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239 lines
8.4 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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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_text">
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<p><b>Written by <a href="http://misha.brukman.net">Misha Brukman</a></b></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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<p>Finally, these are my notes, and since my extensions are not complete, I may
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be missing steps. If you find some omissions, please let me know <a
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href="http://misha.brukman.net/contact.html">directly</a> or post on <a
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href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM-dev</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="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/IntrinsicLowering.cpp</tt>:
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implement the lowering for this intrinsic</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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propagate your intrinsic, add support to it in the
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<tt>canConstantFoldCallTo</tt> and <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>If this intrinsic requires code generator support (ie, it cannot be lowered).
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You should also add support to the code generator in question.</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/i*.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/InstructionReader.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/i*.cpp</tt>:
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implement the class you defined in <tt>llvm/include/llvm/i*.h</tt></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.def</tt>:
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add enum for the type</li>
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<li><tt>llvm/include/llvm/Type.h</tt>:
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add ID number 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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<p>TODO</p>
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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://misha.brukman.net">Misha Brukman</a><br>
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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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</body>
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</html>
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