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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 Structure</a> </li>
<li><a href="#blockdefs">Block Definitions</a>
<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 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>
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<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="uint32_vbr">uint32_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 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">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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<div class="doc_code">
%MyType = type { int, sbyte }<br>
%MyVar = external global %MyType
</div>
<p>there are two definitions. The definition of <tt>%MyVar</tt> uses
<tt>%MyType</tt>. In the C++ IR this linkage between <tt>%MyVar</tt> and
<tt>%MyType</tt> is
explicit through the use of C++ pointers. In bytecode, however, there's no
ability to store memory addresses. Instead, we compute and write out slot
numbers for every Type and Value written to the file.</p>
<p>A slot number is simply an unsigned 32-bit integer encoded in the variable
bit rate scheme (see <a href="#encoding">encoding</a>). This ensures that
low slot numbers are encoded in one byte. Through various bits of magic LLVM
attempts to always keep the slot numbers low. The first attempt is to associate
slot numbers with their "type plane". That is, Values of the same type are
written to the bytecode file in a list (sequentially). Their order in that list
determines their slot number. This means that slot #1 doesn't mean anything
unless you also specify for which type you want slot #1. Types are handled
specially and are always written to the file first (in the
<a href="#globaltypes">Global Type Pool</a>) and
in such a way that both forward and backward references of the types can often be
resolved with a single pass through the type pool. </p>
<p>Slot numbers are also kept small by rearranging their order. Because of the
structure of LLVM, certain values are much more likely to be used frequently
in the body of a function. For this reason, a compaction table is provided in
the body of a function if its use would make the function body smaller.
Suppose you have a function body that uses just the types "int*" and "{double}"
but uses them thousands of time. Its worthwhile to ensure that the slot number
for these types are low so they can be encoded in a single byte (via vbr).
This is exactly what the compaction table does.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="general">General Structure</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section provides the general structur 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>
</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"> <a href="#globaltypes">Global Type 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"> <a href="#globalinfo">Module Globals 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"> <a href="#constantpool">Module Constant 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"> <a href="#functiondefs">Function 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"> <a href="#constantpool">Function Constant 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"> <a href="#compactiontable">Compaction 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"> <a href="#instructionlist">Instruction 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"> <a href="#symtab">Function Symbol 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"> <a href="#symtab">Module Symbol 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 or see <a href="#blocktypes">Block Types</a> for
details about the contents of each of the block types.</p>
</div>
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<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">
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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>
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<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="globaltypes">Global Type Pool</a> </div>
<div class="doc_text">
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<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>
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589
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591
592
593
594
595
596
597
598
599
600
601
602
603
604
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606
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609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
<h3>Array Types</h3>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</td>
<td class="td_left">Type ID for Array Types (15)</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</td>
<td class="td_left">Slot number of array's element type.</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</td>
<td class="td_left">The number of elements in the array.</td>
</tr>
</table>
<h3>Pointer Types</h3>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</td>
<td class="td_left">Type ID For Pointer Types (16)</td>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</td>
<td class="td_left">Slot number of pointer's element type.</td>
</tr>
</table>
<h3>Opaque Types</h3>
<table>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr><tr>
<td><a href="#uint32_vbr">uint32_vbr</td>
<td class="td_left">Type ID For Opaque Types (17)</td>
</tr>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="globalinfo">Module Global Info</a> </div>
<div class="doc_text">
<p>The module global info block contains the definitions of all global
variables including their initializers and the <em>declaration</em> of all
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638
639
640
641
642
643
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698
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701
702
703
704
705
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707
<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">
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811
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813
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815
816
817
818
819
820
821
822
823
<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>
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838
839
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845
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849
850
851
852
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855
856
857
858
859
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861
862
863
<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 linkage type of the function: 0=External, 1=Weak,
2=Appending, 3=Internal, 4=LinkOnce<sup>1</sup></td>
</tr><tr>
<td><a href="#constantpool">constant pool</a></td>
<td class="td_left">The constant pool block for this function.
<sup>2</sup>
</td>
</tr><tr>
<td><a href="#compactiontable">compaction table</a></td>
<td class="td_left">The compaction table block for the function.
<sup>2</sup>
</td>
</tr><tr>
<td><a href="#instructionlist">instruction list</a></td>
<td class="td_left">The list of instructions in the function.</td>
</tr><tr>
<td><a href="#symboltable">symbol table</a></td>
<td class="td_left">The function's slot table 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">
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876
877
878
879
880
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918
<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">
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939
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946
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998
999
1000
<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.<sup>1</sup></td>
</tr>
</table>
Notes:
<ol>
<li>A repeated field with a variety of formats. See
<a href="#instruction">Instructions</a> for details.</li>
</ol>
</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 for the operand(s).
<sup>1,2</sup></td>
</tr>
</table>
Notes:<ol>
<li>Repeatable field (limit given by previous field).</li>
<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