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<div class="doc_title"> LLVM Bytecode File Format </div>
<ol>
<li><a href="#abstract">Abstract</a></li>
<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><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
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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>
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<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
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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>
<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>
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<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 & 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 & 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>
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<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>
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<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 << 1) | 1)" and 3 is encoded as "(3 << 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>
</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
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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 class="td_left">A uint_vbr indicating the type of the character 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">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>
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%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>
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<!-- *********************************************************************** -->
<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">
<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">
<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">
<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">
<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">
<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">
<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">
<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">
<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">
<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.
</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>
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<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>
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<li><a href="#gtypepool">Global Type Pool</a>: This block contains all the
global (module) level types.</li>
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<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>
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<li><a href="#constants">Constants</a>: This block contains all the global
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
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">
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<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>
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<!-- _______________________________________________________________________ -->
<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 & 1.1</li>
<li>#1: LLVM 1.2</li>
<li>#2: LLVM 1.3</li>
</ul>
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<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>
Chris Lattner
committed
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="globaltypes">Global Type Pool</a> </div>
<div class="doc_text">
Chris Lattner
committed
<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="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Number of type definitions that follow in the next
field.</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>
</table>
Notes:
<ol>
<li>Repeated field.</li>
</ol>
</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>
</table>
Notes:
<ol>
<li>See the definition of Type::TypeID in Type.h for the numeric equivalents
of the primitive type ids.</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</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>
</table>
Notes:
<ol>
<li>Repeated field.</li>
<li>Optional field.</li>
</ol>
<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>
</table>
Notes:
<ol>
<li>Repeatable field.</li>
</ol>
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
<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">
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
<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="#globalvar">globalvar</a></td>
<td class="td_left">Definition of the global variable (see below).
<sup>1</sup>
</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Slot number of the global variable's constant
initializer.<sup>1,2</sup>
</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Zero. This terminates the list of global variables.
</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Type slot number of a function defined in this
bytecode file.<sup>3</sup>
</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</a></td>
<td class="td_left">Zero. This terminates the list of function
declarations.
</tr>
</table>
Notes:<ol>
<li>Both these fields are repeatable but in pairs.</li>
<li>Optional field.</li>
<li>Repeatable field.</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="globalvar">Global Variable Field</a>
</div>
<div class="doc_text">
<p>Global variables are written using a single
<a href="#uint32_vbr">uint32_vbr</a> that encodes information about the global
variable. The table below provides the bit layout of the value written for
each global variable.</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">Is constant?</td>
</tr><tr>
<td>1</td><td>bit</td>
<td class="td_left">Has initializer?<sup>1</sup></td>
</tr><tr>
<td>2-4</td><td>enumeration</td>
<td class="td_left">Linkage type: 0=External, 1=Weak, 2=Appending,
3=Internal, 4=LinkOnce</td>
</tr><tr>
<td>5-31</td><td>type slot</td>
<td class="td_left">Slot number of type for the global variable.</td>
</tr>
</table>
Notes:
<ol>
<li>This bit determines whether the constant initializer field follows
immediately after this field</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="constantpool">Constant Pool</a> </div>
<div class="doc_text">
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
<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 the 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>
</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.
</td>
</tr><tr>
<td><a href="#string">string</a></td>
<td class="td_left">Slot number of the constant string's type which
includes the length of the string.<sup>1</sup>
</td>
</tr>
</table>
Notes:
<ol>
<li>Repeated field.</li>
</ol>
<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>To be determined.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="compactiontable">Compaction Table</a> </div>
<div class="doc_text">
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
<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 compresses 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. 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.<sup>1</sup></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.<sup>2</sup></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<sup>1</sup></td>
</tr>
</table>
Notes:<ol>
<li>Repeated field.</li>
<li>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.
</ol>
</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 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>
<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>
</table>
Notes:
<ol>
<li>Maximum length shown, may be smaller</li>
<li>Repeated field.</li>
</ol>
</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