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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="#vbr">Variable Bit-Rate Encoding</a></li>
<li><a href="#encoding">Encoding Primitives</a></li>
<li><a href="#slots">Slots</a></li>
</ol>
</li>
<li><a href="#general">General Structure</a> </li>
<li><a href="#blockdefs">Block Definitions</a>
<ol>
<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_section"> <a name="abstract">Abstract </a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This document describes the LLVM bytecode file format. 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>
<p>The LLVM bytecode format may change in the future, but LLVM will always be
backwards compatible with older formats. This document will only describe
the most current version of the bytecode format. See
<a href="#versiondiffs">Version Differences</a> for the details on how the
current version is different from previous versions.</p>
</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 specific layout details. It is recommended that you read
this section thoroughly before interpreting the detailed descriptions.</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 using
a binary encoding 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 (<a href="#llist">llist</a>), and null terminated lists
(<a href="#zlist">zlist</a>), as described below in the
<a href="#encoding">Encoding Primitives</a>.</p>
</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="vbr">Variable Bit-Rate Encoding</a> </div>
<div class="doc_text">
<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>
<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>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="encoding">Encoding Primitives</a> </div>
<div class="doc_text">
<p>Each field in the bytecode format is encoded into the file using a small
set of primitive formats. The table below defines the encoding rules for the
various primitives used and gives them each a type name. The type names used
in the descriptions of blocks and fields in the <a href="#details">Detailed
Layout</a>next section. Any type name with the suffix <em>_vbr</em> indicates
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"><b>unsigned</b></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="uint32_vbr"><b>uint32_vbr</b></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"><b>uint64_vbr</b></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"><b>int64_vbr</b></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"><b>char</b></a></td>
<td class="td_left">A single unsigned character encoded into one byte</td>
</tr><tr>
<td><a name="bit"><b>bit(n-m)</b></a></td>
<td class="td_left">A set of bit within some larger integer field. The
values of <code>n</code> and <code>m</code> specify the inclusive range
of bits that define the subfield. The value for <code>m</code> may be
omitted if its the same as <code>n</code>.</td>
</tr><tr>
<td><a name="string"><b>string</b></a></td>
<td class="td_left">A uint32_vbr indicating the type of the constant string
which also includes its length, immediately followed by the characters of
the string. There is no terminating null byte in the string.</td>
</tr><tr>
<td><a name="data"><b>data</b></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="llist"><b>llist(x)</b></a></td>
<td class="td_left">A length list of x. This means the list is encoded as
an <a href="#uint32_vbr">uint32_vbr</a> providing the length of the list,
followed by a sequence of that many "x" items. This implies that the reader
should iterate the number of times provided by the length.</td>
</tr><tr>
<td><a name="zlist"><b>zlist(x)</b></a></td>
<td class="td_left">A zero-terminated list of x. This means the list is encoded
as a sequence of an indeterminate number of "x" items, followed by an
<a href="#uint32_vbr">uint32_vbr</a> terminating value. This implies that none
of the "x" items can have a zero value (or else the list terminates).</td>
</tr><tr>
<td><a name="block"><b>block</b></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="notation">Field Notation</a> </div>
<div class="doc_text">
<p>In the detailed block and field descriptions that follow, a regex like
notation is used to describe optional and repeated fields. A very limited
subset of regex is used to describe these, as given in the following table:
</p>
<table class="doc_table" >
<tr>
<th><b>Character</b></th>
<th class="td_left"><b>Meaning</b></th>
</tr><tr>
<td><b><code>?</code></b></td>
<td class="td_left">The question mark indicates 0 or 1 occurrences of
the thing preceding it.</td>
</tr><tr>
<td><b><code>*</code></b></td>
<td class="td_left">The asterisk indicates 0 or more occurrences of the
thing preceding it.</td>
</tr><tr>
<td><b><code>+</code></b></td>
<td class="td_left">The plus sign indicates 1 or more occurrences of the
thing preceding it.</td>
</tr><tr>
<td><b><code>()</code></b></td>
<td class="td_left">Parentheses are used for grouping.</td>
</tr><tr>
<td><b><code>,</code></b></td>
<td class="td_left">The comma separates sequential fields.</td>
</tr>
</table>
<p>So, for example, consider the following specifications:</p>
<div class="doc_code">
<ol>
<li><code>string?</code></li>
<li><code>(uint32_vbr,uin32_vbr)+</code></li>
<li><code>(unsigned?,uint32_vbr)*</code></li>
<li><code>(llist(unsigned))?</code></li>
</ol>
</div>
<p>with the following interpretations:</p>
<ol>
<li>An optional string. Matches either nothing or a single string</li>
<li>One or more pairs of uint32_vbr.</li>
<li>Zero or more occurrences of either an unsigned followed by a uint32_vbr
or just a uint32_vbr.</li>
<li>An optional length list of unsigned values.</li>
</ol>
</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"><code>
%MyType = type { int, sbyte }<br>
%MyVar = external global %MyType
</code></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 Structure</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section provides the general structure of the LLVM bytecode file
format. 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>
<th>Description</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>
<td class="td_left">This contains the file signature (magic number)
that identifies the file as LLVM bytecode.</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>
<td class="td_left">This is the top level block in a bytecode file. It
contains all the other blocks.</li>
</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&nbsp;Type&nbsp;Pool</a></td>
<td class="td_left">This block contains all the global (module) level
types.</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&nbsp;Globals&nbsp;Info</a></td>
<td class="td_left">This block contains the type, constness, and linkage
for each of the global variables in the module. It also contains the
type of the functions and the constant initializers.</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&nbsp;Constant&nbsp;Pool</a></td>
<td class="td_left">This block contains all the global constants
except function arguments, global values and constant strings.</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&nbsp;Definitions</a>*</td>
<td class="td_left">One function block is written for each function in
the module. The function block contains the instructions, compaction
table, type constant pool, and symbol table for the function.</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&nbsp;Constant&nbsp;Pool</a></td>
<td class="td_left">Any constants (including types) used solely
within the function are emitted here in the function constant pool.
</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&nbsp;Table</a></td>
<td class="td_left">This table reduces bytecode size by providing a
funtion-local mapping of type and value slot numbers to their
global slot numbers</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&nbsp;List</a></td>
<td class="td_left">This block contains all the instructions of the
function. The basic blocks are inferred by terminating instructions.
</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="#symtab">Function&nbsp;Symbol&nbsp;Table</a></td>
<td class="td_left">This symbol table provides the names for the
function specific values used (basic block labels mostly).</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="#symtab">Module&nbsp;Symbol&nbsp;Table</a></td>
<td class="td_left">This symbol table provides the names for the various
entries in the file that are not function specific (global vars, and
functions mostly).</td>
</tr>
</table>
<p>Use the links in the table for details about the contents of each of the block types.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="blockdefs">Block Definitions</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section provides the detailed layout of the individual block types
in the LLVM bytecode file format. </p>
</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>
<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>
<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
<a href="#uint32_vbr">uint32_vbr</a> as shown in the following table.</p>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr><tr>
<td><a href="#bit">bit(0)</a></td>
<td class="td_left">Target is big endian?</td>
</tr><tr>
<td><a href="#bit">bit(1)</a></td>
<td class="td_left">On target pointers are 64-bit?</td>
</tr><tr>
<td><a href="#bit">bit(2)</a></td>
<td class="td_left">Target has no endianess?</td>
</tr><tr>
<td><a href="#bit">bit(3)</a></td>
<td class="td_left">Target has no pointer size?</td>
</tr><tr>
<td><a href="#bit">bit(4-31)</a></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>
<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 (0x15)</td>
</tr><tr>
<td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Size in bytes of the type pool block.</td>
</tr><tr>
<td><a href="#llist">llist</a>(<a href="#type">type</a>)</td>
<td class="td_left">A length list of type definitions.</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 kind
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</a></td>
<td class="td_left">Type ID for the primitive types (values 1 to 11)
<sup>1</sup></td>
</tr>
</table>
Notes:
<ol>
<li>The values for the Type IDs for the primitive types are provided by the
definition of the <code>llvm::Type::TypeID</code> enumeration in
<code>include/llvm/Type.h</code>. The enumeration gives the following
mapping:<ol>
<li>bool</li>
<li>ubyte</li>
<li>sbyte</li>
<li>ushort</li>
<li>short</li>
<li>uint</li>
<li>int</li>
<li>ulong</li>
<li>long</li>
<li>float</li>
<li>double</li>
</ol></li>
</ol>
<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</a></td>
<td class="td_left">Type ID for function types (13)</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Slot number of function's return type.</td>
</tr><tr>
<td><a href="#llist">llist</a>(<a href="#uint32_vbr">uint32_vbr</a>)</td>
<td class="td_left">Slot number of each argument's type.</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a>?</td>
<td class="td_left">Value 0 if this is a varargs function, missing otherwise.</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</a></td>
<td class="td_left">Type ID for structure types (14)</td>
</tr><tr>
<td><a href="#zlist">zlist</a>(<a href="#uint32_vbr">uint32_vbr</a>)</td>
<td class="td_left">Slot number of each of the element's fields.</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</a></td>
<td class="td_left">Type ID for Array Types (15)</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Slot number of array's element type.</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></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</a></td>
<td class="td_left">Type ID For Pointer Types (16)</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></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</a></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>The module global info block contains the definitions of all global
variables including their initializers and the <em>declaration</em> of all
functions. The format is shown in the table below:</p>
<table>
<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 global info identifier (0x14)</td>
</tr><tr>
<td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Size in bytes of the module global info block.</td>
</tr><tr>
<td><a href="#zlist">zlist</a>(<a href="#globalvar">globalvar</a>)</td>
<td class="td_left">A zero terminated list of global var definitions
occuring in the module.</td>
</tr><tr>
<td><a href="#zlist">zlist</a>(<a href="#uint32_vbr">uint32_vbr</a>)</td>
<td class="td_left">A zero terminated list of function types occuring in
the module.</td>
</tr>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="globalvar">Global Variable Field</a>
</div>
<div class="doc_text">
<p>Global variables are written using an <a href="#uint32_vbr">uint32_vbr</a>
that encodes information about the global variable and a list of the constant
initializers for the global var, if any.</p>
<p>The table below provides the bit layout of the first
<a href="#uint32_vbr">uint32_vbr</a> that describes the global variable.</p>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr><tr>
<td><a href="#bit">bit(0)</a></td>
<td class="td_left">Is constant?</td>
</tr><tr>
<td><a href="#bit">bit(1)</a></td>
<td class="td_left">Has initializer? Note that this bit determines whether
the constant initializer field (described below) follows.</li>
</tr><tr>
<td><a href="#bit">bit(2-4)</a></td>
<td class="td_left">Linkage type: 0=External, 1=Weak, 2=Appending,
3=Internal, 4=LinkOnce</td>
</tr><tr>
<td><a href="#bit">bit(5-31)</a></td>
<td class="td_left">Slot number of type for the global variable.</td>
</tr>
</table>
<p>The table below provides the format of the constant initializers for the
global variable field, if it has one.</p>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr><tr>
<td>(<a href="#zlist">zlist</a>(<a href="#uint32_vbr">uint32_vbr</a>))?
</a>
</td>
<td class="td_left">An optional zero-terminated list of slot numbers of
the global variable's constant initializer.</td>
</tr>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="constantpool">Constant Pool</a> </div>
<div class="doc_text">
<p>A constant pool defines as set of constant values. There are actually two
types of constant pool blocks: one for modules and one for functions. For
modules, the block begins with the constant strings encountered anywhere in
the module. For functions, the block begins with types only encountered in
the function. In both cases the header is identical. The tables that follow,
show the header, module constant pool preamble, function constant pool
preamble, and the part common to both function and module constant pools.</p>
<p><b>Common Block Header</b></p>
<table>
<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">Constant pool identifier (0x12)</td>
</tr><tr>
<td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Size in bytes of the constant pool block.</td>
</tr>
</table>
<p><b>Module Constant Pool Preamble (constant strings)</b></p>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">The number of constant strings that follow.</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Zero. This identifies the following "plane" as
containing the constant strings. This is needed to identify it
uniquely from other constant planes that follow.
</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a>+</td>
<td class="td_left">Slot number of the constant string's type. Note
that the constant string's type implicitly defines the length of
the string.
</td>
</tr>
</table>
<p><b>Function Constant Pool Preamble (function types)</b></p>
<p>The structure of the types for functions is identical to the
<a href="#globaltypes">Global Type Pool</a>. Please refer to that section
for the details.
<p><b>Common Part (other constants)</b></p>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Number of entries in this type plane.</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Type slot number of this plane.</td>
</tr><tr>
<td><a href="#constant">constant</a>+</td>
<td class="td_left">The definition of a constant (see below).</td>
</tr>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="constant">Constant Field</a></div>
<div class="doc_text">
<p>Constants come in many shapes and flavors. The sections that followe define
the format for each of them. All constants start with a
<a href="#uint32_vbr">uint32_vbr</a> encoded integer that provides the number
of operands for the constant. For primitive, structure, and array constants,
this will always be zero since those types of constants have no operands.
In this case, we have the following field definitions:</p>
<ul>
<li><b>Bool</b>. This is written as an <a href="#uint32_vbr">uint32_vbr</a>
of value 1U or 0U.</li>
<li><b>Signed Integers (sbyte,short,int,long)</b>. These are written as
an <a href="#int64_vbr">int64_vbr</a> with the corresponding value.</li>
<li><b>Unsigned Integers (ubyte,ushort,uint,ulong)</b>. These are written
as an <a href="#uint64_vbr">uint64_vbr</a> with the corresponding value.
</li>
<li><b>Floating Point</b>. Both the float and double types are written
literally in binary format.</li>
<li><b>Arrays</b>. Arrays are written simply as a list of
<a href="#uint32_vbr">uint32_vbr</a> encoded slot numbers to the constant
element values.</li>
<li><b>Structures</b>. Structures are written simply as a list of
<a href="#uint32_vbr">uint32_vbr</a> encoded slot numbers to the constant
field values of the structure.</li>
</ul>
<p>When the number of operands to the constant is non-zero, we have a
constant expression and its field format is provided in the table below.</p>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Op code of the instruction for the constant
expression.</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">The slot number of the constant value for an
operand.<sup>1</sup></td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">The slot number for the type of the constant value
for an operand.<sup>1</sup></td>
</tr>
</table>
Notes:<ol>
<li>Both these fields are repeatable but only in pairs.</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="functiondefs">Function Definition</a></div>
<div class="doc_text">
<p>Function definitions contain the linkage, constant pool or compaction
table, instruction list, and symbol table for a function. The following table
shows the structure of a function definition.</p>
<table>
<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">Function definition block identifier (0x11)</td>
</tr><tr>
<td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Size in bytes of the function definition block.</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">The linkage type of the function: 0=External, 1=Weak,
2=Appending, 3=Internal, 4=LinkOnce<sup>1</sup></td>
</tr><tr>
<td><a href="#block">block</a></td>
<td class="td_left">The <a href="#constantpool">constant pool</a> block
for this function.<sup>2</sup></td>
</tr><tr>
<td><a href="#block">block</a></td>
<td class="td_left">The <a href="#compactiontable">compaction table</a>
block for the function.<sup>2</sup></td>
</tr><tr>
<td><a href="#block">block</a></td>
<td class="td_left">The <a href="#instructionlist">instruction list</a>
for the function.</td>
</tr><tr>
<td><a href="#block">block</a></td>
<td class="td_left">The function's <a href="#symboltable">symbol table</a>
containing only those symbols pertinent to the function (mostly
block labels).</td>
</tr>
</table>
Notes:<ol>
<li>Note that if the linkage type is "External" then none of the other
fields will be present as the function is defined elsewhere.</li>
<li>Note that only one of the constant pool or compaction table will be
written. Compaction tables are only written if they will actually save
bytecode space. If not, then a regular constant pool is written.</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="compactiontable">Compaction Table</a> </div>
<div class="doc_text">
<p>Compaction tables are part of a function definition. They are merely a
device for reducing the size of bytecode files. The size of a bytecode
file is dependent on the <em>value</em> of the slot numbers used because
larger values use more bytes in the variable bit rate encoding scheme.
Furthermore, the compressed instruction format reserves only six bits for
the type of the instruction. In large modules, declaring hundreds or thousands
of types, the values of the slot numbers can be quite large. However,
functions may use only a small fraction of the global types. In such cases
a compaction table is created that maps the global type and value slot
numbers to smaller values used by a function. Functions will contain either
a function-specific constant pool <em>or</em> a compaction table but not
both. Compaction tables have the format shown in the table below.</p>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">The number of types that follow</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a>+</td>
<td class="td_left">The slot number in the global type plane of the
type that will be referenced in the function with the index of
this entry in the compaction table.</td>
</tr><tr>
<td><a href="#type_len">type_len</a></td>
<td class="td_left">An encoding of the type and number of values that
follow. This field's encoding varies depending on the size of
the type plane. See <a href="#type_len">Type and Length</a> for
further details.</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a>+</td>
<td class="td_left">The slot number in the globals of the value that
will be referenced in the function with the index of this entry in
the compaction table</td>
</tr>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="type_len">Type and Length</a></div>
<div class="doc_text">
<p>The type and length of a compaction table type plane is encoded differently
depending on the length of the plane. For planes of length 1 or 2, the length
is encoded into bits 0 and 1 of a <a href="#uint32_vbr">uint32_vbr</a> and the
type is encoded into bits 2-31. Because type numbers are often small, this
often saves an extra byte per plane. If the length of the plane is greater
than 2 then the encoding uses a <a href="#uint32_vbr">uint32_vbr</a> for each
of the length and type, in that order.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="instructionlist">Instruction List</a> </div>
<div class="doc_text">
<p>The instructions in a function are written as a simple list. Basic blocks
are inferred by the terminating instruction types. The format of the block
is given in the following table.</p>
<table>
<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">Instruction list identifier (0x33).</td>
</tr><tr>
<td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Size in bytes of the instruction list.</td>
</tr><tr>
<td><a href="#instruction">instruction</a>+</td>
<td class="td_left">An instruction. Instructions have a variety of formats.
See <a href="#instruction">Instructions</a> for details.</td>
</tr>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="instruction">Instructions</a></div>
<div class="doc_text">
<p>For brevity, instructions are written in one of four formats, depending on
the number of operands to the instruction. Each instruction begins with a
<a href="#uint32_vbr">uint32_vbr</a> that encodes the type of the instruction
as well as other things. The tables that follow describe the format of this
first word of each instruction.</p>
<p><b>Instruction Format 0</b></p>
<p>This format is used for a few instructions that can't easily be optimized
because they have large numbers of operands (e.g. PHI Node or getelementptr).
Each of the opcode, type, and operand fields is as successive fields.</p>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Specifies the opcode of the instruction. Note that for
compatibility with the other instruction formats, the opcode is shifted
left by 2 bits. Bits 0 and 1 must have value zero for this format.</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Provides the slot number of the result type of the
instruction</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">The number of operands that follow.</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a>+</td>
<td class="td_left">The slot number of the value(s) for the operand(s).
<sup>1</sup></td>
</tr>
</table>
Notes:<ol>
<li>Note that if the instruction is a getelementptr and the type of the
operand is a sequential type (array or pointer) then the slot number is
shifted up two bits and the low order bits will encode the type of index
used, as follows: 0=uint, 1=int, 2=ulong, 3=long.</li>
</ol>
<p><b>Instruction Format 1</b></p>
<p>This format encodes the opcode, type and a single operand into a single
<a href="#uint32_vbr">uint32_vbr</a> as follows:</p>
<table>
<tr>
<th><b>Bits</b></th>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr><tr>
<td>0-1</td><td>constant "1"</td>
<td class="td_left">These two bits must be the value 1 which identifies
this as an instruction of format 1.</td>
</td>
</tr><tr>
<td>2-7</td><td><a href="#opcodes">opcode</a></td>
<td class="td_left">Specifies the opcode of the instruction. Note that
the maximum opcode value is 63.</td>
</tr><tr>
<td>8-19</td><td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Specifies the slot number of the type for this
instruction. Maximum slot number is 2<sup>12</sup>-1=4095.</td>
</tr><tr>
<td>20-31</td><td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Specifies the slot number of the value for the
first operand. Maximum slot number is 2<sup>12</sup>-1=4095. Note
that the value 2<sup>12</sup>-1 denotes zero operands.</td>
</tr>
</table>
<p><b>Instruction Format 2</b></p>
<p>This format encodes the opcode, type and two operands into a single
<a href="#uint32_vbr">uint32_vbr</a> as follows:</p>
<table>
<tr>
<th><b>Bits</b></th>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr><tr>
<td>0-1</td><td>constant "2"</td>
<td class="td_left">These two bits must be the value 2 which identifies
this as an instruction of format 2.</td>
</td>
</tr><tr>
<td>2-7</td><td><a href="#opcodes">opcode</a></td>
<td class="td_left">Specifies the opcode of the instruction. Note that
the maximum opcode value is 63.</td>
</tr><tr>
<td>8-15</td><td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Specifies the slot number of the type for this
instruction. Maximum slot number is 2<sup>8</sup>-1=255.</td>
</tr><tr>
<td>16-23</td><td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Specifies the slot number of the value for the
first operand. Maximum slot number is 2<sup>8</sup>-1=255.</td>
</tr><tr>
<td>24-31</td><td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Specifies the slot number of the value for the
second operand. Maximum slot number is 2<sup>8</sup>-1=255.</td>
</tr>
</table>
<p><b>Instruction Format 3</b></p>
<p>This format encodes the opcode, type and three operands into a single
<a href="#uint32_vbr">uint32_vbr</a> as follows:</p>
<table>
<tr>
<th><b>Bits</b></th>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr><tr>
<td>0-1</td><td>constant "3"</td>
<td class="td_left">These two bits must be the value 3 which identifies
this as an instruction of format 3.</td>
</td>
</tr><tr>
<td>2-7</td><td><a href="#opcodes">opcode</a></td>
<td class="td_left">Specifies the opcode of the instruction. Note that
the maximum opcode value is 63.</td>
</tr><tr>
<td>8-13</td><td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Specifies the slot number of the type for this
instruction. Maximum slot number is 2<sup>6</sup>-1=63.</td>
</tr><tr>
<td>14-19</td><td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Specifies the slot number of the value for the
first operand. Maximum slot number is 2<sup>6</sup>-1=63.</td>
</tr><tr>
<td>20-25</td><td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Specifies the slot number of the value for the
second operand. Maximum slot number is 2<sup>6</sup>-1=63.</td>
</tr><tr>
<td>26-31</td><td><a href="#unsigned">unsigned</a></td>
<td class="td_left">Specifies the slot number of the value for the
third operand. Maximum slot number is 2<sup>6</sup>-1=63.</td>
</tr>
</table>
</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>
<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">Symbol Table 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="#symtab_entry">symtab_entry</a>*</td>
<td class="td_left">Provides the slot number of the type and its name.</td>
</tr><tr>
<td><a href="#symtab_plane">symtab_plane</a>*</td>
<td class="td_left">A type plane containing value slot number and name
for all values of the same type.</td>
</tr>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="symtab_plane">Symbol Table Plane</a>
</div>
<div class="doc_text">
<p>A symbol table plane provides the symbol table entries for all values of
a common type. The encoding is given in the following table:</p>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Number of entries in this plane.</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Slot number of type for this plane.</td>
</tr><tr>
<td><a href="#symtab_entry">symtab_entry</a>+</td>
<td class="td_left">The symbol table entries for this plane.</td>
</tr>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="symtab_entry">Symbol Table Entry</a>
</div>
<div class="doc_text">
<p>A symbol table entry provides the assocation between a type or value's
slot number and the name given to that type or value. The format is given
in the following table:</p>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Slot number of the type or value being given a name.
</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Length of the character array that follows.</td>
</tr><tr>
<td><a href="#char">char</a>+</td>
<td class="td_left">The characters of the name.</td>
</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="vers10">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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