slot number (so the type can be
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looked up in the global type pool). For each entry in a type plane, the slot
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number of the value and the name associated with that value are written. The
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format is given in the table below. </p>
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<table class="doc_table_nw" >
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<tr>
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<th><b>Byte(s)</b></th>
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<th><b>Bit(s)</b></th>
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<th><b>Align?</b></th>
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<th><b>Type</b></th>
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<th align="left"><b>Field Description</b></th>
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</tr><tr>
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<td>00-03</td><td>-</td><td>No</td><td>unsigned</td>
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<td align="left">Symbol Table Identifier (0x13)</td>
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</tr><tr>
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<td>04-07</td><td>-</td><td>No</td><td>unsigned</td>
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<td align="left">Size in bytes of the symbol table block.</td>
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</tr><tr>
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<td>08-11<sup>1</sup></td><td>-</td><td>No</td><td>uint32_vbr</td>
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<td align="left">Number of entries in type plane</td>
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</tr><tr>
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<td>12-15<sup>1</sup></td><td>-</td><td>No</td><td>uint32_vbr</td>
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<td align="left">Type plane index for following entries</td>
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</tr><tr>
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<td>16-19<sup>1,2</sup></td><td>-</td><td>No</td><td>uint32_vbr</td>
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<td align="left">Slot number of a value.</td>
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</tr><tr>
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<td>variable<sup>1,2</sup></td><td>-</td><td>No</td><td>string</td>
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<td align="left">Name of the value in the symbol table.</td>
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</tr>
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<tr>
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<td align="left" colspan="5"><sup>1</sup>Maximum length shown,
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may be smaller<br><sup>2</sup>Repeated field.
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</tr>
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</table>
|
2004-05-22 02:28:36 +00:00
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<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:rspencer@x10sys.com">Reid Spencer</a> and
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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<a href="http://llvm.cs.uiuc.edu">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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