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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<html>
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<head>
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<title>Extending LLVM: Adding instructions, intrinsics, types, etc.</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title">
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Extending LLVM: Adding instructions, intrinsics, types, etc.
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</div>
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<ol>
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<li><a href="#introduction">Introduction and Warning</a></li>
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<li><a href="#intrinsic">Adding a new intrinsic function</a></li>
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<li><a href="#instruction">Adding a new instruction</a></li>
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<li><a href="#type">Adding a new type</a>
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<ol>
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<li><a href="#fund_type">Adding a new fundamental type</a></li>
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<li><a href="#derived_type">Adding a new derived type</a></li>
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</ol></li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="http://misha.brukman.net">Misha Brukman</a>,
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Brad Jones, and <a href="http://nondot.org/sabre">Chris Lattner</a></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="introduction">Introduction and Warning</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>During the course of using LLVM, you may wish to customize it for your
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research project or for experimentation. At this point, you may realize that
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you need to add something to LLVM, whether it be a new fundamental type, a new
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intrinsic function, or a whole new instruction.</p>
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<p>When you come to this realization, stop and think. Do you really need to
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extend LLVM? Is it a new fundamental capability that LLVM does not support at
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its current incarnation or can it be synthesized from already pre-existing LLVM
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elements? If you are not sure, ask on the <a
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href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM-dev</a> list. The
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reason is that extending LLVM will get involved as you need to update all the
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different passes that you intend to use with your extension, and there are
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<em>many</em> LLVM analyses and transformations, so it may be quite a bit of
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work.</p>
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<p>Adding an <a href="#intrinsic">intrinsic function</a> is easier than adding
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an instruction, and is transparent to optimization passes which treat it as an
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unanalyzable function. If your added functionality can be expressed as a
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function call, an intrinsic function is the method of choice for LLVM
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extension.</p>
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<p>Before you invest a significant amount of effort into a non-trivial
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extension, <span class="doc_warning">ask on the list</span> if what you are
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looking to do can be done with already-existing infrastructure, or if maybe
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someone else is already working on it. You will save yourself a lot of time and
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effort by doing so.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="intrinsic">Adding a new intrinsic function</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>Adding a new intrinsic function to LLVM is much easier than adding a new
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instruction. Almost all extensions to LLVM should start as an intrinsic
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function and then be turned into an instruction if warranted.</p>
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<ol>
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<li><tt>llvm/docs/LangRef.html</tt>:
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Document the intrinsic. Decide whether it is code generator specific and
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what the restrictions are. Talk to other people about it so that you are
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sure it's a good idea.</li>
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<li><tt>llvm/include/llvm/Intrinsics.h</tt>:
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add an enum in the <tt>llvm::Intrinsic</tt> namespace</li>
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<li><tt>llvm/lib/VMCore/Verifier.cpp</tt>:
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Add code to check the invariants of the intrinsic are respected.</li>
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<li><tt>llvm/lib/VMCore/Function.cpp (<tt>Function::getIntrinsicID()</tt>)</tt>:
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Identify the new intrinsic function, returning the enum for the intrinsic
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that you added.</li>
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<li><tt>llvm/lib/Analysis/BasicAliasAnalysis.cpp</tt>: If the new intrinsic does
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not access memory or does not write to memory, add it to the relevant list
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of functions.</li>
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<li><tt>llvm/lib/Transforms/Utils/Local.cpp</tt>: If it is possible to constant
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fold your intrinsic, add support to it in the <tt>canConstantFoldCallTo</tt> and
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<tt>ConstantFoldCall</tt> functions.</li>
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<li>Test your intrinsic</li>
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<li><tt>llvm/test/Regression/*</tt>: add your test cases to the test suite</li>
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</ol>
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<p>Once the intrinsic has been added to the system, you must add code generator
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support for it. Generally you must do the following steps:</p>
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<dl>
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<dt>Add support to the C backend in <tt>lib/Target/CBackend/</tt></dt>
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<dd>Depending on the intrinsic, there are a few ways to implement this. First,
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if it makes sense to lower the intrinsic to an expanded sequence of C code in
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all cases, just emit the expansion in <tt>visitCallInst</tt>. Second, if the
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intrinsic has some way to express it with GCC (or any other compiler)
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extensions, it can be conditionally supported based on the compiler compiling
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the CBE output (see llvm.prefetch for an example). Third, if the intrinsic
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really has no way to be lowered, just have the code generator emit code that
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prints an error message and calls abort if executed.
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</dd>
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<dt>Add a enum value for the SelectionDAG node in
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<tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt></dt>
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<dd>Also, add code to <tt>lib/CodeGen/SelectionDAG/SelectionDAG.cpp</tt> (and
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<tt>SelectionDAGPrinter.cpp</tt>) to print the node.</dd>
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<dt>Add code to <tt>SelectionDAG/SelectionDAGISel.cpp</tt> to recognize the
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intrinsic.</dt>
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<dd>Presumably the intrinsic should be recognized and turned into the node you
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added above.</dd>
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<dt>Add code to <tt>SelectionDAG/LegalizeDAG.cpp</tt> to <a
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href="CodeGenerator.html#selectiondag_legalize">legalize, promote, and
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expand</a> the node as necessary.</dt>
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<dd>If the intrinsic can be expanded to primitive operations, legalize can break
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the node down into other elementary operations that are be supported.</dd>
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<dt>Add target-specific support to specific code generators.</dt>
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<dd>Extend the code generators you are interested in to recognize and support
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the node, emitting the code you want.</dd>
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</dl>
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<p>
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Unfortunately, the process of extending the code generator to support a new node
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is not extremely well documented. As such, it is often helpful to look at other
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intrinsics (e.g. <tt>llvm.ctpop</tt>) to see how they are recognized and turned
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into a node by <tt>SelectionDAGISel.cpp</tt>, legalized by
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<tt>LegalizeDAG.cpp</tt>, then finally emitted by the various code generators.
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="instruction">Adding a new instruction</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p><span class="doc_warning">WARNING: adding instructions changes the bytecode
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format, and it will take some effort to maintain compatibility with
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the previous version.</span> Only add an instruction if it is absolutely
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necessary.</p>
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<ol>
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<li><tt>llvm/include/llvm/Instruction.def</tt>:
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add a number for your instruction and an enum name</li>
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<li><tt>llvm/include/llvm/Instructions.h</tt>:
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add a definition for the class that will represent your instruction</li>
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<li><tt>llvm/include/llvm/Support/InstVisitor.h</tt>:
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add a prototype for a visitor to your new instruction type</li>
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<li><tt>llvm/lib/AsmParser/Lexer.l</tt>:
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add a new token to parse your instruction from assembly text file</li>
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<li><tt>llvm/lib/AsmParser/llvmAsmParser.y</tt>:
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add the grammar on how your instruction can be read and what it will
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construct as a result</li>
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<li><tt>llvm/lib/Bytecode/Reader/Reader.cpp</tt>:
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add a case for your instruction and how it will be parsed from bytecode</li>
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<li><tt>llvm/lib/VMCore/Instruction.cpp</tt>:
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add a case for how your instruction will be printed out to assembly</li>
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<li><tt>llvm/lib/VMCore/Instructions.cpp</tt>:
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implement the class you defined in
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<tt>llvm/include/llvm/Instructions.h</tt></li>
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<li>Test your instruction</li>
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<li><tt>llvm/lib/Target/*</tt>:
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Add support for your instruction to code generators, or add a lowering
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pass.</li>
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<li><tt>llvm/test/Regression/*</tt>: add your test cases to the test suite.</li>
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</ol>
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<p>Also, you need to implement (or modify) any analyses or passes that you want
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to understand this new instruction.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="type">Adding a new type</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p><span class="doc_warning">WARNING: adding new types changes the bytecode
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format, and will break compatibility with currently-existing LLVM
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installations.</span> Only add new types if it is absolutely necessary.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="fund_type">Adding a fundamental type</a>
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</div>
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<div class="doc_text">
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<ol>
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<li><tt>llvm/include/llvm/Type.h</tt>:
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add enum for the new type; add static <tt>Type*</tt> for this type</li>
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<li><tt>llvm/lib/VMCore/Type.cpp</tt>:
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add mapping from <tt>TypeID</tt> => <tt>Type*</tt>;
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initialize the static <tt>Type*</tt></li>
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<li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
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add ability to parse in the type from text assembly</li>
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<li><tt>llvm/lib/AsmReader/llvmAsmParser.y</tt>:
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add a token for that type</li>
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</ol>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="derived_type">Adding a derived type</a>
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</div>
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<div class="doc_text">
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<ol>
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<li><tt>llvm/include/llvm/Type.h</tt>:
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add enum for the new type; add a forward declaration of the type
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also</li>
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<li><tt>llvm/include/llvm/DerivedType.h</tt>:
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add new class to represent new class in the hierarchy; add forward
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declaration to the TypeMap value type</li>
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<li><tt>llvm/lib/VMCore/Type.cpp</tt>:
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add support for derived type to:
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<div class="doc_code">
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<pre>
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std::string getTypeDescription(const Type &Ty,
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std::vector<const Type*> &TypeStack)
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bool TypesEqual(const Type *Ty, const Type *Ty2,
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std::map<const Type*, const Type*> & EqTypes)
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</pre>
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</div>
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add necessary member functions for type, and factory methods</li>
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<li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
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add ability to parse in the type from text assembly</li>
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<li><tt>llvm/lib/ByteCode/Writer/Writer.cpp</tt>:
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modify <tt>void BytecodeWriter::outputType(const Type *T)</tt> to serialize
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your type</li>
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<li><tt>llvm/lib/ByteCode/Reader/Reader.cpp</tt>:
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modify <tt>const Type *BytecodeReader::ParseType()</tt> to read your data
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type</li>
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<li><tt>llvm/lib/VMCore/AsmWriter.cpp</tt>:
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modify
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<div class="doc_code">
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<pre>
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void calcTypeName(const Type *Ty,
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std::vector<const Type*> &TypeStack,
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std::map<const Type*,std::string> &TypeNames,
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std::string & Result)
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</pre>
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</div>
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to output the new derived type
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</li>
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</ol>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
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<br>
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Last modified: $Date$
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</address>
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