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docs/BitCodeFormat.html
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@ -18,6 +18,7 @@
<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>
</ol>
</li>
<li><a href="#llvmir">LLVM IR Encoding</a></li>
@ -213,12 +214,14 @@ The set of builtin abbrev IDs is:
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>
<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>
</ul>
<p>Abbreviation IDs 4 and above are defined by the stream itself.</p>
<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>
@ -303,10 +306,110 @@ 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>
</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 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>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>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="abbrev_records">Abbreviated Record
Encoding</a></div>
<div class="doc_text">
<p><tt>[&lt;abbrevid&gt;, 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>
</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>
blah
Abbreviations can be determined dynamically per client, per file. Since 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>An abbreviation definition consists of the DEFINE_ABBREV 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>
- blah
</li>
<li>Encoding info with data - <tt>[0<sub>1</sub>, encoding<sub>3</sub>,
value<sub>vbr5</sub>]</tt> -
</li>
</ol>
</div>