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| <div class="doc_title"> LLVM Bytecode File Format </div> |
| <ol> |
| <li><a href="#abstract">Abstract</a></li> |
| <li><a href="#concepts">Concepts</a> |
| <ol> |
| <li><a href="#blocks">Blocks</a></li> |
| <li><a href="#lists">Lists</a></li> |
| <li><a href="#fields">Fields</a></li> |
| <li><a href="#align">Alignment</a></li> |
| <li><a href="#vbr">Variable Bit-Rate Encoding</a></li> |
| <li><a href="#encoding">Encoding Primitives</a></li> |
| <li><a href="#slots">Slots</a></li> |
| </ol> |
| </li> |
| <li><a href="#general">General Structure</a> </li> |
| <li><a href="#blockdefs">Block Definitions</a> |
| <ol> |
| <li><a href="#signature">Signature Block</a></li> |
| <li><a href="#module">Module Block</a></li> |
| <li><a href="#globaltypes">Global Type Pool</a></li> |
| <li><a href="#globalinfo">Module Info Block</a></li> |
| <li><a href="#constantpool">Global Constant Pool</a></li> |
| <li><a href="#functiondefs">Function Definition</a></li> |
| <li><a href="#compactiontable">Compaction Table</a></li> |
| <li><a href="#instructionlist">Instruction List</a></li> |
| <li><a href="#opcodes">Instruction Opcodes</a></li> |
| <li><a href="#symtab">Symbol Table</a></li> |
| </ol> |
| </li> |
| <li><a href="#versiondiffs">Version Differences</a> |
| <ol> |
| <li><a href="#vers13">Version 1.3 Differences From 1.4</a></li> |
| <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> |
| <div class="doc_text"> |
| <p>To support cross-platform differences, the bytecode file is aligned on |
| certain boundaries. This means that a small amount of padding (at most 3 |
| bytes) will be added to ensure that the next entry is aligned to a 32-bit |
| boundary.</p> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="vbr">Variable Bit-Rate Encoding</a> |
| </div> |
| <div class="doc_text"> |
| <p>Most of the values written to LLVM bytecode files are small integers. To |
| minimize the number of bytes written for these quantities, an encoding scheme |
| similar to UTF-8 is used to write integer data. The scheme is known as |
| variable bit rate (vbr) encoding. In this encoding, the high bit of |
| each byte is used to indicate if more bytes follow. If (byte & |
| 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> |
| <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> |
| </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 |
| 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> |
| <p>In summary then, a slot number can be though of as just a vbr encoded index |
| into a list of Type* or Value*. To keep slot numbers low, Value* are indexed by |
| two slot numbers: the "type plane index" (type slot) and the "value index" |
| (value slot).</p> |
| </div> |
| <!-- *********************************************************************** --> |
| <div class="doc_section"> <a name="general">General Structure</a> </div> |
| <!-- *********************************************************************** --> |
| <div class="doc_text"> |
| <p>This section provides the general structure of the LLVM bytecode |
| file format. The bytecode file format requires blocks to be in a |
| certain order and nested in a particular way so that an LLVM module can |
| be constructed efficiently from the contents of the file. This ordering |
| defines a general structure for bytecode files as shown below. The |
| table below shows the order in which all block types may appear. Please |
| note that some of the blocks are optional and some may be repeated. The |
| structure is fairly loose because optional blocks, if empty, are |
| completely omitted from the file.</p> |
| <table> |
| <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> |
| </table> |
| <p>Use the links in the table for details about the contents of each of |
| the block types.</p> |
| </div> |
| <!-- *********************************************************************** --> |
| <div class="doc_section"> <a name="blockdefs">Block Definitions</a> </div> |
| <!-- *********************************************************************** --> |
| <div class="doc_text"> |
| <p>This section provides the detailed layout of the individual block |
| types in the LLVM bytecode file format. </p> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="signature">Signature Block</a> </div> |
| <div class="doc_text"> |
| <p>The signature occurs in every LLVM bytecode file and is always first. |
| It simply provides a few bytes of data to identify the file as being an LLVM |
| bytecode file. This block is always four bytes in length and differs from the |
| other blocks because there is no identifier and no block length at the start |
| of the block. Essentially, this block is just the "magic number" for the file. |
| </p> |
| <p>There are two types of signatures for LLVM bytecode: uncompressed and |
| compressed as shown in the table below. </p> |
| <table> |
| <tbody> |
| <tr> |
| <th><b>Type</b></th> |
| <th class="td_left"><b>Uncompressed</b></th> |
| <th class="td_left"><b>Compressed</b></th> |
| </tr> |
| <tr> |
| <td><a href="#char">char</a></td> |
| <td class="td_left">Constant "l" (0x6C)</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> |
| <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> |
| <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> |
| <td class="td_left">Constant "c" (0x63)</td> |
| </tr> |
| <tr> |
| <td><a href="#char">char</a></td> |
| <td class="td_left">N/A</td> |
| <td class="td_left">'0'=null,'1'=gzip,'2'=bzip2</td> |
| </tr> |
| </tbody> |
| </table> |
| <p>In other words, the uncompressed signature is just the characters 'llvm' |
| while the compressed signature is the characters 'llvc' followed by an ascii |
| digit ('0', '1', or '2') that indicates the kind of compression used. A value of |
| '0' indicates that null compression was used. This can happen when compression |
| was requested on a platform that wasn't configured for gzip or bzip2. A value of |
| '1' means that the rest of the file is compressed using the gzip algorithm and |
| should be uncompressed before interpretation. A value of '2' means that the rest |
| of the file is compressed using the bzip2 algorithm and should be uncompressed |
| before interpretation. In all cases, the data resulting from uncompression |
| should be interpreted as if it occurred immediately after the 'llvm' |
| signature (i.e. the uncompressed data begins with the |
| <a href="#module">Module Block</a></p> |
| <p><b>NOTE:</b> As of LLVM 1.4, all bytecode files produced by the LLVM tools |
| are compressed by default. To disable compression, pass the |
| <tt>--disable-compression</tt> option to the tool, if it supports it. |
| </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> |
| <tbody> |
| <tr> |
| <th><b>Type</b></th> |
| <th class="td_left"><b>Field Description</b></th> |
| </tr> |
| <tr> |
| <td><a href="#unsigned">unsigned</a><br></td> |
| <td class="td_left"><a href="#mod_header">Module Block Identifier |
| (0x01)</a></td> |
| </tr> |
| <tr> |
| <td><a href="#unsigned">unsigned</a></td> |
| <td class="td_left"><a href="#mod_header">Module Block Size</a></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="#symtab">Module Symbol Table</a></td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection"><a name="mod_header">Module Block Header</a></div> |
| <div class="doc_text"> |
| <p>The block header for the module block uses a longer format than the other |
| blocks in a bytecode file. Specifically, instead of encoding the type and size |
| of the block into a 32-bit integer with 5-bits for type and 27-bits for size, |
| the module block header uses two 32-bit unsigned values, one for type, and one |
| for size. While the 2<sup>27</sup> byte limit on block size is sufficient for the blocks |
| contained in the module, it isn't sufficient for the module block itself |
| because we want to ensure that bytecode files as large as 2<sup>32</sup> bytes |
| are possible. For this reason, the module block (and only the module block) |
| uses a long format header.</p> |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection"><a name="format">Format Information</a></div> |
| <div class="doc_text"> |
| <p>The format information field is encoded into a <a href="#uint32_vbr">uint32_vbr</a> |
| as shown in the following table.</p> |
| <table> |
| <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> |
| <ul> |
| <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> |
| </ul> |
| <p>Note that we plan to eventually expand the target description |
| capabilities |
| of bytecode files to <a href="http://llvm.cs.uiuc.edu/PR263">target |
| triples</a>. |
| </p> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="globaltypes">Global Type Pool</a> </div> |
| <div class="doc_text"> |
| <p>The global type pool consists of type definitions. Their order of appearance |
| in the file determines their type 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> |
| <h3>Primitive Types</h3> |
| <p>The primitive types encompass the basic integer and floating point |
| types. They are encoded simply as their TypeID.</p> |
| <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 the primitive types (values 1 to |
| 11) <sup>1</sup></td> |
| </tr> |
| </tbody> |
| </table> |
| Notes: |
| <ol> |
| <li>The values for the Type IDs for the primitive types are provided |
| by the definition of the <code>llvm::Type::TypeID</code> enumeration |
| in <code>include/llvm/Type.h</code>. The enumeration gives the |
| following mapping: |
| <ol> |
| <li>bool</li> |
| <li>ubyte</li> |
| <li>sbyte</li> |
| <li>ushort</li> |
| <li>short</li> |
| <li>uint</li> |
| <li>int</li> |
| <li>ulong</li> |
| <li>long</li> |
| <li>float</li> |
| <li>double</li> |
| </ol> |
| </li> |
| </ol> |
| <h3>Function Types</h3> |
| <table> |
| <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">Type 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">Type 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">Type 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">Type 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> |
| <h3>Packed 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 Packed Types (18)</td> |
| </tr> |
| <tr> |
| <td><a href="#uint24_vbr">uint24_vbr</a></td> |
| <td class="td_left">Slot number of packed vector'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 packed vector.</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> |
| <td class="td_left">A zero terminated list of global var |
| definitions occurring in the module.</td> |
| </tr> |
| <tr> |
| <td><a href="#zlist">zlist</a>(<a href="#funcfield">funcfield</a>)</td> |
| <td class="td_left">A zero terminated list of function definitions |
| occurring 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> |
| </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">Type 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 value slot |
| numbers of the global variable's constant initializer.</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection"><a name="funcfield">Function Field</a> |
| </div> |
| <div class="doc_text"> |
| <p>Functions are written using an <a href="#uint32_vbr">uint32_vbr</a> |
| that encodes information about the function and a set of flags.</p> |
| |
| <p>The table below provides the bit layout of the <a |
| href="#uint32_vbr">uint32_vbr</a> that describes the function.</p> |
| |
| <table> |
| <tbody> |
| <tr> |
| <th><b>Type</b></th> |
| <th class="td_left"><b>Description</b></th> |
| </tr> |
| <tr> |
| <td><a href="#bit">bit(0-3)</a></td> |
| <td class="td_left">Reserved for future use. Currently set to 0001.</td> |
| </tr> |
| <tr> |
| <td><a href="#bit">bit(4)</a></td> |
| <td class="td_left">If this bit is set to 1, the indicated function is |
| external, and there is no <a href="#functiondefs">Function Definiton |
| Block</a> in the bytecode file for the function.</td> |
| </tr> |
| <tr> |
| <td><a href="#bit">bit(5-)</a></td> |
| <td class="td_left">Type slot number of type for the function.</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> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint32_vbr</a></td> |
| <td class="td_left">The number of constant strings that follow.</td> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint32_vbr</a></td> |
| <td class="td_left">Zero. This identifies the following "plane" |
| as containing the constant strings. This is needed to identify it |
| uniquely from other constant planes that follow. </td> |
| </tr> |
| <tr> |
| <td><a href="#uint24_vbr">uint24_vbr</a>+</td> |
| <td class="td_left">Type slot number of the constant string's type. |
| Note that the constant string's type implicitly defines the length of |
| the string. </td> |
| </tr> |
| </tbody> |
| </table> |
| <p><b>Function Constant Pool Preamble (function types)</b></p> |
| <p>The structure of the types for functions is identical to the <a |
| href="#globaltypes">Global Type Pool</a>. Please refer to that section |
| for the details. </p> |
| <p><b>Common Part (other constants)</b></p> |
| <table> |
| <tbody> |
| <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="#uint24_vbr">uint24_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> |
| </tbody> |
| </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 follow |
| 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 value 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 value slot numbers to the constant |
| field values of the structure.</li> |
| </ul> |
| |
| <p>When the number of operands to the constant is one, we have an 'undef' value |
| of the specified type.</p> |
| |
| <p>When the number of operands to the constant is greater than one, we have a |
| constant expression and its field format is provided in the table below, and the |
| number is equal to the number of operands+1.</p> |
| <table> |
| <tbody> |
| <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 value slot number of the constant value for an |
| operand.<sup>1</sup></td> |
| </tr> |
| <tr> |
| <td><a href="#uint24_vbr">uint24_vbr</a></td> |
| <td class="td_left">The type slot number for the type of the constant |
| value for an operand.<sup>1</sup></td> |
| </tr> |
| </tbody> |
| </table> |
| Notes: |
| <ol> |
| <li>Both these fields are repeatable but only in pairs.</li> |
| </ol> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="functiondefs">Function Definition</a></div> |
| <div class="doc_text"> |
| <p>Function definitions contain the linkage, constant pool or |
| compaction table, instruction list, and symbol table for a function. |
| The following table shows the structure of a function definition.</p> |
| <table> |
| <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">Function definition block identifier (0x02) + |
| size<br> |
| </td> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint32_vbr</a></td> |
| <td class="td_left">The linkage type of the function: 0=External, |
| 1=Weak, 2=Appending, 3=Internal, 4=LinkOnce<sup>1</sup></td> |
| </tr> |
| <tr> |
| <td><a href="#block">block</a></td> |
| <td class="td_left">The <a href="#constantpool">constant pool</a> |
| block for this function.<sup>2</sup></td> |
| </tr> |
| <tr> |
| <td><a href="#block">block</a></td> |
| <td class="td_left">The <a href="#compactiontable">compaction |
| table</a> block for the function.<sup>2</sup></td> |
| </tr> |
| <tr> |
| <td><a href="#block">block</a></td> |
| <td class="td_left">The <a href="#instructionlist">instruction |
| list</a> for the function.</td> |
| </tr> |
| <tr> |
| <td><a href="#block">block</a></td> |
| <td class="td_left">The function's <a href="#symtab">symbol |
| table</a> containing only those symbols pertinent to the function |
| (mostly block labels).</td> |
| </tr> |
| </tbody> |
| </table> |
| Notes: |
| <ol> |
| <li>Note that if the linkage type is "External" then none of the |
| other fields will be present as the function is defined elsewhere.</li> |
| <li>Note that only one of the constant pool or compaction table will |
| be written. Compaction tables are only written if they will actually |
| save bytecode space. If not, then a regular constant pool is written.</li> |
| </ol> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="compactiontable">Compaction Table</a> |
| </div> |
| <div class="doc_text"> |
| <p>Compaction tables are part of a function definition. They are merely |
| a device for reducing the size of bytecode files. The size of a |
| bytecode file is dependent on the <em>values</em> of the slot numbers |
| used because larger values use more bytes in the variable bit rate |
| encoding scheme. Furthermore, the compressed instruction format |
| reserves only six bits for the type of the instruction. In large |
| modules, declaring hundreds or thousands of types, the values of the |
| slot numbers can be quite large. However, functions may use only a |
| small fraction of the global types. In such cases a compaction table is |
| created that maps the global type and value slot numbers to smaller |
| values used by a function. Functions will contain either a |
| function-specific constant pool <em>or</em> a compaction table but not |
| both. Compaction tables have the format shown in the table below.</p> |
| <table> |
| <tbody> |
| <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="#uint24_vbr">uint24_vbr</a>+</td> |
| <td class="td_left">The type slot number in the global types of |
| the type that will be referenced in the function with the index of this |
| entry in the compaction table.</td> |
| </tr> |
| <tr> |
| <td><a href="#type_len">type_len</a></td> |
| <td class="td_left">An encoding of the type and number of values |
| that follow. This field's encoding varies depending on the size of the |
| type plane. See <a href="#type_len">Type and Length</a> for further |
| details.</td> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint32_vbr</a>+</td> |
| <td class="td_left">The value slot number in the global values |
| that will be referenced in the function with the index of this entry in |
| the compaction table.</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection"><a name="type_len">Type and Length</a></div> |
| <div class="doc_text"> |
| <p>The type and length of a compaction table type plane is encoded |
| differently depending on the length of the plane. For planes of length |
| 1 or 2, the length is encoded into bits 0 and 1 of a <a |
| href="#uint32_vbr">uint32_vbr</a> and the type is encoded into bits |
| 2-31. Because type numbers are often small, this often saves an extra |
| byte per plane. If the length of the plane is greater than 2 then the |
| encoding uses a <a href="#uint32_vbr">uint32_vbr</a> for each of the |
| length and type, in that order.</p> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="instructionlist">Instruction List</a></div> |
| <div class="doc_text"> |
| <p>The instructions in a function are written as a simple list. Basic |
| blocks are inferred by the terminating instruction types. The format of |
| the block is given in the following table.</p> |
| <table> |
| <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">Instruction list identifier (0x07) + size<br> |
| </td> |
| </tr> |
| <tr> |
| <td><a href="#instruction">instruction</a>+</td> |
| <td class="td_left">An instruction. Instructions have a variety |
| of formats. See <a href="#instruction">Instructions</a> for details.</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection"><a name="instruction">Instructions</a></div> |
| <div class="doc_text"> |
| <p>For brevity, instructions are written in one of four formats, |
| depending on the number of operands to the instruction. Each |
| instruction begins with a <a href="#uint32_vbr">uint32_vbr</a> that |
| encodes the type of the instruction as well as other things. The tables |
| that follow describe the format of this first part 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 |
| shortened because they have large numbers of operands (e.g. PHI Node or |
| getelementptr). Each of the opcode, type, and operand fields is found in |
| successive fields.</p> |
| <table> |
| <tbody> |
| <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="#uint24_vbr">uint24_vbr</a></td> |
| <td class="td_left">Provides the type slot number of the result type of |
| the instruction.</td> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint32_vbr</a></td> |
| <td class="td_left">The number of operands that follow.</td> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint32_vbr</a>+</td> |
| <td class="td_left">The slot number of the value(s) for the operand(s). |
| <sup>1</sup></td> |
| </tr> |
| </tbody> |
| </table> |
| Notes: |
| <ol> |
| <li>Note that if the instruction is a getelementptr and the type of |
| the operand is a sequential type (array or pointer) then the slot |
| number is shifted up two bits and the low order bits will encode the |
| type of index used, as follows: 0=uint, 1=int, 2=ulong, 3=long.</li> |
| </ol> |
| <p><b>Instruction Format 1</b></p> |
| <p>This format encodes the opcode, type and a single operand into a |
| single <a href="#uint32_vbr">uint32_vbr</a> as follows:</p> |
| <table> |
| <tbody> |
| <tr> |
| <th><b>Bits</b></th> |
| <th><b>Type</b></th> |
| <th class="td_left"><b>Field Description</b></th> |
| </tr> |
| <tr> |
| <td>0-1</td> |
| <td>constant "1"</td> |
| <td class="td_left">These two bits must be the value 1 which identifies |
| this as an instruction of format 1.</td> |
| </tr> |
| <tr> |
| <td>2-7</td> |
| <td><a href="#opcode">opcode</a></td> |
| <td class="td_left">Specifies the opcode of the instruction. Note that |
| the maximum opcode value is 63.</td> |
| </tr> |
| <tr> |
| <td>8-19</td> |
| <td><a href="#unsigned">unsigned</a></td> |
| <td class="td_left">Specifies the slot number of the type for this |
| instruction. Maximum slot number is 2<sup>12</sup>-1=4095.</td> |
| </tr> |
| <tr> |
| <td>20-31</td> |
| <td><a href="#unsigned">unsigned</a></td> |
| <td class="td_left">Specifies the slot number of the value for the |
| first operand. Maximum slot number is 2<sup>12</sup>-1=4095. Note that |
| the value 2<sup>12</sup>-1 denotes zero operands.</td> |
| </tr> |
| </tbody> |
| </table> |
| <p><b>Instruction Format 2</b></p> |
| <p>This format encodes the opcode, type and two operands into a single <a |
| href="#uint32_vbr">uint32_vbr</a> as follows:</p> |
| <table> |
| <tbody> |
| <tr> |
| <th><b>Bits</b></th> |
| <th><b>Type</b></th> |
| <th class="td_left"><b>Field Description</b></th> |
| </tr> |
| <tr> |
| <td>0-1</td> |
| <td>constant "2"</td> |
| <td class="td_left">These two bits must be the value 2 which identifies |
| this as an instruction of format 2.</td> |
| </tr> |
| <tr> |
| <td>2-7</td> |
| <td><a href="#opcodes">opcode</a></td> |
| <td class="td_left">Specifies the opcode of the instruction. Note that |
| the maximum opcode value is 63.</td> |
| </tr> |
| <tr> |
| <td>8-15</td> |
| <td><a href="#unsigned">unsigned</a></td> |
| <td class="td_left">Specifies the slot number of the type for this |
| instruction. Maximum slot number is 2<sup>8</sup>-1=255.</td> |
| </tr> |
| <tr> |
| <td>16-23</td> |
| <td><a href="#unsigned">unsigned</a></td> |
| <td class="td_left">Specifies the slot number of the value for the first |
| operand. Maximum slot number is 2<sup>8</sup>-1=255.</td> |
| </tr> |
| <tr> |
| <td>24-31</td> |
| <td><a href="#unsigned">unsigned</a></td> |
| <td class="td_left">Specifies the slot number of the value for the second |
| operand. Maximum slot number is 2<sup>8</sup>-1=255.</td> |
| </tr> |
| </tbody> |
| </table> |
| <p><b>Instruction Format 3</b></p> |
| <p>This format encodes the opcode, type and three operands into a |
| single <a href="#uint32_vbr">uint32_vbr</a> as follows:</p> |
| <table> |
| <tbody> |
| <tr> |
| <th><b>Bits</b></th> |
| <th><b>Type</b></th> |
| <th class="td_left"><b>Field Description</b></th> |
| </tr> |
| <tr> |
| <td>0-1</td> |
| <td>constant "3"</td> |
| <td class="td_left">These two bits must be the value 3 which identifies |
| this as an instruction of format 3.</td> |
| </tr> |
| <tr> |
| <td>2-7</td> |
| <td><a href="#opcodes">opcode</a></td> |
| <td class="td_left">Specifies the opcode of the instruction. Note that |
| the maximum opcode value is 63.</td> |
| </tr> |
| <tr> |
| <td>8-13</td> |
| <td><a href="#unsigned">unsigned</a></td> |
| <td class="td_left">Specifies the slot number of the type for this |
| instruction. Maximum slot number is 2<sup>6</sup>-1=63.</td> |
| </tr> |
| <tr> |
| <td>14-19</td> |
| <td><a href="#unsigned">unsigned</a></td> |
| <td class="td_left">Specifies the slot number of the value for the first |
| operand. Maximum slot number is 2<sup>6</sup>-1=63.</td> |
| </tr> |
| <tr> |
| <td>20-25</td> |
| <td><a href="#unsigned">unsigned</a></td> |
| <td class="td_left">Specifies the slot number of the value for the second |
| operand. Maximum slot number is 2<sup>6</sup>-1=63.</td> |
| </tr> |
| <tr> |
| <td>26-31</td> |
| <td><a href="#unsigned">unsigned</a></td> |
| <td class="td_left">Specifies the slot number of the value for the third |
| operand. Maximum slot number is 2<sup>6</sup>-1=63.</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="opcodes">Instruction Opcodes</a></div> |
| <div class="doc_text"> |
| <p>Instructions encode an opcode that identifies the kind of instruction. |
| Opcodes are an enumerated integer value. The specific values used depend on |
| the version of LLVM you're using. The opcode values are defined in the |
| <a href="http://llvm.cs.uiuc.edu/cvsweb/cvsweb.cgi/llvm/include/llvm/Instruction.def"> |
| <tt>include/llvm/Instruction.def</tt></a> file. You should check there for the |
| most recent definitions. The table below provides the opcodes defined as of |
| the writing of this document. The table associates each opcode mnemonic with |
| its enumeration value and the bytecode and LLVM version numbers in which the |
| opcode was introduced.</p> |
| <table> |
| <tbody> |
| <tr> |
| <th>Opcode</th> |
| <th>Number</th> |
| <th>Bytecode Version</th> |
| <th>LLVM Version</th> |
| </tr> |
| <tr><td colspan="4"><b>Terminator Instructions</b></td></tr> |
| <tr><td>Ret</td><td>1</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Br</td><td>2</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Switch</td><td>3</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Invoke</td><td>4</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Unwind</td><td>5</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Unreachable</td><td>6</td><td>1</td><td>1.4</td></tr> |
| <tr><td colspan="4"><b>Binary Operators</b></td></tr> |
| <tr><td>Add</td><td>7</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Sub</td><td>8</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Mul</td><td>9</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Div</td><td>10</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Rem</td><td>11</td><td>1</td><td>1.0</td></tr> |
| <tr><td colspan="4"><b>Logical Operators</b></td></tr> |
| <tr><td>And</td><td>12</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Or</td><td>13</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Xor</td><td>14</td><td>1</td><td>1.0</td></tr> |
| <tr><td colspan="4"><b>Binary Comparison Operators</b></td></tr> |
| <tr><td>SetEQ</td><td>15</td><td>1</td><td>1.0</td></tr> |
| <tr><td>SetNE</td><td>16</td><td>1</td><td>1.0</td></tr> |
| <tr><td>SetLE</td><td>17</td><td>1</td><td>1.0</td></tr> |
| <tr><td>SetGE</td><td>18</td><td>1</td><td>1.0</td></tr> |
| <tr><td>SetLT</td><td>19</td><td>1</td><td>1.0</td></tr> |
| <tr><td>SetGT</td><td>20</td><td>1</td><td>1.0</td></tr> |
| <tr><td colspan="4"><b>Memory Operators</b></td></tr> |
| <tr><td>Malloc</td><td>21</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Free</td><td>22</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Alloca</td><td>23</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Load</td><td>24</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Store</td><td>25</td><td>1</td><td>1.0</td></tr> |
| <tr><td>GetElementPtr</td><td>26</td><td>1</td><td>1.0</td></tr> |
| <tr><td colspan="4"><b>Other Operators</b></td></tr> |
| <tr><td>PHI</td><td>27</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Cast</td><td>28</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Call</td><td>29</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Shl</td><td>30</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Shr</td><td>31</td><td>1</td><td>1.0</td></tr> |
| <tr><td>VANext</td><td>32</td><td>1</td><td>1.0</td></tr> |
| <tr><td>VAArg</td><td>33</td><td>1</td><td>1.0</td></tr> |
| <tr><td>Select</td><td>34</td><td>2</td><td>1.2</td></tr> |
| <tr><td>UserOp1</td><td>35</td><td>1</td><td>1.0</td></tr> |
| <tr><td>UserOp2</td><td>36</td><td>1</td><td>1.0</td></tr> |
| </tbody> |
| </table> |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="symtab">Symbol Table</a> </div> |
| <div class="doc_text"> |
| <p>A symbol table can be put out in conjunction with a module or a function. A |
| symbol table has a list of name/type associations followed by a list of |
| name/value associations. The name/value associations are organized into "type |
| planes" so that all values of a common type are listed together. Each type |
| plane starts with the number of entries in the plane and the type slot number |
| for all the values in that plane (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> |
| <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">Symbol Table Identifier (0x04)</td> |
| </tr> |
| <tr> |
| <td><a href="#llist">llist</a>(<a href="#symtab_entry">type_entry</a>)</td> |
| <td class="td_left">A length list of symbol table entries for |
| <tt>Type</tt>s |
| </td> |
| </tr> |
| <tr> |
| <td><a href="#zlist">llist</a>(<a href="#symtab_plane">symtab_plane</a>)</td> |
| <td class="td_left">A length list of "type planes" of symbol table |
| entries for <tt>Value</tt>s</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection"> <a name="type_entry">Symbol Table Type |
| Entry</a> |
| </div> |
| <div class="doc_text"> |
| <p>A symbol table type entry associates a name with a type. The name is provided |
| simply as an array of chars. The type is provided as a type slot number (index) |
| into the global type pool. The format is given in the following table:</p> |
| <table> |
| <tbody> |
| <tr> |
| <th><b>Type</b></th> |
| <th class="td_left"><b>Field Description</b></th> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint24_vbr</a></td> |
| <td class="td_left">Type slot number of the type being given a |
| name relative to the global type pool. |
| </td> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint32_vbr</a></td> |
| <td class="td_left">Length of the character array that follows.</td> |
| </tr> |
| <tr> |
| <td><a href="#char">char</a>+</td> |
| <td class="td_left">The characters of the name.</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection"> <a name="symtab_plane">Symbol Table |
| Plane</a> |
| </div> |
| <div class="doc_text"> |
| <p>A symbol table plane provides the symbol table entries for all |
| values of a common type. The encoding is given in the following table:</p> |
| <table> |
| <tbody> |
| <tr> |
| <th><b>Type</b></th> |
| <th class="td_left"><b>Field Description</b></th> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint32_vbr</a></td> |
| <td class="td_left">Number of entries in this plane.</td> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint32_vbr</a></td> |
| <td class="td_left">Type slot number of type for all values in this plane..</td> |
| </tr> |
| <tr> |
| <td><a href="#value_entry">value_entry</a>+</td> |
| <td class="td_left">The symbol table entries for to associate values with |
| names.</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection"><a name="value_entry">Symbol Table Value |
| Entry</a> |
| </div> |
| <div class="doc_text"> |
| <p>A symbol table value entry provides the assocation between a value and the |
| name given to the value. The value is referenced by its slot number. The |
| format is given in the following table:</p> |
| <table> |
| <tbody> |
| <tr> |
| <th><b>Type</b></th> |
| <th class="td_left"><b>Field Description</b></th> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint24_vbr</a></td> |
| <td class="td_left">Value slot number of the value being given a name. |
| </td> |
| </tr> |
| <tr> |
| <td><a href="#uint32_vbr">uint32_vbr</a></td> |
| <td class="td_left">Length of the character array that follows.</td> |
| </tr> |
| <tr> |
| <td><a href="#char">char</a>+</td> |
| <td class="td_left">The characters of the name.</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <div class="doc_section"> <a name="versiondiffs">Version Differences</a> |
| </div> |
| <!-- *********************************************************************** --> |
| <div class="doc_text"> |
| <p>This section describes the differences in the Bytecode Format across |
| LLVM |
| versions. The versions are listed in reverse order because it assumes |
| the current version is as documented in the previous sections. Each |
| section here |
| describes the differences between that version and the one that <i>follows</i>. |
| </p> |
| </div> |
| |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="vers13">Version 1.3 Differences From |
| 1.4</a></div> |
| <!-- _______________________________________________________________________ --> |
| |
| <div class="doc_subsubsection">Unreachable Instruction</div> |
| <div class="doc_text"> |
| <p>The LLVM <a href="LangRef.html#i_unreachable">Unreachable</a> instruction |
| was added in version 1.4 of LLVM. This caused all instruction numbers after |
| it to shift down by one.</p> |
| </div> |
| |
| <div class="doc_subsubsection">Function Flags</div> |
| <div class="doc_text"> |
| <p>LLVM bytecode versions prior to 1.4 did not include the 5 bit offset |
| in <a href="#funcfield">the function list</a> in the <a |
| href="#globalinfo">Module Global Info</a> block.</p> |
| </div> |
| |
| <div class="doc_subsubsection">Function Flags</div> |
| <div class="doc_text"> |
| <p>LLVM bytecode versions prior to 1.4 did not include the 'undef' constant |
| value, which affects the encoding of <a href="#constant">Constant |
| Fields</a>.</p> |
| </div> |
| |
| <!-- |
| <div class="doc_subsubsection">Aligned Data</div> |
| <div class="doc_text"> |
| <p>In version 1.3, certain data items were aligned to 32-bit boundaries. In |
| version 1.4, alignment of data was done away with completely. The need for |
| alignment has gone away and the only thing it adds is bytecode file size |
| overhead. In most cases this overhead was small. However, in functions with |
| large numbers of format 0 instructions (GEPs and PHIs with lots of parameters) |
| or regular instructions with large valued operands (e.g. because there's just |
| a lot of instructions in the function) the overhead can be extreme. In one |
| test case, the overhead was 44,000 bytes (34% of the total file size). |
| Consequently in release 1.4, the decision was made to eliminate alignment |
| altogether.</p> |
| <p>In version 1.3 format, the following bytecode constructs were aligned (i.e. |
| they were followed by one to three bytes of padding):</p> |
| <ul> |
| <li>All blocks.</li> |
| <li>Instructions using the long format (format 0).</li> |
| <li>All call instructions that called a var args function.</li> |
| <li>The target triple (a string field at the end of the module block).</li> |
| <li>The version field (immediately following the signature).</li> |
| </ul> |
| <p>None of these constructs are aligned in version 1.4</p> |
| </div> |
| --> |
| |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="vers12">Version 1.2 Differences |
| From 1.3</a></div> |
| <!-- _______________________________________________________________________ --> |
| |
| <div class="doc_subsubsection">Type Derives From Value</div> |
| <div class="doc_text"> |
| <p>In version 1.2, the Type class in the LLVM IR derives from the Value |
| class. This is not the case in version 1.3. Consequently, in version |
| 1.2 the notion of a "Type Type" was used to write out values that were |
| Types. The types always occuped plane 12 (corresponding to the |
| TypeTyID) of any type planed set of values. In 1.3 this representation |
| is not convenient because the TypeTyID (12) is not present and its |
| value is now used for LabelTyID. Consequently, the data structures |
| written that involve types do so by writing all the types first and |
| then each of the value planes according to those types. In version 1.2, |
| the types would have been written intermingled with the values.</p> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection">Restricted getelementptr Types</div> |
| <div class="doc_text"> |
| <p>In version 1.2, the getelementptr instruction required a ubyte type |
| index for accessing a structure field and a long type index for |
| accessing an array element. Consequently, it was only possible to |
| access structures of 255 or fewer elements. Starting in version 1.3, |
| this restriction was lifted. Structures must now be indexed with uint |
| constants. Arrays may now be indexed with int, uint, long, or ulong |
| typed values. The consequence of this was that the bytecode format had |
| to change in order to accommodate the larger range of structure indices.</p> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection">Short Block Headers</div> |
| <div class="doc_text"> |
| <p>In version 1.2, block headers were always 8 bytes being comprised of |
| both an unsigned integer type and an unsigned integer size. For very |
| small modules, these block headers turn out to be a large fraction of |
| the total bytecode file size. In an attempt to make these small files |
| smaller, the type and size information was encoded into a single |
| unsigned integer (4 bytes) comprised of 5 bits for the block type |
| (maximum 31 block types) and 27 bits for the block size (max |
| ~134MBytes). These limits seemed sufficient for any blocks or sizes |
| forseen in the future. Note that the module block, which encloses all |
| the other blocks is still written as 8 bytes since bytecode files |
| larger than 134MBytes might be possible.</p> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection">Dependent Libraries and Target Triples</div> |
| <div class="doc_text"> |
| <p>In version 1.2, the bytecode format does not store module's target |
| triple or dependent. These fields have been added to the end of the <a |
| href="#globalinfo">module global info block</a>. The purpose of these |
| fields is to allow a front end compiler to specifiy that the generated |
| module is specific to a particular target triple (operating |
| system/manufacturer/processor) which makes it non-portable; and to |
| allow front end compilers to specify the list of libraries that the |
| module depends on for successful linking.</p> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection">Types Restricted to 24-bits</div> |
| <div class="doc_text"> |
| <p>In version 1.2, type slot identifiers were written as 32-bit VBR |
| quantities. In 1.3 this has been reduced to 24-bits in order to ensure |
| that it is not possible to overflow the type field of a global variable |
| definition. 24-bits for type slot numbers is deemed sufficient for any |
| practical use of LLVM.</p> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="vers11">Version 1.1 Differences |
| From 1.2 </a></div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection">Explicit Primitive Zeros</div> |
| <div class="doc_text"> |
| <p>In version 1.1, the zero value for primitives was explicitly encoded |
| into the bytecode format. Since these zero values are constant values |
| in the LLVM IR and never change, there is no reason to explicitly |
| encode them. This explicit encoding was removed in version 1.2.</p> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsubsection">Inconsistent Module Global Info</div> |
| <div class="doc_text"> |
| <p>In version 1.1, the Module Global Info block was not aligned causing |
| the next block to be read in on an unaligned boundary. This problem was |
| corrected in version 1.2.<br> |
| <br> |
| </p> |
| </div> |
| <!-- _______________________________________________________________________ --> |
| <div class="doc_subsection"><a name="vers10">Version 1.0 Differences |
| From 1.1</a></div> |
| <div class="doc_text"> |
| <p>None. Version 1.0 and 1.1 bytecode formats are identical.</p> |
| </div> |
| <!-- *********************************************************************** --> |
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