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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<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="#intrinsic">Adding a new intrinsic function</a></li>
<li><a href="#instruction">Adding a new instruction</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>Adding an <a href="#intrinsic">intrinsic function</a> 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>
<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>
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<div class="doc_section">
<a name="intrinsic">Adding a new intrinsic function</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Adding a new intrinsic function to LLVM is much easier than adding a new
instruction. Almost all extensions to LLVM should start as an intrinsic
function and then be turned into an instruction if warranted.</p>
<ol>
<li><tt>llvm/docs/LangRef.html</tt>:
Document the intrinsic. Decide whether it is code generator specific and
what the restrictions are. Talk to other people about it so that you are
sure it's a good idea.</li>
<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>:
Add code to check the invariants of the intrinsic are respected.</li>
<li><tt>llvm/lib/VMCore/Function.cpp (<tt>Function::getIntrinsicID()</tt>)</tt>:
Identify the new intrinsic function, returning the enum for the intrinsic
that you added.</li>
<li><tt>llvm/lib/Analysis/BasicAliasAnalysis.cpp</tt>: If the new intrinsic does
not access memory, or does not write to memory, add it to the relevant list
of functions.</li>
<li><tt>llvm/lib/Transforms/Utils/Local.cpp</tt>: If it is possible to constant
propagate your intrinsic, add support to it in the
<tt>canConstantFoldCallTo</tt> and <tt>ConstantFoldCall</tt> functions.</li>
<li>Test your intrinsic</li>
<li><tt>llvm/test/Regression/*</tt>: add your test cases to the test suite.</li>
</ol>
<p>If this intrinsic requires code generator support (ie, it cannot be lowered).
You should also add support to the code generator in question.</p>
</div>
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<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 it will take some effort to maintain compatibility with
the previous version.</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>
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<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> => <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>
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