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add a bunch of content.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37002 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2007-05-12 05:37:42 +00:00
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docs/BitCodeFormat.html
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@ -1,4 +1,5 @@
<!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">
@ -13,6 +14,10 @@
<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>
</ol>
</li>
<li><a href="#llvmir">LLVM IR Encoding</a></li>
@ -71,10 +76,13 @@ structure. This structure consists of the following concepts:
</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>
@ -91,21 +99,218 @@ understanding the encoding.</p>
<div class="doc_text">
<p>LLVM </p>
<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>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="wellformed">Well-Formedness</a> </div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="primitives">Primitives</a>
</div>
<div class="doc_text">
<p>blah
<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
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:<s/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>
<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>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 - DEFINE_ABBREV - This defines a new abbreviation.</li>
<li>3 - UNABBREV_RECORD - This ID specifies the definition of an unabbreviated
record.</li>
</ul>
<p>Abbreviation IDs 4 and above are defined by the stream itself.</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). 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, or
they may be associated with all blocks of a particular ID.
</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>,
&lt;align32bits&gt;, 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.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="END_BLOCK">END_BLOCK
Encoding</a></div>
<div class="doc_text">
<p><tt>[END_BLOCK, &lt;align32bits&gt;]</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>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="datarecord">Data Records</a>
</div>
<div class="doc_text">
<p>
blah
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="llvmir">LLVM IR Encoding</a></div>
<!-- *********************************************************************** -->