llvm-project/llvm/docs/BytecodeFormat.html

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  <div class="doc_title"> LLVM Bytecode File Format </div>
<ol>
  <li><a href="#abstract">Abstract</a></li>
  <li><a href="#concepts">Concepts</a>
    <ol>
      <li><a href="#blocks">Blocks</a></li>
      <li><a href="#lists">Lists</a></li>
      <li><a href="#fields">Fields</a></li>
      <li><a href="#align">Alignment</a></li>
      <li><a href="#encoding">Encoding Primitives</a></li>
      <li><a href="#slots">Slots</a></li>
    </ol>
  </li>
  <li><a href="#general">General Layout</a>
    <ol>
      <li><a href="#structure">Structure</a></li>
    </ol>
  </li>
  <li><a href="#details">Detailed Layout</a>
    <ol>
      <li><a href="#notation">Notation</a></li>
      <li><a href="#blocktypes">Blocks Types</a></li>
      <li><a href="#signature">Signature Block</a></li>
      <li><a href="#module">Module Block</a></li>
      <li><a href="#globaltypes">Global Type Pool</a></li>
      <li><a href="#globalinfo">Module Info Block</a></li>
      <li><a href="#constantpool">Global Constant Pool</a></li>
      <li><a href="#functiondefs">Function Definition</a></li>
      <li><a href="#compactiontable">Compaction Table</a></li>
      <li><a href="#instructionlist">Instruction List</a></li>
      <li><a href="#symtab">Symbol Table</a></li>
    </ol>
  </li>
  <li><a href="#versiondiffs">Version Differences</a>
    <ol>
      <li><a href="#vers12">Version 1.2 Differences From 1.3</a></li>
      <li><a href="#vers11">Version 1.1 Differences From 1.2</a></li>
      <li><a href="#vers10">Version 1.0 Differences From 1.1</a></li>
    </ol>
  </li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
</p>
</div>
<div class="doc_warning">
  <p>Warning: This is a work in progress.</p>
</div>

<!-- *********************************************************************** -->
<div class="doc_section"> <a name="abstract">Abstract </a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This document describes the LLVM bytecode file format as of version 1.3. 
It specifies the binary encoding rules of the bytecode file format
so that equivalent systems can encode bytecode files correctly.  The LLVM 
bytecode representation is used to store the intermediate representation on 
disk in compacted form.
</p>
</div>

<!-- *********************************************************************** -->
<div class="doc_section"> <a name="concepts">Concepts</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section describes the general concepts of the bytecode file format 
without getting into bit and byte level specifics.  Note that the LLVM bytecode
format may change in the future, but will always be backwards compatible with
older formats.  This document only describes the most current version of the
bytecode format.</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="blocks">Blocks</a> </div>
<div class="doc_text">
<p>LLVM bytecode files consist simply of a sequence of blocks of bytes. 
Each block begins with an header of two unsigned integers. The first value 
identifies the type of block and the second value provides the size of the 
block in bytes.  The block identifier is used because it is possible for entire 
blocks to be omitted from the file if they are empty. The block identifier helps 
the reader determine which kind of block is next in the file.  Note that blocks 
can be nested within other blocks.</p>
<p> All blocks are variable length, and the block header specifies the size of 
the block.  All blocks begin on a byte index that is aligned to an even 32-bit 
boundary. That is, the first block is 32-bit aligned because it starts at offset
0. Each block is padded with zero fill bytes to ensure that the next block also
starts on a 32-bit boundary.</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="lists">Lists</a> </div>
<div class="doc_text">
  <p>LLVM Bytecode blocks often contain lists of things of a similar type. For
  example, a function contains a list of instructions and a function type 
  contains a list of argument types.  There are two basic types of lists: 
  length lists, and null terminated lists, as described here:</p>
  <ul>
    <li><b>Length Lists</b>.  Length lists are simply preceded by the number 
    of items in the list. The bytecode reader will read the count first and 
    then iterate that many times to read in the list contents.</li>
    <li><b>Null Terminated Lists</b>. For some lists, the number of elements 
    in the list is not readily available at the time of writing the bytecode. 
    In these cases, the list is terminated by some null value. What constitutes 
    a null value differs, but it almost always boils down to a zero value.</li>
  </ul>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="fields">Fields</a> </div>
<div class="doc_text">
<p>Fields are units of information that LLVM knows how to write atomically.
Most fields have a uniform length or some kind of length indication built into
their encoding. For example, a constant string (array of bytes) is
written simply as the length followed by the characters. Although this is 
similar to a list, constant strings are treated atomically and are thus
fields.</p>
<p>Fields use a condensed bit format specific to the type of information
they must contain. As few bits as possible are written for each field. The
sections that follow will provide the details on how these fields are 
written and how the bits are to be interpreted.</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="align">Alignment</a> </div>
<div class="doc_text">
  <p>To support cross-platform differences, the bytecode file is aligned on 
  certain boundaries. This means that a small amount of padding (at most 3 
  bytes) will be added to ensure that the next entry is aligned to a 32-bit 
  boundary.</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="encoding">Encoding Primitives</a> </div>
<div class="doc_text">
<p>Each field that can be put out is encoded into the file using a small set 
of primitives. The rules for these primitives are described below.</p>
<h3>Variable Bit Rate Encoding</h3>
<p>Most of the values written to LLVM bytecode files are small integers.  To 
minimize the number of bytes written for these quantities, an encoding
scheme similar to UTF-8 is used to write integer data. The scheme is known as
variable bit rate (vbr) encoding.  In this encoding, the high bit of each 
byte is used to indicate if more bytes follow. If (byte &amp; 0x80) is non-zero 
in any given byte, it means there is another byte immediately following that 
also contributes to the value. For the final byte (byte &amp; 0x80) is false 
(the high bit is not set). In each byte only the low seven bits contribute to 
the value. Consequently 32-bit quantities can take from one to <em>five</em> 
bytes to encode. In general, smaller quantities will encode in fewer bytes, 
as follows:</p>
<table class="doc_table_nw">
  <tr>
    <th>Byte #</th>
    <th>Significant Bits</th>
    <th>Maximum Value</th>
  </tr>
  <tr><td>1</td><td>0-6</td><td>127</td></tr>
  <tr><td>2</td><td>7-13</td><td>16,383</td></tr>
  <tr><td>3</td><td>14-20</td><td>2,097,151</td></tr>
  <tr><td>4</td><td>21-27</td><td>268,435,455</td></tr>
  <tr><td>5</td><td>28-34</td><td>34,359,738,367</td></tr>
  <tr><td>6</td><td>35-41</td><td>4,398,046,511,103</td></tr>
  <tr><td>7</td><td>42-48</td><td>562,949,953,421,311</td></tr>
  <tr><td>8</td><td>49-55</td><td>72,057,594,037,927,935</td></tr>
  <tr><td>9</td><td>56-62</td><td>9,223,372,036,854,775,807</td></tr>
  <tr><td>10</td><td>63-69</td><td>1,180,591,620,717,411,303,423</td></tr>
</table>
<p>Note that in practice, the tenth byte could only encode bit 63 
since the maximum quantity to use this encoding is a 64-bit integer.</p>

<p><em>Signed</em> VBR values are encoded with the standard vbr encoding, but 
with the sign bit as the low order bit instead of the high order bit.  This 
allows small negative quantities to be encoded efficiently.  For example, -3
is encoded as "((3 &lt;&lt; 1) | 1)" and 3 is encoded as "(3 &lt;&lt; 1) | 
0)", emitted with the standard vbr encoding above.</p>

<p>The table below defines the encoding rules for type names used in the
descriptions of blocks and fields in the next section. Any type name with
the suffix <em>_vbr</em> indicate a quantity that is encoded using 
variable bit rate encoding as described above.</p>
<table class="doc_table" >
  <tr>
    <th><b>Type</b></th>
    <th class="td_left"><b>Rule</b></th>
  </tr>
  <tr>
    <td><a name="unsigned">unsigned</a></td>
    <td class="td_left">A 32-bit unsigned integer that always occupies four 
      consecutive bytes. The unsigned integer is encoded using LSB first 
      ordering. That is bits 2<sup>0</sup> through 2<sup>7</sup> are in the 
      byte with the lowest file offset (little endian).</td>
  </tr><tr>
    <td><a name="uint_vbr">uint_vbr</a></td>
    <td class="td_left">A 32-bit unsigned integer that occupies from one to five 
    bytes using variable bit rate encoding.</td>
  </tr><tr>
    <td><a name="uint64_vbr">uint64_vbr</a></td>
    <td class="td_left">A 64-bit unsigned integer that occupies from one to ten 
    bytes using variable bit rate encoding.</td>
  </tr><tr>
    <td><a name="int64_vbr">int64_vbr</a></td>
    <td class="td_left">A 64-bit signed integer that occupies from one to ten 
    bytes using the signed variable bit rate encoding.</td>
  </tr><tr>
    <td><a name="char">char</a></td>
    <td class="td_left">A single unsigned character encoded into one byte</td>
  </tr><tr>
    <td><a name="bit">bit</a></td>
    <td class="td_left">A single bit within some larger integer field.</td>
  </tr><tr>
    <td><a name="string">string</a></td>
    <td class="td_left">A uint_vbr indicating the length of the character string 
    immediately followed by the characters of the string. There is no 
    terminating null byte in the string.</td>
  </tr><tr>
    <td><a name="data">data</a></td>
    <td class="td_left">An arbitrarily long segment of data to which no 
    interpretation is implied. This is used for float, double, and constant 
    initializers.</td>
  </tr><tr>
    <td><a name="block">block</a></td>
    <td class="td_left">A block of data that is logically related. A block 
      begins with an <a href="#unsigned">unsigned</a> that provides the block
      identifier (constant value) and an <a href="#unsigned">unsigned</a> that
      provides the length of the block. Blocks may compose other blocks.
    </td>
  </tr>
</table>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="slots">Slots</a> </div>
<div class="doc_text">
<p>The bytecode format uses the notion of a "slot" to reference Types and
Values. Since the bytecode file is a <em>direct</em> representation of LLVM's
intermediate representation, there is a need to represent pointers in the file.
Slots are used for this purpose. For example, if one has the following assembly:
</p>
<div class="doc_code">
  %MyType = type { int, sbyte }<br>
  %MyVar = external global %MyType
</div>
<p>there are two definitions. The definition of <tt>%MyVar</tt> uses 
<tt>%MyType</tt>. In the C++ IR this linkage between <tt>%MyVar</tt> and 
<tt>%MyType</tt> is
explicit through the use of C++ pointers. In bytecode, however, there's no
ability to store memory addresses. Instead, we compute and write out slot 
numbers for every Type and Value written to the file.</p>
<p>A slot number is simply an unsigned 32-bit integer encoded in the variable
bit rate scheme (see <a href="#encoding">encoding</a>). This ensures that
low slot numbers are encoded in one byte. Through various bits of magic LLVM
attempts to always keep the slot numbers low. The first attempt is to associate
slot numbers with their "type plane". That is, Values of the same type are 
written to the bytecode file in a list (sequentially). Their order in that list
determines their slot number. This means that slot #1 doesn't mean anything
unless you also specify for which type you want slot #1. Types are handled
specially and are always written to the file first (in the 
<a href="#globaltypes">Global Type Pool</a>) and
in such a way that both forward and backward references of the types can often be
resolved with a single pass through the type pool. </p>
<p>Slot numbers are also kept small by rearranging their order. Because of the
structure of LLVM, certain values are much more likely to be used frequently
in the body of a function. For this reason, a compaction table is provided in
the body of a function if its use would make the function body smaller. 
Suppose you have a function body that uses just the types "int*" and "{double}"
but uses them thousands of time. Its worthwhile to ensure that the slot number
for these types are low so they can be encoded in a single byte (via vbr).
This is exactly what the compaction table does.</p>
</div>

<!-- *********************************************************************** -->
<div class="doc_section"> <a name="general">General Layout</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
  <p>This section provides the general layout of the LLVM bytecode file format.
  The detailed layout can be found in the <a href="#details">next section</a>.
</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="structure">Structure</a> </div>
<div class="doc_text">
<p>The bytecode file format requires blocks to be in a certain order and 
nested in a particular way  so that an LLVM module can be constructed 
efficiently from the contents of the file.  This ordering defines a general 
structure for bytecode files as shown below. The table below shows the order
in which all block types may appear. Please note that some of the blocks are
optional and some may be repeated. The structure is fairly loose because 
optional blocks, if empty, are completely omitted from the file. 
</p>
<table>
  <tr>
    <th>ID</th>
    <th>Parent</th>
    <th>Optional?</th>
    <th>Repeated?</th>
    <th>Level</th>
    <th>Block Type</th>
  </tr>
  <tr><td>N/A</td><td>File</td><td>No</td><td>No</td><td>0</td>
    <td class="td_left"><a href="#signature">Signature</a></td>
  </tr>
  <tr><td>0x01</td><td>File</td><td>No</td><td>No</td><td>0</td>
    <td class="td_left"><a href="#module">Module</a></td>
  </tr>
  <tr><td>0x15</td><td>Module</td><td>No</td><td>No</td><td>1</td>
    <td class="td_left">&nbsp;&nbsp;&nbsp;
      <a href="#globaltypes">Global Type Pool</a></td>
  </tr>
  <tr><td>0x14</td><td>Module</td><td>No</td><td>No</td><td>1</td>
    <td class="td_left">&nbsp;&nbsp;&nbsp;
      <a href="#globalinfo">Module Globals Info</a></td>
  </tr>
  <tr><td>0x12</td><td>Module</td><td>Yes</td><td>No</td><td>1</td>
    <td class="td_left">&nbsp;&nbsp;&nbsp;
      <a href="#constantpool">Module Constant Pool</a></td>
  </tr>
  <tr><td>0x11</td><td>Module</td><td>Yes</td><td>Yes</td><td>1</td>
    <td class="td_left">&nbsp;&nbsp;&nbsp;
      <a href="#functiondefs">Function Definitions</a></td>
  </tr>
  <tr><td>0x12</td><td>Function</td><td>Yes</td><td>No</td><td>2</td>
    <td class="td_left">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
      <a href="#constantpool">Function Constant Pool</a></td>
  </tr>
  <tr><td>0x33</td><td>Function</td><td>Yes</td><td>No</td><td>2</td>
    <td class="td_left">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
      <a href="#compactiontable">Compaction Table</a></td>
  </tr>
  <tr><td>0x32</td><td>Function</td><td>No</td><td>No</td><td>2</td>
    <td class="td_left">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
      <a href="#instructionlist">Instruction List</a></td>
  </tr>
  <tr><td>0x13</td><td>Function</td><td>Yes</td><td>No</td><td>2</td>
    <td class="td_left">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
      <a href="#symboltable">Function Symbol Table</a></td>
  </tr>
  <tr><td>0x13</td><td>Module</td><td>Yes</td><td>No</td><td>1</td>
    <td class="td_left">&nbsp;&nbsp;&nbsp;
      <a href="#symboltable">Module Symbol Table</a></td>
  </tr>
</table>
<p>Use the links in the table or see <a href="#blocktypes">Block Types</a> for 
details about the contents of each of the block types.</p>
</div>

<!-- *********************************************************************** -->
<div class="doc_section"> <a name="details">Detailed Layout</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section provides the detailed layout of the LLVM bytecode file format.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="notation">Notation</a></div>
<div class="doc_text">
<p>The descriptions of the bytecode format that follow describe the order, type
and bit fields in detail. These descriptions are provided in tabular form. 
Each table has four columns that specify:</p>
<ol>
  <li><b>Byte(s)</b>: The offset in bytes of the field from the start of
  its container (block, list, other field).</li>
  <li><b>Bit(s)</b>: The offset in bits of the field from the start of
  the byte field. Bits are always little endian. That is, bit addresses with
  smaller values have smaller address (i.e. 2<sup>0</sup> is at bit 0, 
  2<sup>1</sup> at 1, etc.)
  </li>
  <li><b>Align?</b>: Indicates if this field is aligned to 32 bits or not.
  This indicates where the <em>next</em> field starts, always on a 32 bit
  boundary.</li>
  <li><b>Type</b>: The basic type of information contained in the field.</li>
  <li><b>Description</b>: Describes the contents of the field.</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="blocktypes">Block Types</a></div>
<div class="doc_text">
  <p>The bytecode format encodes the intermediate representation into groups
  of bytes known as blocks. The blocks are written sequentially to the file in
  the following order:</p>
<ol>
  <li><a href="#signature">Signature</a>: This contains the file signature 
  (magic number) that identifies the file as LLVM bytecode and the bytecode 
  version number.</li>
  <li><a href="#module">Module Block</a>: This is the top level block in a
  bytecode file. It contains all the other blocks.</li>
  <li><a href="#gtypepool">Global Type Pool</a>: This block contains all the
  global (module) level types.</li>
  <li><a href="#modinfo">Module Info</a>: This block contains the types of the
  global variables and functions in the module as well as the constant
  initializers for the global variables</li>
  <li><a href="#constants">Constants</a>: This block contains all the global
  constants except function arguments, global values and constant strings.</li>
  <li><a href="#functions">Functions</a>: One function block is written for
  each function in the module. </li>
  <li><a href="#symtab">Symbol Table</a>: The module level symbol table that
  provides names for the various other entries in the file is the final block 
  written.</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="signature">Signature Block</a> </div>
<div class="doc_text">
<p>The signature occurs in every LLVM bytecode file and is always first.
It simply provides a few bytes of data to identify the file as being an LLVM
bytecode file. This block is always four bytes in length and differs from the
other blocks because there is no identifier and no block length at the start
of the block. Essentially, this block is just the "magic number" for the file.
<table class="doc_table_nw" >
  <tr>
    <th><b>Type</b></th>
    <th class="td_left"><b>Field Description</b></th>
  </tr><tr>
    <td><a href="#char">char</a></td>
    <td class="td_left">Constant "l" (0x6C)</td>
  </tr><tr>
    <td><a href="#char">char</a></td>
    <td class="td_left">Constant "l" (0x6C)</td>
  </tr><tr>
    <td><a href="#char">char</a></td>
    <td class="td_left">Constant "v" (0x76)</td>
  </tr><tr>
    <td><a href="#char">char</a></td>
    <td class="td_left">Constant "m" (0x6D)</td>
  </tr>
</table>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="module">Module Block</a> </div>
<div class="doc_text">
<p>The module block contains a small pre-amble and all the other blocks in
the file. The table below shows the structure of the module block. Note that it
only provides the module identifier, size of the module block, and the format
information. Everything else is contained in other blocks, described in other
sections.</p>
<table class="doc_table_nw" >
  <tr>
    <th><b>Type</b></th>
    <th class="td_left"><b>Field Description</b></th>
  </tr><tr>
    <td><a href="#unsigned">unsigned</a></td>
    <td class="td_left">Module Identifier (0x01)</td>
  </tr><tr>
    <td><a href="#unsigned">unsigned</a></td>
    <td class="td_left">Size of the module block in bytes</td>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</a></td>
    <td class="td_left"><a href="#format">Format Information</a></td>
  </tr><tr>
    <td><a href="#block">block</a></td>
    <td class="td_left"><a href="#globaltypes">Global Type Pool</a></td>
  </tr><tr>
    <td><a href="#block">block</a></td>
    <td class="td_left"><a href="#globalinfo">Module Globals Info</a></td>
  </tr><tr>
    <td><a href="#block">block</a></td>
    <td class="td_left"><a href="#constantpool">Module Constant Pool</a></td>
  </tr><tr>
    <td><a href="#block">block</a></td>
    <td class="td_left"><a href="#functiondefs">Function Definitions</a></td>
  </tr><tr>
    <td><a href="#block">block</a></td>
    <td class="td_left"><a href="#symboltable">Module Symbol Table</a></td>
  </tr>
</table>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="format">Format Information</a></div>
<div class="doc_text">
<p>The format information field is encoded into a 32-bit vbr-encoded unsigned 
integer as shown in the following table.</p>
<table>
  <tr>
    <th><b>Bit(s)</b></th>
    <th><b>Type</b></th>
    <th class="td_left"><b>Description</b></th>
  </tr><tr>
    <td>0</td><td>bit</td>
    <td class="td_left">Big Endian?</td>
  </tr><tr>
    <td>1</td><td>bit</td>
    <td class="td_left">Pointers Are 64-bit?</td>
  </tr><tr>
    <td>2</td><td>bit</td>
    <td class="td_left">Has No Endianess?</td>
  </tr><tr>
    <td>3</td><td>bit</td>
    <td class="td_left">Has No Pointer Size?</td>
  </tr><tr>
    <td>4-31</td><td>bit</td>
    <td class="td_left">Bytecode Format Version</td>
  </tr>
</table>
<p>
Of particular note, the bytecode format number is simply a 28-bit
monotonically increase integer that identifies the version of the bytecode
format (which is not directly related to the LLVM release number).  The 
bytecode versions defined so far are (note that this document only describes 
the latest version, 1.3):</p>
<ul>
<li>#0: LLVM 1.0 &amp; 1.1</li>
<li>#1: LLVM 1.2</li>
<li>#2: LLVM 1.3</li>
</ul>
<p>Note that we plan to eventually expand the target description capabilities
of bytecode files to <a href="http://llvm.cs.uiuc.edu/PR263">target triples</a>.
</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="globaltypes">Global Type Pool</a> </div>
<div class="doc_text">
<p>The global type pool consists of type definitions. Their order of appearance
in the file determines their slot number (0 based). Slot numbers are used to 
replace pointers in the intermediate representation. Each slot number uniquely
identifies one entry in a type plane (a collection of values of the same type).
Since all values have types and are associated with the order in which the type
pool is written, the global type pool <em>must</em> be written as the first 
block of a module. If it is not, attempts to read the file will fail because
both forward and backward type resolution will not be possible.</p>
<p>The type pool is simply a list of type definitions, as shown in the table 
below.</p>
<table class="doc_table_nw" >
  <tr>
    <th><b>Type</b></th>
    <th class="td_left"><b>Field Description</b></th>
  </tr><tr>
    <td><a href="#unsigned">unsigned</a></td>
    <td class="td_left">Type Pool Identifier (0x13)</td>
  </tr><tr>
    <td><a href="#unsigned">unsigned</a></td>
    <td class="td_left">Size in bytes of the symbol table block.</td>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</a></td>
    <td class="td_left">Number of entries in type plane</td>
  </tr><tr>
    <td><a href="#type">type</a></td>
    <td class="td_left">Each of the type definitions (see below)<sup>1</sup></td>
  </tr><tr>
    <td class="td_left" colspan="2">
      <sup>1</sup>Repeated field.<br/>
    </td>
  </tr>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="type">Type Definitions</a></div>
<div class="doc_text">
<p>Types in the type pool are defined using a different format for each
basic type of type as given in the following sections.</p>
<h3>Primitive Types</h3>
<p>The primitive types encompass the basic integer and floating point types</p>
<table>
  <tr>
    <th><b>Type</b></th>
    <th class="td_left"><b>Description</b></th>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Type ID For The Primitive (1-11)<sup>1</sup></td>
  </tr><tr>
    <td class="td_left" colspan="2">
      <sup>1</sup>See the definition of Type::TypeID in Type.h for the numeric
      equivalents of the primitive type ids.<br/>
    </td>
  </tr>
</table>
<h3>Function Types</h3>
<table>
  <tr>
    <th><b>Type</b></th>
    <th class="td_left"><b>Description</b></th>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Type ID for function types (13)</td>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Slot number of function's return type.</td>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">The number of arguments in the function.</td>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
      <td class="td_left">Slot number of each argument's type.<sup>1</sup></td>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Value 0 if this is a varargs function.<sup>2</sup></td>
  </tr><tr>
    <td class="td_left" colspan="2">
      <sup>1</sup>Repeated field.<br/>
      <sup>2</sup>Optional field.
    </td>
  </tr>
</table>
<h3>Structure Types</h3>
<table>
  <tr>
    <th><b>Type</b></th>
    <th class="td_left"><b>Description</b></th>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Type ID for structure types (14)</td>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Slot number of each of the element's fields.<sup>1</sup></td>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Null Terminator (VoidTy type id)</td>
  </tr><tr>
    <td class="td_left" colspan="2">
      <sup>1</sup>Repeated field.<br/>
    </td>
  </tr>
</table>
<h3>Array Types</h3>
<table>
  <tr>
    <th><b>Type</b></th>
    <th class="td_left"><b>Description</b></th>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Type ID for Array Types (15)</td>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Slot number of array's element type.</td>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">The number of elements in the array.</td>
  </tr>
</table>
<h3>Pointer Types</h3>
<table>
  <tr>
    <th><b>Type</b></th>
    <th class="td_left"><b>Description</b></th>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Type ID For Pointer Types (16)</td>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Slot number of pointer's element type.</td>
  </tr>
</table>
<h3>Opaque Types</h3>
<table>
  <tr>
    <th><b>Type</b></th>
    <th class="td_left"><b>Description</b></th>
  </tr><tr>
    <td><a href="#uint32_vbr">uint32_vbr</td>
    <td class="td_left">Type ID For Opaque Types (17)</td>
  </tr>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="globalinfo">Module Global Info</a> </div>
<div class="doc_text">
  <p>To be determined.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="constantpool">Constant Pool</a> </div>
<div class="doc_text">
  <p>To be determined.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="functiondefs">Function Definition</a> </div>
<div class="doc_text">
  <p>To be determined.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="compactiontable">Compaction Table</a> </div>
<div class="doc_text">
  <p>To be determined.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="instructionlist">Instruction List</a> </div>
<div class="doc_text">
  <p>To be determined.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="symtab">Symbol Table</a> </div>
<div class="doc_text">
<p>A symbol table can be put out in conjunction with a module or a function.
A symbol table is a list of type planes. Each type plane starts with the number
of entries in the plane and the type plane's slot number (so the type can be 
looked up in the global type pool). For each entry in a type plane, the slot 
number of the value and the name associated with that value are written.  The 
format is given in the table below. </p>
<table class="doc_table_nw" >
  <tr>
    <th><b>Byte(s)</b></th>
    <th><b>Bit(s)</b></th>
    <th><b>Align?</b></th>
    <th><b>Type</b></th>
    <th class="td_left"><b>Field Description</b></th>
  </tr><tr>
    <td>00-03</td><td>-</td><td>No</td><td>unsigned</td>
    <td class="td_left">Symbol Table Identifier (0x13)</td>
  </tr><tr>
    <td>04-07</td><td>-</td><td>No</td><td>unsigned</td>
    <td class="td_left">Size in bytes of the symbol table block.</td>
  </tr><tr>
    <td>08-11<sup>1</sup></td><td>-</td><td>No</td><td>uint32_vbr</td>
    <td class="td_left">Number of entries in type plane</td>
  </tr><tr>
    <td>12-15<sup>1</sup></td><td>-</td><td>No</td><td>uint32_vbr</td>
    <td class="td_left">Type plane index for following entries</td>
  </tr><tr>
    <td>16-19<sup>1,2</sup></td><td>-</td><td>No</td><td>uint32_vbr</td>
    <td class="td_left">Slot number of a value.</td>
  </tr><tr>
    <td>variable<sup>1,2</sup></td><td>-</td><td>No</td><td>string</td>
    <td class="td_left">Name of the value in the symbol table.</td>
  </tr>
  <tr>
    <td class="td_left" colspan="5"><sup>1</sup>Maximum length shown, 
      may be smaller<br><sup>2</sup>Repeated field.
  </tr>
</table>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="versiondiffs">Version Differences</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section describes the differences in the Bytecode Format across LLVM
versions. The versions are listed in reverse order because it assumes the 
current version is as documented in the previous sections. Each section here
describes the differences between that version and the one that <i>follows</i>
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection">
<a name="vers12">Version 1.2 Differences From 1.3</a></div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Type Derives From Value</div>
<div class="doc_text">
  <p>In version 1.2, the Type class in the LLVM IR derives from the Value class.
  This is not the case in version 1.3. Consequently, in version 1.2 the notion
  of a "Type Type" was used to write out values that were Types. The types 
  always occuped plane 12 (corresponding to the TypeTyID) of any type planed
  set of values. In 1.3 this representation is not convenient because the 
  TypeTyID (12) is not present and its value is now used for LabelTyID. 
  Consequently, the data structures written that involve types do so by writing
  all the types first and then each of the value planes according to those
  types. In version 1.2, the types would have been written intermingled with
  the values.</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Restricted getelementptr Types</a></div>
<div class="doc_text">
  <p>In version 1.2, the getelementptr instruction required a ubyte type index
  for accessing a structure field and a long type index for accessing an array
  element. Consequently, it was only possible to access structures of 255 or
  fewer elements. Starting in version 1.3, this restriction was lifted. 
  Structures must now be indexed with uint constants. Arrays may now be 
  indexed with int, uint, long, or ulong typed values. 
  The consequence of this was that the bytecode format had to 
  change in order to accommodate the larger range of structure indices.</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection">
<a name="vers11">Version 1.1 Differences From 1.2 </a></div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Explicit Primitive Zeros</div>
<div class="doc_text">
  <p>In version 1.1, the zero value for primitives was explicitly encoded into
  the bytecode format. Since these zero values are constant values in the
  LLVM IR and never change, there is no reason to explicitly encode them. This
  explicit encoding was removed in version 1.2.</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Inconsistent Module Global Info</div>
<div class="doc_text">
  <p>In version 1.1, the Module Global Info block was not aligned causing the
  next block to be read in on an unaligned boundary. This problem was corrected
  in version 1.2.</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection">
<a name="vers11">Version 1.0 Differences From 1.1</a></div>
<div class="doc_text">
<p>None. Version 1.0 and 1.1 bytecode formats are identical.</p>
</div>

<!-- *********************************************************************** -->
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