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add initial description of llvm top-level stuff.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37017 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2007-05-13 01:39:44 +00:00
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docs/BitCodeFormat.html
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@ -22,7 +22,11 @@
<li><a href="#stdblocks">Standard Blocks</a></li>
</ol>
</li>
<li><a href="#llvmir">LLVM IR Encoding</a></li>
<li><a href="#llvmir">LLVM IR Encoding</a>
<ol>
<li><a href="#basics">Basics</a></li>
</ol>
</li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>.
@ -114,7 +118,8 @@ is used by a reader to know what is contained in the file.</p>
<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
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>.
@ -505,7 +510,92 @@ abbreviation.
<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>
<p><tt>['B'<sub>8</sub>, 'C'<sub>8</sub>, 0x0<sub>4</sub>, 0xC<sub>4</sub>,
0xE<sub>4</sub>, 0xD<sub>4</sub>]</tt></p>
<p>When viewed as bytes, this is "BC 0xC0DE".</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 - MODULE_BLOCK - This is the top-level block that contains the
entire module, and describes a variety of per-module information.</li>
<li>9 - PARAMATTR_BLOCK - This enumerates the parameter attributes.</li>
<li>10 - TYPE_BLOCK - This describes all of the types in the module.</li>
<li>11 - CONSTANTS_BLOCK - This describes constants for a module or
function.</li>
<li>12 - FUNCTION_BLOCK - This describes a function body.</li>
<li>13 - TYPE_SYMTAB_BLOCK - This describes the type symbol table.</li>
<li>14 - VALUE_SYMTAB_BLOCK - 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>