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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="#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>
<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>
</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>
<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 &
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>
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161
162
163
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191
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196
197
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200
201
<tbody>
<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>
</tbody>
</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 << 1) | 1)" and 3 is encoded as "(3 <<
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">
<tbody>
<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 style="vertical-align: top;"><a name="uint24_vbr">
<b>uint24_vbr</b></a></td>
<td style="vertical-align: top; text-align: left;">A 24-bit unsigned
integer that occupies from one to four bytes using variable bit rate
encoding.</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 style="vertical-align: top;"><b><a name="float"><b>float</b></a></b></td>
<td style="vertical-align: top; text-align: left;">A floating point value encoded
as a 32-bit IEEE value written in little-endian form.<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;"><b><b><a name="double"><b>double</b></a></b></b></td>
<td style="vertical-align: top; text-align: left;">A floating point value encoded
as a64-bit IEEE value written in little-endian form</td>
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291
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295
296
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301
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</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 constant initializers.<br>
</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 is an unsigned 32-bit integer that encodes the type of the block
in the low 5 bits and the size of the block in the high 27 bits. The
length does not include the block header or any alignment bytes at the
end of the block. Blocks may compose other blocks. </td>
</tr>
</tbody>
</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">
<tbody>
<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>
</tbody>
</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>
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555
556
<tbody>
<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. </td>
</tr>
<tr>
<td>0x06</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>0x05</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>0x03</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>0x02</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>0x03</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>0x08</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>0x07</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>0x04</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>0x04</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>
</tbody>
<p>Use the links in the table for details about the contents of each of
the block types.</p>
<!-- *********************************************************************** -->
<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">
Chris Lattner
committed
<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>
<tbody>
<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>
</tbody>
</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>
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr>
<tr>
<td><a href="#block">block</a><br>
</td>
<td class="td_left">Module Block Identifier (0x01) and Size<br>
</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>
</tbody>
</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>
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
<tbody>
<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>
</tbody>
</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>
<li>#0: LLVM 1.0 & 1.1</li>
<li>#1: LLVM 1.2</li>
<li>#2: LLVM 1.2.5 (not released)</li>
<li>#3: LLVM 1.3<br>
</li>
<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>.
</div>
<!-- _______________________________________________________________________ -->
<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>
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr>
<tr>
<td><a href="#unsigned">block</a></td>
<td class="td_left">Type Pool Identifier (0x06) + Size<br>
</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>
</tbody>
</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>
<p>The primitive types encompass the basic integer and floating point
types</p>
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr>
<tr>
<td><a href="#uint24_vbr">uint24_vbr</a></td>
<td class="td_left">Type ID for the primitive types (values 1 to
11) <sup>1</sup></td>
</tr>
</tbody>
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>
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr>
<tr>
<td><a href="#uint24_vbr">uint24_vbr</a></td>
<td class="td_left">Type ID for function types (13)</td>
</tr>
<tr>
<td><a href="#uint24_vbr">uint24_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="#uint24_vbr">uint24_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>
</tbody>
</table>
<h3>Structure Types</h3>
<table>
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr>
<tr>
<td><a href="#uint24_vbr">uint24_vbr</a></td>
<td class="td_left">Type ID for structure types (14)</td>
</tr>
<tr>
<td><a href="#zlist">zlist</a>(<a href="#uint24_vbr">uint24_vbr</a>)</td>
<td class="td_left">Slot number of each of the element's fields.</td>
</tr>
</tbody>
</table>
<h3>Array Types</h3>
<table>
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr>
<tr>
<td><a href="#uint24_vbr">uint24_vbr</a></td>
<td class="td_left">Type ID for Array Types (15)</td>
</tr>
<tr>
<td><a href="#uint24_vbr">uint24_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>
</tbody>
</table>
<h3>Pointer Types</h3>
<table>
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr>
<tr>
<td><a href="#uint24_vbr">uint24_vbr</a></td>
<td class="td_left">Type ID For Pointer Types (16)</td>
</tr>
<tr>
<td><a href="#uint24_vbr">uint24_vbr</a></td>
<td class="td_left">Slot number of pointer's element type.</td>
</tr>
</tbody>
</table>
<h3>Opaque Types</h3>
<table>
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
</tr>
<tr>
<td><a href="#uint24_vbr">uint24_vbr</a></td>
<td class="td_left">Type ID For Opaque Types (17)</td>
</tr>
</tbody>
</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>
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr>
<tr>
<td><a href="#block">block</a></td>
<td class="td_left">Module global info identifier (0x05) + size<br>
</td>
</tr>
<tr>
<td><a href="#zlist">zlist</a>(<a href="#globalvar">globalvar</a>)</td>
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
<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="#uint24_vbr">uint24_vbr</a>)</td>
<td class="td_left">A zero terminated list of function types
occuring in the module.</td>
</tr>
<tr>
<td style="vertical-align: top;"><a href="#llist">llist</a>(<a
href="#string">string</a>)<br>
</td>
<td style="vertical-align: top; text-align: left;">A length list
of strings that specify the names of the libraries that this module
depends upon.<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;"><a href="#string">string</a><br>
</td>
<td style="vertical-align: top; text-align: left;">The target
triple for the module (blank means no target triple specified, i.e. a
platform independent module).<br>
</td>
</tr>
</tbody>
</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>
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Description</b></th>
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
</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. </td>
</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>
</tbody>
</table>
<p>The table below provides the format of the constant initializers for
the global variable field, if it has one.</p>
<table>
<tbody>
<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>))?
</td>
<td class="td_left">An optional zero-terminated list of slot
numbers of the global variable's constant initializer.</td>
</tr>
</tbody>
</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>
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>
</tr>
<tr>
<td><a href="#block">block</a></td>
<td class="td_left">Constant pool identifier (0x03) + size<br>
</td>
</tr>
</tbody>
</table>
<p><b>Module Constant Pool Preamble (constant strings)</b></p>
<table>
<tbody>
<tr>
<th><b>Type</b></th>
<th class="td_left"><b>Field Description</b></th>