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+<html>
+<head>
+  <title>Extending LLVM: Adding instructions, intrinsics, types, etc.</title>
+  <link rel="stylesheet" href="llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">
+  Extending LLVM: Adding instructions, intrinsics, types, etc.
+</div>
+
+<ol>
+  <li><a href="#introduction">Introduction and Warning</a></li>
+  <li><a href="#instruction">Adding a new instruction</a></li>
+  <li><a href="#intrinsic">Adding a new intrinsic function</a></li>
+  <li><a href="#type">Adding a new type</a>
+  <ol>
+    <li><a href="#fund_type">Adding a new fundamental type</a></li>
+    <li><a href="#derived_type">Adding a new derived type</a></li>
+  </ol></li>
+</ol>
+
+<div class="doc_text">    
+  <p><b>Written by <a href="http://misha.brukman.net">Misha Brukman</a></b></p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section">
+  <a name="introduction">Introduction and Warning</a>
+</div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>During the course of using LLVM, you may wish to customize it for your
+research project or for experimentation. At this point, you may realize that
+you need to add something to LLVM, whether it be a new fundamental type, a new
+intrinsic function, or a whole new instruction.</p>
+
+<p>When you come to this realization, stop and think. Do you really need to
+extend LLVM? Is it a new fundamental capability that LLVM does not support at
+its current incarnation or can it be synthesized from already pre-existing LLVM
+elements? If you are not sure, ask on the <a
+href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM-dev</a> list. The
+reason is that extending LLVM will get involved as you need to update all the
+different passes that you intend to use with your extension, and there are
+<em>many</em> LLVM analyses and transformations, so it may be quite a bit of
+work.</p>
+
+<p>Before you invest a significant amount of effort into a non-trivial
+extension, <span class="doc_warning">ask on the list</span> if what you are
+looking to do can be done with already-existing infrastructure, or if maybe
+someone else is already working on it. You will save yourself a lot of time and
+effort by doing so.</p>
+
+<p>Finally, these are my notes, and since my extensions are not complete, I may
+be missing steps. If you find some omissions, please let me know <a
+href="http://misha.brukman.net/contact.html">directly</a> or post on <a
+href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM-dev</a>.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section">
+  <a name="instruction">Adding a new instruction</a>
+</div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p><span class="doc_warning">WARNING: adding instructions changes the bytecode
+format, and will break compatibility with currently-existing LLVM
+installations.</span> Only add an instruction if it is absolutely
+necessary.</p>
+
+<ol>
+<li><tt>llvm/include/llvm/Instruction.def</tt>:
+    add a number for your instruction and an enum name</li>
+
+<li><tt>llvm/include/llvm/i*.h</tt>:
+    add a definition for the class that will represent your instruction</li>
+
+<li><tt>llvm/include/llvm/Support/InstVisitor.h</tt>:
+    add a prototype for a visitor to your new instruction type</li>
+
+<li><tt>llvm/lib/AsmParser/Lexer.l</tt>:
+    add a new token to parse your instruction from assembly text file</li>
+
+<li><tt>llvm/lib/AsmParser/llvmAsmParser.y</tt>:
+    add the grammar on how your instruction can be read and what it will
+    construct as a result</li>
+
+<li><tt>llvm/lib/Bytecode/Reader/InstructionReader.cpp</tt>:
+    add a case for your instruction and how it will be parsed from bytecode</li>
+
+<li><tt>llvm/lib/VMCore/Instruction.cpp</tt>:
+    add a case for how your instruction will be printed out to assembly</li>
+
+<li><tt>llvm/lib/VMCore/i*.cpp</tt>:
+    implement the class you defined in <tt>llvm/include/llvm/i*.h</tt></li>
+
+</ol>
+
+<p>Also, you need to implement (or modify) any analyses or passes that you want
+to understand this new instruction.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section">
+  <a name="intrinsic">Adding a new intrinsic function</a>
+</div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Adding an intrinsic function is easier than adding an instruction, and is
+transparent to optimization passes which treat it as an unanalyzable function.
+If your added functionality can be expressed as a function call, an intrinsic
+function is the method of choice for LLVM extension.</p>
+
+<ol>
+
+<li><tt>llvm/include/llvm/Intrinsics.h</tt>:
+    add an enum in the <tt>llvm::Intrinsic</tt> namespace</li>
+
+<li><tt>llvm/lib/VMCore/IntrinsicLowering.cpp</tt>:
+    implement the lowering for this intrinsic</li>
+
+<li><tt>llvm/lib/VMCore/Verifier.cpp</tt>:
+    handle the new intrinsic</li>
+
+<li><tt>llvm/lib/VMCore/Function.cpp</tt>:
+    handle the new intrinsic</li>
+
+</ol>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section">
+  <a name="type">Adding a new type</a>
+</div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p><span class="doc_warning">WARNING: adding new types changes the bytecode
+format, and will break compatibility with currently-existing LLVM
+installations.</span> Only add new types if it is absolutely necessary.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+  <a name="fund_type">Adding a fundamental type</a>
+</div>
+
+<div class="doc_text">
+
+<ol>
+
+<li><tt>llvm/include/llvm/Type.def</tt>:
+    add enum for the type</li>
+
+<li><tt>llvm/include/llvm/Type.h</tt>:
+    add ID number for the new type; add static <tt>Type*</tt> for this type</li>
+
+<li><tt>llvm/lib/VMCore/Type.cpp</tt>:
+    add mapping from <tt>TypeID</tt> =&gt; <tt>Type*</tt>;
+    initialize the static <tt>Type*</tt></li>
+
+<li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
+    add ability to parse in the type from text assembly</li>
+
+<li><tt>llvm/lib/AsmReader/llvmAsmParser.y</tt>:
+    add a token for that type</li>
+
+</ol>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+  <a name="derived_type">Adding a derived type</a>
+</div>
+
+<div class="doc_text">
+
+<p>TODO</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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+
+  <a href="http://misha.brukman.net">Misha Brukman</a><br>
+  <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
+  <br>
+  Last modified: $Date$
+</address>
+
+</body>
+</html>