-<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
+<!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">
<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>
</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>
</p>
<ul>
-<li>A magic number that identifies the stream.</li>
-<li>Encoding primitives like variable bit-rate integers.</li>
-<li>Blocks, which define nested content.</li>
-<li>Data Records, which describe entities within the file.</li>
+<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>
<div class="doc_text">
-<p>LLVM </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_subsection"><a name="primitives">Primitives</a>
+</div>
+
+<div class="doc_text">
+
+<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 are encoded in two ways: either as <a href="#fixedwidth">Fixed
+Width Integers</a> or as <a href="#variablewidth">Variable Width
+Integers</a>.
+</p>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"> <a name="fixedwidth">Fixed Width Integers</a>
+</div>
+
+<div class="doc_text">
+
+<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"> <a name="variablewidth">Variable Width
+Integers</a></div>
+
+<div class="doc_text">
+
+<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"> <a name="char6">6-bit characters</a></div>
+
+<div class="doc_text">
+
+<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"> <a name="wordalign">Word Alignment</a></div>
+
+<div class="doc_text">
+
+<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 class="doc_subsection"><a name="abbrevid">Abbreviation IDs</a>
+</div>
+
+<div class="doc_text">
+
+<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 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>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="blocks">Blocks</a>
+</div>
+
+<div class="doc_text">
+
+<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 hierachical 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
+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, 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>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"> <a name="ENTER_SUBBLOCK">ENTER_SUBBLOCK
+Encoding</a></div>
+
+<div class="doc_text">
+
+<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"> <a name="END_BLOCK">END_BLOCK
+Encoding</a></div>
+
+<div class="doc_text">
+
+<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 class="doc_subsection"><a name="datarecord">Data Records</a>
+</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
+<tt>MODULE_CODE_TRIPLE</tt>, and the values of the record are the ASCII codes
+for the characters in the string.
+</p>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"> <a name="UNABBREV_RECORD">UNABBREV_RECORD
+Encoding</a></div>
+
+<div class="doc_text">
+
+<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"> <a name="abbrev_records">Abbreviated Record
+Encoding</a></div>
+
+<div class="doc_text">
+
+<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>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="abbreviations">Abbreviations</a>
+</div>
+
+<div class="doc_text">
+<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 if the specific stream does
+not need it. 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>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection"><a name="DEFINE_ABBREV">DEFINE_ABBREV
+ Encoding</a></div>
+
+<div class="doc_text">
+
+<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 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>
+
+<ol>
+<li>Fixed: 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: 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: This field is an array of values. The array operand
+ has no extra data, but expects another operand to follow it which indicates
+ 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: This field should be emitted as
+ a <a href="#char6">char6-encoded value</a>. This operand type takes no
+ extra data.</li>
+<li>Blob: 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>
+</ol>
+
+<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 code for <tt>TRIPLE</tt> in LLVM IR files.</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 builtin 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="wellformed">Well-Formedness</a> </div>
+<div class="doc_subsubsection"><a name="BLOCKINFO">#0 - BLOCKINFO
+Block</a></div>
<div class="doc_text">
-<p>blah
+<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 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> can optionally occur in this block. The elements of
+the record are the bytes for the string name of the block. llvm-bcanalyzer uses
+this to dump out bitcode files symbolically.</p>
+
+<p>The <tt>SETRECORDNAME</tt> record can optionally occur in this block. The
+first entry 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 uses
+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 a 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_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 Integers</a> are an efficient way to
+encode arbitrary sized unsigned values, but is an extremely inefficient way to
+encode 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.
+</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 — <tt>MODULE_BLOCK</tt> — This is the top-level block that
+ contains the entire module, and describes a variety of per-module
+ information.</li>
+<li>9 — <tt>PARAMATTR_BLOCK</tt> — This enumerates the parameter
+ attributes.</li>
+<li>10 — <tt>TYPE_BLOCK</tt> — This describes all of the types in
+ the module.</li>
+<li>11 — <tt>CONSTANTS_BLOCK</tt> — This describes constants for a
+ module or function.</li>
+<li>12 — <tt>FUNCTION_BLOCK</tt> — This describes a function
+ body.</li>
+<li>13 — <tt>TYPE_SYMTAB_BLOCK</tt> — This describes the type symbol
+ table.</li>
+<li>14 — <tt>VALUE_SYMTAB_BLOCK</tt> — This describes a value symbol
+ table.</li>
+</ul>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="MODULE_BLOCK">MODULE_BLOCK Contents</a>
+</div>
+
+<div class="doc_text">
+
+<p>
+</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: $Date$
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