<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>
- <li><a href="#llvmir">LLVM IR Encoding</a></li>
</ol>
<div class="doc_author">
- <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>.
+ <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
+ and <a href="http://www.reverberate.org">Joshua Haberman</a>.
</p>
</div>
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, like very
+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>This document first describes the LLVM bitstream format, then describes the
-record structure used by LLVM IR files.
+<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>
<div class="doc_text">
-<p>The first four bytes of the stream identify the encoding of the file. This
-is used by a reader to know what is contained in the file.</p>
+<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>
<div class="doc_text">
<p>
-A bitstream literally consists of a stream of bits. This stream is made up of a
-number of primitive values that encode a stream of integer values. These
-integers are are encoded in two ways: either as <a href="#fixedwidth">Fixed
+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>
<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>
-<ul>
-<li>'a' .. 'z' - 0 .. 25</li>
-<li>'A' .. 'Z' - 26 .. 52</li>
-<li>'0' .. '9' - 53 .. 61</li>
-<li>'.' - 62</li>
-<li>'_' - 63</li>
-</ul>
+<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
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 by the stream itself.
+one of the abbreviation IDs defined for the current block by the stream itself.
</p>
<p>
</p>
<ul>
-<li>0 - <a href="#END_BLOCK">END_BLOCK</a> - This abbrev ID marks the end of the
- current block.</li>
-<li>1 - <a href="#ENTER_SUBBLOCK">ENTER_SUBBLOCK</a> - This abbrev ID marks the
- beginning of a new block.</li>
-<li>2 - <a href="#DEFINE_ABBREV">DEFINE_ABBREV</a> - This defines a new
- abbreviation.</li>
-<li>3 - <a href="#UNABBREV_RECORD">UNABBREV_RECORD</a> - This ID specifies the
- definition of an unabbreviated record.</li>
+<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
<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). Nested blocks capture the hierachical structure of the data
+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 they do not understand. The LLVM IR reader uses this
+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>
<ol>
-<li>A current abbrev id width. This value starts at 2, and is set every time a
+<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, or
- they may be associated with all blocks of a particular ID.
+<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>
+<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>
</div>
<align32bits>, blocklen<sub>32</sub>]</tt></p>
<p>
-The ENTER_SUBBLOCK abbreviation ID specifies the start of a new block record.
-The <tt>blockid</tt> value is encoded as a 8-bit VBR identifier, and indicates
-the type of block being entered (which is application specific). The
-<tt>newabbrevlen</tt> value is a 4-bit VBR which specifies the
-abbrev id width for the sub-block. The <tt>blocklen</tt> 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.
+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>
<p><tt>[END_BLOCK, <align32bits>]</tt></p>
<p>
-The END_BLOCK 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>
+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 class="doc_text">
<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
-there are multiple different ways to encode a record (with 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 MODULE_CODE_TRIPLE,
-and the values of the record are the ascii codes for the characters in the
-string.</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>
</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 UNABBREV_RECORD provides a default fallback encoding, which is both
-completely general and also extremely inefficient. It can describe an arbitrary
-record, by emitting the code and operands as vbrs.</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 UNABBREV_RECORD abbrevid, a vbr6 for the
-MODULE_CODE_TRIPLE code, a vbr6 for the length of the string (which is equal to
-the number of operands), and a vbr6 for each character. Since there are no
-letters with value 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>
+<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>
<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 <a href="#UNABBREV_RECORD">UNABBREV_RECORD</a>
-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>
+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>
<!-- ======================================================================= -->
</p>
<p>
-Abbreviations can be determined dynamically per client, per file. Since the
+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 if the specific stream does
-not need it. As a concrete example, LLVM IR files usually emit an abbreviation
+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>
<p><tt>[DEFINE_ABBREV, numabbrevops<sub>vbr5</sub>, abbrevop0, abbrevop1,
...]</tt></p>
-<p>An abbreviation definition consists of the DEFINE_ABBREV abbrevid followed
-by a VBR that specifies the number of abbrev operands, then the abbrev
+<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>
+(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>
- - blah
+<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> -
-
+<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 class="doc_subsection"><a name="stdblocks">Standard Blocks</a>
+</div>
+
+<div class="doc_text">
+
+<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>
+
</div>
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="BLOCKINFO">#0 - BLOCKINFO
+Block</a></div>
+
+<div class="doc_text">
+
+<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 class="doc_section"> <a name="wrapper">Bitcode Wrapper Format</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<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>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="llvmir">LLVM IR Encoding</a></div>
<div class="doc_text">
-<p></p>
+<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>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="basics">Basics</a>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="ir_magic">LLVM IR Magic Number</a></div>
+
+<div class="doc_text">
+
+<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="ir_signed_vbr">Signed VBRs</a></div>
+
+<div class="doc_text">
+
+<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>
+
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="ir_blocks">LLVM IR Blocks</a></div>
+
+<div class="doc_text">
+
+<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 class="doc_subsection"><a name="MODULE_BLOCK">MODULE_BLOCK Contents</a>
+</div>
+
+<div class="doc_text">
+
+<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>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="MODULE_CODE_VERSION">MODULE_CODE_VERSION Record</a>
+</div>
+
+<div class="doc_text">
+
+<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="MODULE_CODE_TRIPLE">MODULE_CODE_TRIPLE Record</a>
+</div>
+
+<div class="doc_text">
+<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="MODULE_CODE_DATALAYOUT">MODULE_CODE_DATALAYOUT Record</a>
+</div>
+
+<div class="doc_text">
+<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="MODULE_CODE_ASM">MODULE_CODE_ASM Record</a>
+</div>
+
+<div class="doc_text">
+<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="MODULE_CODE_SECTIONNAME">MODULE_CODE_SECTIONNAME Record</a>
+</div>
+
+<div class="doc_text">
+<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="MODULE_CODE_DEPLIB">MODULE_CODE_DEPLIB Record</a>
+</div>
+
+<div class="doc_text">
+<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
+string, one of the libraries mentioned in a <tt>deplibs</tt>
+declaration. There should be one <tt>DEPLIB</tt> record for each
+library name referenced.</p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="MODULE_CODE_GLOBALVAR">MODULE_CODE_GLOBALVAR Record</a>
+</div>
+
+<div class="doc_text">
+<p><tt>[GLOBALVAR, pointer type, isconst, initid, linkage, alignment, section, visibility, threadlocal]</tt></p>
+
+<p>The <tt>GLOBALVAR</tt> record (code 7) marks the declaration or
+definition of a global variable. The operand fields are:</p>
+
+<ul>
+<li><i>pointer type</i>: The type index of the pointer type used to point to
+this global variable</li>
+
+<li><i>isconst</i>: Non-zero if the variable is treated as constant within
+the module, or zero if it is not</li>
+
+<li><i>initid</i>: If non-zero, the value index of the initializer for this
+variable, plus 1.</li>
+
+<li><a name="linkage"><i>linkage</i></a>: An encoding of the linkage
+type for this variable:
+ <ul>
+ <li><tt>external</tt>: code 0</li>
+ <li><tt>weak</tt>: code 1</li>
+ <li><tt>appending</tt>: code 2</li>
+ <li><tt>internal</tt>: code 3</li>
+ <li><tt>linkonce</tt>: code 4</li>
+ <li><tt>dllimport</tt>: code 5</li>
+ <li><tt>dllexport</tt>: code 6</li>
+ <li><tt>extern_weak</tt>: code 7</li>
+ <li><tt>common</tt>: code 8</li>
+ <li><tt>private</tt>: code 9</li>
+ <li><tt>weak_odr</tt>: code 10</li>
+ <li><tt>linkonce_odr</tt>: code 11</li>
+ <li><tt>available_externally</tt>: code 12</li>
+ <li><tt>linker_private</tt>: code 13</li>
+ </ul>
+</li>
+
+<li><i>alignment</i>: The logarithm base 2 of the variable's requested
+alignment, plus 1</li>
+
+<li><i>section</i>: If non-zero, the 1-based section index in the
+table of <a href="#MODULE_CODE_SECTIONNAME">MODULE_CODE_SECTIONNAME</a>
+entries.</li>
+
+<li><a name="visibility"><i>visibility</i></a>: If present, an
+encoding of the visibility of this variable:
+ <ul>
+ <li><tt>default</tt>: code 0</li>
+ <li><tt>hidden</tt>: code 1</li>
+ <li><tt>protected</tt>: code 2</li>
+ </ul>
+</li>
+
+<li><i>threadlocal</i>: If present and non-zero, indicates that the variable
+is <tt>thread_local</tt></li>
+
+</ul>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="MODULE_CODE_FUNCTION">MODULE_CODE_FUNCTION Record</a>
+</div>
+
+<div class="doc_text">
+
+<p><tt>[FUNCTION, type, callingconv, isproto, linkage, paramattr, alignment, section, visibility, gc]</tt></p>
+
+<p>The <tt>FUNCTION</tt> record (code 8) marks the declaration or
+definition of a function. The operand fields are:</p>
+
+<ul>
+<li><i>type</i>: The type index of the function type describing this function</li>
+
+<li><i>callingconv</i>: The calling convention number:
+ <ul>
+ <li><tt>ccc</tt>: code 0</li>
+ <li><tt>fastcc</tt>: code 8</li>
+ <li><tt>coldcc</tt>: code 9</li>
+ <li><tt>x86_stdcallcc</tt>: code 64</li>
+ <li><tt>x86_fastcallcc</tt>: code 65</li>
+ <li><tt>arm_apcscc</tt>: code 66</li>
+ <li><tt>arm_aapcscc</tt>: code 67</li>
+ <li><tt>arm_aapcs_vfpcc</tt>: code 68</li>
+ </ul>
+</li>
+
+<li><i>isproto</i>: Non-zero if this entry represents a declaration
+rather than a definition</li>
+
+<li><i>linkage</i>: An encoding of the <a href="#linkage">linkage type</a>
+for this function</li>
+
+<li><i>paramattr</i>: If nonzero, the 1-based parameter attribute index
+into the table of <a href="#PARAMATTR_CODE_ENTRY">PARAMATTR_CODE_ENTRY</a>
+entries.</li>
+
+<li><i>alignment</i>: The logarithm base 2 of the function's requested
+alignment, plus 1</li>
+
+<li><i>section</i>: If non-zero, the 1-based section index in the
+table of <a href="#MODULE_CODE_SECTIONNAME">MODULE_CODE_SECTIONNAME</a>
+entries.</li>
+
+<li><i>visibility</i>: An encoding of the <a href="#visibility">visibility</a>
+ of this function</li>
+
+<li><i>gc</i>: If present and nonzero, the 1-based garbage collector
+index in the table of
+<a href="#MODULE_CODE_GCNAME">MODULE_CODE_GCNAME</a> entries.</li>
+</ul>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="MODULE_CODE_ALIAS">MODULE_CODE_ALIAS Record</a>
+</div>
+
+<div class="doc_text">
+
+<p><tt>[ALIAS, alias type, aliasee val#, linkage, visibility]</tt></p>
+
+<p>The <tt>ALIAS</tt> record (code 9) marks the definition of an
+alias. The operand fields are</p>
+
+<ul>
+<li><i>alias type</i>: The type index of the alias</li>
+
+<li><i>aliasee val#</i>: The value index of the aliased value</li>
+
+<li><i>linkage</i>: An encoding of the <a href="#linkage">linkage type</a>
+for this alias</li>
+
+<li><i>visibility</i>: If present, an encoding of the
+<a href="#visibility">visibility</a> of the alias</li>
+
+</ul>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="MODULE_CODE_PURGEVALS">MODULE_CODE_PURGEVALS Record</a>
+</div>
+
+<div class="doc_text">
+<p><tt>[PURGEVALS, numvals]</tt></p>
+
+<p>The <tt>PURGEVALS</tt> record (code 10) resets the module-level
+value list to the size given by the single operand value. Module-level
+value list items are added by <tt>GLOBALVAR</tt>, <tt>FUNCTION</tt>,
+and <tt>ALIAS</tt> records. After a <tt>PURGEVALS</tt> record is seen,
+new value indices will start from the given <i>numvals</i> value.</p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="MODULE_CODE_GCNAME">MODULE_CODE_GCNAME Record</a>
+</div>
+
+<div class="doc_text">
+<p><tt>[GCNAME, ...string...]</tt></p>
+
+<p>The <tt>GCNAME</tt> record (code 11) contains a variable number of
+values representing the bytes of a single garbage collector name
+string. There should be one <tt>GCNAME</tt> record for each garbage
+collector name referenced in function <tt>gc</tt> attributes within
+the module. These records can be referenced by 1-based index in the <i>gc</i>
+fields of <tt>FUNCTION</tt> records.</p>
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="PARAMATTR_BLOCK">PARAMATTR_BLOCK Contents</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>PARAMATTR_BLOCK</tt> block (id 9) ...
+</p>
+
+</div>
+
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="PARAMATTR_CODE_ENTRY">PARAMATTR_CODE_ENTRY Record</a>
+</div>
+
+<div class="doc_text">
+
+<p><tt>[ENTRY, paramidx0, attr0, paramidx1, attr1...]</tt></p>
+
+<p>The <tt>ENTRY</tt> record (code 1) ...
+</p>
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="TYPE_BLOCK">TYPE_BLOCK Contents</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>TYPE_BLOCK</tt> block (id 10) ...
+</p>
+
+</div>
+
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="CONSTANTS_BLOCK">CONSTANTS_BLOCK Contents</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>CONSTANTS_BLOCK</tt> block (id 11) ...
+</p>
+
+</div>
+
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="FUNCTION_BLOCK">FUNCTION_BLOCK Contents</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>FUNCTION_BLOCK</tt> block (id 12) ...
+</p>
+
+<p>In addition to the record types described below, a
+<tt>FUNCTION_BLOCK</tt> block may contain the following sub-blocks:
+</p>
+
+<ul>
+<li><a href="#CONSTANTS_BLOCK"><tt>CONSTANTS_BLOCK</tt></a></li>
+<li><a href="#VALUE_SYMTAB_BLOCK"><tt>VALUE_SYMTAB_BLOCK</tt></a></li>
+<li><a href="#METADATA_ATTACHMENT"><tt>METADATA_ATTACHMENT</tt></a></li>
+</ul>
+
+</div>
+
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="TYPE_SYMTAB_BLOCK">TYPE_SYMTAB_BLOCK Contents</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>TYPE_SYMTAB_BLOCK</tt> block (id 13) ...
+</p>
+
+</div>
+
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="VALUE_SYMTAB_BLOCK">VALUE_SYMTAB_BLOCK Contents</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>VALUE_SYMTAB_BLOCK</tt> block (id 14) ...
+</p>
+
+</div>
+
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="METADATA_BLOCK">METADATA_BLOCK Contents</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>METADATA_BLOCK</tt> block (id 15) ...
+</p>
+
+</div>
+
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="METADATA_ATTACHMENT">METADATA_ATTACHMENT Contents</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>METADATA_ATTACHMENT</tt> block (id 16) ...
+</p>
</div>
<!-- *********************************************************************** -->
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
Last modified: $Date$