diff options
| author | Bill Wendling <isanbard@gmail.com> | 2012-06-28 08:43:12 +0000 |
|---|---|---|
| committer | Bill Wendling <isanbard@gmail.com> | 2012-06-28 08:43:12 +0000 |
| commit | 0ca9927a71fed311eea7459b4c85c98cc7ed0352 (patch) | |
| tree | 4e950553c2a69c6db06092a5dc69138c0e2e6378 /docs | |
| parent | 87dc7a4c8d21bd638465881f3b6d091f22d9767c (diff) | |
Sphinxify the bitcode format document.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159340 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/BitCodeFormat.html | 1490 | ||||
| -rw-r--r-- | docs/BitCodeFormat.rst | 1045 | ||||
| -rw-r--r-- | docs/subsystems.rst | 3 |
3 files changed, 1047 insertions, 1491 deletions
diff --git a/docs/BitCodeFormat.html b/docs/BitCodeFormat.html deleted file mode 100644 index 6a670f56b9..0000000000 --- a/docs/BitCodeFormat.html +++ /dev/null @@ -1,1490 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> -<html> -<head> - <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> - <title>LLVM Bitcode File Format</title> - <link rel="stylesheet" href="_static/llvm.css" type="text/css"> -</head> -<body> -<h1> LLVM Bitcode File Format</h1> -<ol> - <li><a href="#abstract">Abstract</a></li> - <li><a href="#overview">Overview</a></li> - <li><a href="#bitstream">Bitstream Format</a> - <ol> - <li><a href="#magic">Magic Numbers</a></li> - <li><a href="#primitives">Primitives</a></li> - <li><a href="#abbrevid">Abbreviation IDs</a></li> - <li><a href="#blocks">Blocks</a></li> - <li><a href="#datarecord">Data Records</a></li> - <li><a href="#abbreviations">Abbreviations</a></li> - <li><a href="#stdblocks">Standard Blocks</a></li> - </ol> - </li> - <li><a href="#wrapper">Bitcode Wrapper Format</a> - </li> - <li><a href="#llvmir">LLVM IR Encoding</a> - <ol> - <li><a href="#basics">Basics</a></li> - <li><a href="#MODULE_BLOCK">MODULE_BLOCK Contents</a></li> - <li><a href="#PARAMATTR_BLOCK">PARAMATTR_BLOCK Contents</a></li> - <li><a href="#TYPE_BLOCK">TYPE_BLOCK Contents</a></li> - <li><a href="#CONSTANTS_BLOCK">CONSTANTS_BLOCK Contents</a></li> - <li><a href="#FUNCTION_BLOCK">FUNCTION_BLOCK Contents</a></li> - <li><a href="#TYPE_SYMTAB_BLOCK">TYPE_SYMTAB_BLOCK Contents</a></li> - <li><a href="#VALUE_SYMTAB_BLOCK">VALUE_SYMTAB_BLOCK Contents</a></li> - <li><a href="#METADATA_BLOCK">METADATA_BLOCK Contents</a></li> - <li><a href="#METADATA_ATTACHMENT">METADATA_ATTACHMENT Contents</a></li> - </ol> - </li> -</ol> -<div class="doc_author"> - <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>, - <a href="http://www.reverberate.org">Joshua Haberman</a>, - and <a href="mailto:housel@acm.org">Peter S. Housel</a>. -</p> -</div> - -<!-- *********************************************************************** --> -<h2><a name="abstract">Abstract</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p>This document describes the LLVM bitstream file format and the encoding of -the LLVM IR into it.</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="overview">Overview</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p> -What is commonly known as the LLVM bitcode file format (also, sometimes -anachronistically known as bytecode) is actually two things: a <a -href="#bitstream">bitstream container format</a> -and an <a href="#llvmir">encoding of LLVM IR</a> into the container format.</p> - -<p> -The bitstream format is an abstract encoding of structured data, very -similar to XML in some ways. Like XML, bitstream files contain tags, and nested -structures, and you can parse the file without having to understand the tags. -Unlike XML, the bitstream format is a binary encoding, and unlike XML it -provides a mechanism for the file to self-describe "abbreviations", which are -effectively size optimizations for the content.</p> - -<p>LLVM IR files may be optionally embedded into a <a -href="#wrapper">wrapper</a> structure that makes it easy to embed extra data -along with LLVM IR files.</p> - -<p>This document first describes the LLVM bitstream format, describes the -wrapper format, then describes the record structure used by LLVM IR files. -</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="bitstream">Bitstream Format</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p> -The bitstream format is literally a stream of bits, with a very simple -structure. This structure consists of the following concepts: -</p> - -<ul> -<li>A "<a href="#magic">magic number</a>" that identifies the contents of - the stream.</li> -<li>Encoding <a href="#primitives">primitives</a> like variable bit-rate - integers.</li> -<li><a href="#blocks">Blocks</a>, which define nested content.</li> -<li><a href="#datarecord">Data Records</a>, which describe entities within the - file.</li> -<li>Abbreviations, which specify compression optimizations for the file.</li> -</ul> - -<p>Note that the <a -href="CommandGuide/html/llvm-bcanalyzer.html">llvm-bcanalyzer</a> tool can be -used to dump and inspect arbitrary bitstreams, which is very useful for -understanding the encoding.</p> - -<!-- ======================================================================= --> -<h3> - <a name="magic">Magic Numbers</a> -</h3> - -<div> - -<p>The first two bytes of a bitcode file are 'BC' (0x42, 0x43). -The second two bytes are an application-specific magic number. Generic -bitcode tools can look at only the first two bytes to verify the file is -bitcode, while application-specific programs will want to look at all four.</p> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="primitives">Primitives</a> -</h3> - -<div> - -<p> -A bitstream literally consists of a stream of bits, which are read in order -starting with the least significant bit of each byte. The stream is made up of a -number of primitive values that encode a stream of unsigned integer values. -These integers are encoded in two ways: either as <a href="#fixedwidth">Fixed -Width Integers</a> or as <a href="#variablewidth">Variable Width -Integers</a>. -</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="fixedwidth">Fixed Width Integers</a> -</h4> - -<div> - -<p>Fixed-width integer values have their low bits emitted directly to the file. - For example, a 3-bit integer value encodes 1 as 001. Fixed width integers - are used when there are a well-known number of options for a field. For - example, boolean values are usually encoded with a 1-bit wide integer. -</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="variablewidth">Variable Width Integers</a> -</h4> - -<div> - -<p>Variable-width integer (VBR) values encode values of arbitrary size, -optimizing for the case where the values are small. Given a 4-bit VBR field, -any 3-bit value (0 through 7) is encoded directly, with the high bit set to -zero. Values larger than N-1 bits emit their bits in a series of N-1 bit -chunks, where all but the last set the high bit.</p> - -<p>For example, the value 27 (0x1B) is encoded as 1011 0011 when emitted as a -vbr4 value. The first set of four bits indicates the value 3 (011) with a -continuation piece (indicated by a high bit of 1). The next word indicates a -value of 24 (011 << 3) with no continuation. The sum (3+24) yields the value -27. -</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4><a name="char6">6-bit characters</a></h4> - -<div> - -<p>6-bit characters encode common characters into a fixed 6-bit field. They -represent the following characters with the following 6-bit values:</p> - -<div class="doc_code"> -<pre> -'a' .. 'z' — 0 .. 25 -'A' .. 'Z' — 26 .. 51 -'0' .. '9' — 52 .. 61 - '.' — 62 - '_' — 63 -</pre> -</div> - -<p>This encoding is only suitable for encoding characters and strings that -consist only of the above characters. It is completely incapable of encoding -characters not in the set.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4><a name="wordalign">Word Alignment</a></h4> - -<div> - -<p>Occasionally, it is useful to emit zero bits until the bitstream is a -multiple of 32 bits. This ensures that the bit position in the stream can be -represented as a multiple of 32-bit words.</p> - -</div> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="abbrevid">Abbreviation IDs</a> -</h3> - -<div> - -<p> -A bitstream is a sequential series of <a href="#blocks">Blocks</a> and -<a href="#datarecord">Data Records</a>. Both of these start with an -abbreviation ID encoded as a fixed-bitwidth field. The width is specified by -the current block, as described below. The value of the abbreviation ID -specifies either a builtin ID (which have special meanings, defined below) or -one of the abbreviation IDs defined for the current block by the stream itself. -</p> - -<p> -The set of builtin abbrev IDs is: -</p> - -<ul> -<li><tt>0 - <a href="#END_BLOCK">END_BLOCK</a></tt> — This abbrev ID marks - the end of the current block.</li> -<li><tt>1 - <a href="#ENTER_SUBBLOCK">ENTER_SUBBLOCK</a></tt> — This - abbrev ID marks the beginning of a new block.</li> -<li><tt>2 - <a href="#DEFINE_ABBREV">DEFINE_ABBREV</a></tt> — This defines - a new abbreviation.</li> -<li><tt>3 - <a href="#UNABBREV_RECORD">UNABBREV_RECORD</a></tt> — This ID - specifies the definition of an unabbreviated record.</li> -</ul> - -<p>Abbreviation IDs 4 and above are defined by the stream itself, and specify -an <a href="#abbrev_records">abbreviated record encoding</a>.</p> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="blocks">Blocks</a> -</h3> - -<div> - -<p> -Blocks in a bitstream denote nested regions of the stream, and are identified by -a content-specific id number (for example, LLVM IR uses an ID of 12 to represent -function bodies). Block IDs 0-7 are reserved for <a href="#stdblocks">standard blocks</a> -whose meaning is defined by Bitcode; block IDs 8 and greater are -application specific. Nested blocks capture the hierarchical structure of the data -encoded in it, and various properties are associated with blocks as the file is -parsed. Block definitions allow the reader to efficiently skip blocks -in constant time if the reader wants a summary of blocks, or if it wants to -efficiently skip data it does not understand. The LLVM IR reader uses this -mechanism to skip function bodies, lazily reading them on demand. -</p> - -<p> -When reading and encoding the stream, several properties are maintained for the -block. In particular, each block maintains: -</p> - -<ol> -<li>A current abbrev id width. This value starts at 2 at the beginning of - the stream, and is set every time a - block record is entered. The block entry specifies the abbrev id width for - the body of the block.</li> - -<li>A set of abbreviations. Abbreviations may be defined within a block, in - which case they are only defined in that block (neither subblocks nor - enclosing blocks see the abbreviation). Abbreviations can also be defined - inside a <tt><a href="#BLOCKINFO">BLOCKINFO</a></tt> block, in which case - they are defined in all blocks that match the ID that the BLOCKINFO block is - describing. -</li> -</ol> - -<p> -As sub blocks are entered, these properties are saved and the new sub-block has -its own set of abbreviations, and its own abbrev id width. When a sub-block is -popped, the saved values are restored. -</p> - -<!-- _______________________________________________________________________ --> -<h4><a name="ENTER_SUBBLOCK">ENTER_SUBBLOCK Encoding</a></h4> - -<div> - -<p><tt>[ENTER_SUBBLOCK, blockid<sub>vbr8</sub>, newabbrevlen<sub>vbr4</sub>, - <align32bits>, blocklen<sub>32</sub>]</tt></p> - -<p> -The <tt>ENTER_SUBBLOCK</tt> abbreviation ID specifies the start of a new block -record. The <tt>blockid</tt> value is encoded as an 8-bit VBR identifier, and -indicates the type of block being entered, which can be -a <a href="#stdblocks">standard block</a> or an application-specific block. -The <tt>newabbrevlen</tt> value is a 4-bit VBR, which specifies the abbrev id -width for the sub-block. The <tt>blocklen</tt> value is a 32-bit aligned value -that specifies the size of the subblock in 32-bit words. This value allows the -reader to skip over the entire block in one jump. -</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4><a name="END_BLOCK">END_BLOCK Encoding</a></h4> - -<div> - -<p><tt>[END_BLOCK, <align32bits>]</tt></p> - -<p> -The <tt>END_BLOCK</tt> abbreviation ID specifies the end of the current block -record. Its end is aligned to 32-bits to ensure that the size of the block is -an even multiple of 32-bits. -</p> - -</div> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="datarecord">Data Records</a> -</h3> - -<div> -<p> -Data records consist of a record code and a number of (up to) 64-bit -integer values. The interpretation of the code and values is -application specific and may vary between different block types. -Records can be encoded either using an unabbrev record, or with an -abbreviation. In the LLVM IR format, for example, there is a record -which encodes the target triple of a module. The code is -<tt>MODULE_CODE_TRIPLE</tt>, and the values of the record are the -ASCII codes for the characters in the string. -</p> - -<!-- _______________________________________________________________________ --> -<h4><a name="UNABBREV_RECORD">UNABBREV_RECORD Encoding</a></h4> - -<div> - -<p><tt>[UNABBREV_RECORD, code<sub>vbr6</sub>, numops<sub>vbr6</sub>, - op0<sub>vbr6</sub>, op1<sub>vbr6</sub>, ...]</tt></p> - -<p> -An <tt>UNABBREV_RECORD</tt> provides a default fallback encoding, which is both -completely general and extremely inefficient. It can describe an arbitrary -record by emitting the code and operands as VBRs. -</p> - -<p> -For example, emitting an LLVM IR target triple as an unabbreviated record -requires emitting the <tt>UNABBREV_RECORD</tt> abbrevid, a vbr6 for the -<tt>MODULE_CODE_TRIPLE</tt> code, a vbr6 for the length of the string, which is -equal to the number of operands, and a vbr6 for each character. Because there -are no letters with values less than 32, each letter would need to be emitted as -at least a two-part VBR, which means that each letter would require at least 12 -bits. This is not an efficient encoding, but it is fully general. -</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4><a name="abbrev_records">Abbreviated Record Encoding</a></h4> - -<div> - -<p><tt>[<abbrevid>, fields...]</tt></p> - -<p> -An abbreviated record is a abbreviation id followed by a set of fields that are -encoded according to the <a href="#abbreviations">abbreviation definition</a>. -This allows records to be encoded significantly more densely than records -encoded with the <tt><a href="#UNABBREV_RECORD">UNABBREV_RECORD</a></tt> type, -and allows the abbreviation types to be specified in the stream itself, which -allows the files to be completely self describing. The actual encoding of -abbreviations is defined below. -</p> - -<p>The record code, which is the first field of an abbreviated record, -may be encoded in the abbreviation definition (as a literal -operand) or supplied in the abbreviated record (as a Fixed or VBR -operand value).</p> - -</div> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="abbreviations">Abbreviations</a> -</h3> - -<div> -<p> -Abbreviations are an important form of compression for bitstreams. The idea is -to specify a dense encoding for a class of records once, then use that encoding -to emit many records. It takes space to emit the encoding into the file, but -the space is recouped (hopefully plus some) when the records that use it are -emitted. -</p> - -<p> -Abbreviations can be determined dynamically per client, per file. Because the -abbreviations are stored in the bitstream itself, different streams of the same -format can contain different sets of abbreviations according to the needs -of the specific stream. -As a concrete example, LLVM IR files usually emit an abbreviation -for binary operators. If a specific LLVM module contained no or few binary -operators, the abbreviation does not need to be emitted. -</p> - -<!-- _______________________________________________________________________ --> -<h4><a name="DEFINE_ABBREV">DEFINE_ABBREV Encoding</a></h4> - -<div> - -<p><tt>[DEFINE_ABBREV, numabbrevops<sub>vbr5</sub>, abbrevop0, abbrevop1, - ...]</tt></p> - -<p> -A <tt>DEFINE_ABBREV</tt> record adds an abbreviation to the list of currently -defined abbreviations in the scope of this block. This definition only exists -inside this immediate block — it is not visible in subblocks or enclosing -blocks. Abbreviations are implicitly assigned IDs sequentially starting from 4 -(the first application-defined abbreviation ID). Any abbreviations defined in a -<tt>BLOCKINFO</tt> record for the particular block type -receive IDs first, in order, followed by any -abbreviations defined within the block itself. Abbreviated data records -reference this ID to indicate what abbreviation they are invoking. -</p> - -<p> -An abbreviation definition consists of the <tt>DEFINE_ABBREV</tt> abbrevid -followed by a VBR that specifies the number of abbrev operands, then the abbrev -operands themselves. Abbreviation operands come in three forms. They all start -with a single bit that indicates whether the abbrev operand is a literal operand -(when the bit is 1) or an encoding operand (when the bit is 0). -</p> - -<ol> -<li>Literal operands — <tt>[1<sub>1</sub>, litvalue<sub>vbr8</sub>]</tt> -— Literal operands specify that the value in the result is always a single -specific value. This specific value is emitted as a vbr8 after the bit -indicating that it is a literal operand.</li> -<li>Encoding info without data — <tt>[0<sub>1</sub>, - encoding<sub>3</sub>]</tt> — Operand encodings that do not have extra - data are just emitted as their code. -</li> -<li>Encoding info with data — <tt>[0<sub>1</sub>, encoding<sub>3</sub>, -value<sub>vbr5</sub>]</tt> — Operand encodings that do have extra data are -emitted as their code, followed by the extra data. -</li> -</ol> - -<p>The possible operand encodings are:</p> - -<ul> -<li>Fixed (code 1): The field should be emitted as - a <a href="#fixedwidth">fixed-width value</a>, whose width is specified by - the operand's extra data.</li> -<li>VBR (code 2): The field should be emitted as - a <a href="#variablewidth">variable-width value</a>, whose width is - specified by the operand's extra data.</li> -<li>Array (code 3): This field is an array of values. The array operand - has no extra data, but expects another operand to follow it, indicating - the element type of the array. When reading an array in an abbreviated - record, the first integer is a vbr6 that indicates the array length, - followed by the encoded elements of the array. An array may only occur as - the last operand of an abbreviation (except for the one final operand that - gives the array's type).</li> -<li>Char6 (code 4): This field should be emitted as - a <a href="#char6">char6-encoded value</a>. This operand type takes no - extra data. Char6 encoding is normally used as an array element type. - </li> -<li>Blob (code 5): This field is emitted as a vbr6, followed by padding to a - 32-bit boundary (for alignment) and an array of 8-bit objects. The array of - bytes is further followed by tail padding to ensure that its total length is - a multiple of 4 bytes. This makes it very efficient for the reader to - decode the data without having to make a copy of it: it can use a pointer to - the data in the mapped in file and poke directly at it. A blob may only - occur as the last operand of an abbreviation.</li> -</ul> - -<p> -For example, target triples in LLVM modules are encoded as a record of the -form <tt>[TRIPLE, 'a', 'b', 'c', 'd']</tt>. Consider if the bitstream emitted -the following abbrev entry: -</p> - -<div class="doc_code"> -<pre> -[0, Fixed, 4] -[0, Array] -[0, Char6] -</pre> -</div> - -<p> -When emitting a record with this abbreviation, the above entry would be emitted -as: -</p> - -<div class="doc_code"> -<p> -<tt>[4<sub>abbrevwidth</sub>, 2<sub>4</sub>, 4<sub>vbr6</sub>, 0<sub>6</sub>, -1<sub>6</sub>, 2<sub>6</sub>, 3<sub>6</sub>]</tt> -</p> -</div> - -<p>These values are:</p> - -<ol> -<li>The first value, 4, is the abbreviation ID for this abbreviation.</li> -<li>The second value, 2, is the record code for <tt>TRIPLE</tt> records within LLVM IR file <tt>MODULE_BLOCK</tt> blocks.</li> -<li>The third value, 4, is the length of the array.</li> -<li>The rest of the values are the char6 encoded values - for <tt>"abcd"</tt>.</li> -</ol> - -<p> -With this abbreviation, the triple is emitted with only 37 bits (assuming a -abbrev id width of 3). Without the abbreviation, significantly more space would -be required to emit the target triple. Also, because the <tt>TRIPLE</tt> value -is not emitted as a literal in the abbreviation, the abbreviation can also be -used for any other string value. -</p> - -</div> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="stdblocks">Standard Blocks</a> -</h3> - -<div> - -<p> -In addition to the basic block structure and record encodings, the bitstream -also defines specific built-in block types. These block types specify how the -stream is to be decoded or other metadata. In the future, new standard blocks -may be added. Block IDs 0-7 are reserved for standard blocks. -</p> - -<!-- _______________________________________________________________________ --> -<h4><a name="BLOCKINFO">#0 - BLOCKINFO Block</a></h4> - -<div> - -<p> -The <tt>BLOCKINFO</tt> block allows the description of metadata for other -blocks. The currently specified records are: -</p> - -<div class="doc_code"> -<pre> -[SETBID (#1), blockid] -[DEFINE_ABBREV, ...] -[BLOCKNAME, ...name...] -[SETRECORDNAME, RecordID, ...name...] -</pre> -</div> - -<p> -The <tt>SETBID</tt> record (code 1) indicates which block ID is being -described. <tt>SETBID</tt> records can occur multiple times throughout the -block to change which block ID is being described. There must be -a <tt>SETBID</tt> record prior to any other records. -</p> - -<p> -Standard <tt>DEFINE_ABBREV</tt> records can occur inside <tt>BLOCKINFO</tt> -blocks, but unlike their occurrence in normal blocks, the abbreviation is -defined for blocks matching the block ID we are describing, <i>not</i> the -<tt>BLOCKINFO</tt> block itself. The abbreviations defined -in <tt>BLOCKINFO</tt> blocks receive abbreviation IDs as described -in <tt><a href="#DEFINE_ABBREV">DEFINE_ABBREV</a></tt>. -</p> - -<p>The <tt>BLOCKNAME</tt> record (code 2) can optionally occur in this block. The elements of -the record are the bytes of the string name of the block. llvm-bcanalyzer can use -this to dump out bitcode files symbolically.</p> - -<p>The <tt>SETRECORDNAME</tt> record (code 3) can also optionally occur in this block. The -first operand value is a record ID number, and the rest of the elements of the record are -the bytes for the string name of the record. llvm-bcanalyzer can use -this to dump out bitcode files symbolically.</p> - -<p> -Note that although the data in <tt>BLOCKINFO</tt> blocks is described as -"metadata," the abbreviations they contain are essential for parsing records -from the corresponding blocks. It is not safe to skip them. -</p> - -</div> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="wrapper">Bitcode Wrapper Format</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p> -Bitcode files for LLVM IR may optionally be wrapped in a simple wrapper -structure. This structure contains a simple header that indicates the offset -and size of the embedded BC file. This allows additional information to be -stored alongside the BC file. The structure of this file header is: -</p> - -<div class="doc_code"> -<p> -<tt>[Magic<sub>32</sub>, Version<sub>32</sub>, Offset<sub>32</sub>, -Size<sub>32</sub>, CPUType<sub>32</sub>]</tt> -</p> -</div> - -<p> -Each of the fields are 32-bit fields stored in little endian form (as with -the rest of the bitcode file fields). The Magic number is always -<tt>0x0B17C0DE</tt> and the version is currently always <tt>0</tt>. The Offset -field is the offset in bytes to the start of the bitcode stream in the file, and -the Size field is the size in bytes of the stream. CPUType is a target-specific -value that can be used to encode the CPU of the target. -</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="llvmir">LLVM IR Encoding</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p> -LLVM IR is encoded into a bitstream by defining blocks and records. It uses -blocks for things like constant pools, functions, symbol tables, etc. It uses -records for things like instructions, global variable descriptors, type -descriptions, etc. This document does not describe the set of abbreviations -that the writer uses, as these are fully self-described in the file, and the -reader is not allowed to build in any knowledge of this. -</p> - -<!-- ======================================================================= --> -<h3> - <a name="basics">Basics</a> -</h3> - -<div> - -<!-- _______________________________________________________________________ --> -<h4><a name="ir_magic">LLVM IR Magic Number</a></h4> - -<div> - -<p> -The magic number for LLVM IR files is: -</p> - -<div class="doc_code"> -<p> -<tt>[0x0<sub>4</sub>, 0xC<sub>4</sub>, 0xE<sub>4</sub>, 0xD<sub>4</sub>]</tt> -</p> -</div> - -<p> -When combined with the bitcode magic number and viewed as bytes, this is -<tt>"BC 0xC0DE"</tt>. -</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4><a name="ir_signed_vbr">Signed VBRs</a></h4> - -<div> - -<p> -<a href="#variablewidth">Variable Width Integer</a> encoding is an efficient way to -encode arbitrary sized unsigned values, but is an extremely inefficient for -encoding signed values, as signed values are otherwise treated as maximally large -unsigned values. -</p> - -<p> -As such, signed VBR values of a specific width are emitted as follows: -</p> - -<ul> -<li>Positive values are emitted as VBRs of the specified width, but with their - value shifted left by one.</li> -<li>Negative values are emitted as VBRs of the specified width, but the negated - value is shifted left by one, and the low bit is set.</li> -</ul> - -<p> -With this encoding, small positive and small negative values can both -be emitted efficiently. Signed VBR encoding is used in -<tt>CST_CODE_INTEGER</tt> and <tt>CST_CODE_WIDE_INTEGER</tt> records -within <tt>CONSTANTS_BLOCK</tt> blocks. -</p> - -</div> - - -<!-- _______________________________________________________________________ --> -<h4><a name="ir_blocks">LLVM IR Blocks</a></h4> - -<div> - -<p> -LLVM IR is defined with the following blocks: -</p> - -<ul> -<li>8 — <a href="#MODULE_BLOCK"><tt>MODULE_BLOCK</tt></a> — This is the top-level block that - contains the entire module, and describes a variety of per-module - information.</li> -<li>9 — <a href="#PARAMATTR_BLOCK"><tt>PARAMATTR_BLOCK</tt></a> — This enumerates the parameter - attributes.</li> -<li>10 — <a href="#TYPE_BLOCK"><tt>TYPE_BLOCK</tt></a> — This describes all of the types in - the module.</li> -<li>11 — <a href="#CONSTANTS_BLOCK"><tt>CONSTANTS_BLOCK</tt></a> — This describes constants for a - module or function.</li> -<li>12 — <a href="#FUNCTION_BLOCK"><tt>FUNCTION_BLOCK</tt></a> — This describes a function - body.</li> -<li>13 — <a href="#TYPE_SYMTAB_BLOCK"><tt>TYPE_SYMTAB_BLOCK</tt></a> — This describes the type symbol - table.</li> -<li>14 — <a href="#VALUE_SYMTAB_BLOCK"><tt>VALUE_SYMTAB_BLOCK</tt></a> — This describes a value symbol - table.</li> -<li>15 — <a href="#METADATA_BLOCK"><tt>METADATA_BLOCK</tt></a> — This describes metadata items.</li> -<li>16 — <a href="#METADATA_ATTACHMENT"><tt>METADATA_ATTACHMENT</tt></a> — This contains records associating metadata with function instruction values.</li> -</ul> - -</div> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="MODULE_BLOCK">MODULE_BLOCK Contents</a> -</h3> - -<div> - -<p>The <tt>MODULE_BLOCK</tt> block (id 8) is the top-level block for LLVM -bitcode files, and each bitcode file must contain exactly one. In -addition to records (described below) containing information -about the module, a <tt>MODULE_BLOCK</tt> block may contain the -following sub-blocks: -</p> - -<ul> -<li><a href="#BLOCKINFO"><tt>BLOCKINFO</tt></a></li> -<li><a href="#PARAMATTR_BLOCK"><tt>PARAMATTR_BLOCK</tt></a></li> -<li><a href="#TYPE_BLOCK"><tt>TYPE_BLOCK</tt></a></li> -<li><a href="#TYPE_SYMTAB_BLOCK"><tt>TYPE_SYMTAB_BLOCK</tt></a></li> -<li><a href="#VALUE_SYMTAB_BLOCK"><tt>VALUE_SYMTAB_BLOCK</tt></a></li> -<li><a href="#CONSTANTS_BLOCK"><tt>CONSTANTS_BLOCK</tt></a></li> -<li><a href="#FUNCTION_BLOCK"><tt>FUNCTION_BLOCK</tt></a></li> -<li><a href="#METADATA_BLOCK"><tt>METADATA_BLOCK</tt></a></li> -</ul> - -<!-- _______________________________________________________________________ --> -<h4><a name="MODULE_CODE_VERSION">MODULE_CODE_VERSION Record</a></h4> - -<div> - -<p><tt>[VERSION, version#]</tt></p> - -<p>The <tt>VERSION</tt> record (code 1) contains a single value -indicating the format version. Only version 0 is supported at this -time.</p> -</div> - -<!-- _______________________________________________________________________ --> -<h4><a name="MODULE_CODE_TRIPLE">MODULE_CODE_TRIPLE Record</a></h4> - -<div> -<p><tt>[TRIPLE, ...string...]</tt></p> - -<p>The <tt>TRIPLE</tt> record (code 2) contains a variable number of -values representing the bytes of the <tt>target triple</tt> -specification string.</p> -</div> - -<!-- _______________________________________________________________________ --> -<h4><a name="MODULE_CODE_DATALAYOUT">MODULE_CODE_DATALAYOUT Record</a></h4> - -<div> -<p><tt>[DATALAYOUT, ...string...]</tt></p> - -<p>The <tt>DATALAYOUT</tt> record (code 3) contains a variable number of -values representing the bytes of the <tt>target datalayout</tt> -specification string.</p> -</div> - -<!-- _______________________________________________________________________ --> -<h4><a name="MODULE_CODE_ASM">MODULE_CODE_ASM Record</a></h4> - -<div> -<p><tt>[ASM, ...string...]</tt></p> - -<p>The <tt>ASM</tt> record (code 4) contains a variable number of -values representing the bytes of <tt>module asm</tt> strings, with -individual assembly blocks separated by newline (ASCII 10) characters.</p> -</div> - -<!-- _______________________________________________________________________ --> -<h4><a name="MODULE_CODE_SECTIONNAME">MODULE_CODE_SECTIONNAME Record</a></h4> - -<div> -<p><tt>[SECTIONNAME, ...string...]</tt></p> - -<p>The <tt>SECTIONNAME</tt> record (code 5) contains a variable number -of values representing the bytes of a single section name -string. There should be one <tt>SECTIONNAME</tt> record for each -section name referenced (e.g., in global variable or function -<tt>section</tt> attributes) within the module. These records can be -referenced by the 1-based index in the <i>section</i> fields of -<tt>GLOBALVAR</tt> or <tt>FUNCTION</tt> records.</p> -</div> - -<!-- _______________________________________________________________________ --> -<h4><a name="MODULE_CODE_DEPLIB">MODULE_CODE_DEPLIB Record</a></h4> - -<div> -<p><tt>[DEPLIB, ...string...]</tt></p> - -<p>The <tt>DEPLIB</tt> record (code 6) contains a variable number of -values representing the bytes of a single dependent library name |