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5 <title>Extending LLVM: Adding instructions, intrinsics, types, etc.</title>
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11 <div class="doc_title">
12 Extending LLVM: Adding instructions, intrinsics, types, etc.
16 <li><a href="#introduction">Introduction and Warning</a></li>
17 <li><a href="#intrinsic">Adding a new intrinsic function</a></li>
18 <li><a href="#instruction">Adding a new instruction</a></li>
19 <li><a href="#type">Adding a new type</a>
21 <li><a href="#fund_type">Adding a new fundamental type</a></li>
22 <li><a href="#derived_type">Adding a new derived type</a></li>
26 <div class="doc_author">
27 <p>Written by <a href="http://misha.brukman.net">Misha Brukman</a></p>
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31 <div class="doc_section">
32 <a name="introduction">Introduction and Warning</a>
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36 <div class="doc_text">
38 <p>During the course of using LLVM, you may wish to customize it for your
39 research project or for experimentation. At this point, you may realize that
40 you need to add something to LLVM, whether it be a new fundamental type, a new
41 intrinsic function, or a whole new instruction.</p>
43 <p>When you come to this realization, stop and think. Do you really need to
44 extend LLVM? Is it a new fundamental capability that LLVM does not support at
45 its current incarnation or can it be synthesized from already pre-existing LLVM
46 elements? If you are not sure, ask on the <a
47 href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM-dev</a> list. The
48 reason is that extending LLVM will get involved as you need to update all the
49 different passes that you intend to use with your extension, and there are
50 <em>many</em> LLVM analyses and transformations, so it may be quite a bit of
53 <p>Adding an <a href="#intrinsic">intrinsic function</a> is easier than adding
54 an instruction, and is transparent to optimization passes which treat it as an
55 unanalyzable function. If your added functionality can be expressed as a
56 function call, an intrinsic function is the method of choice for LLVM
59 <p>Before you invest a significant amount of effort into a non-trivial
60 extension, <span class="doc_warning">ask on the list</span> if what you are
61 looking to do can be done with already-existing infrastructure, or if maybe
62 someone else is already working on it. You will save yourself a lot of time and
63 effort by doing so.</p>
65 <p>Finally, these are my notes, and since my extensions are not complete, I may
66 be missing steps. If you find some omissions, please let me know <a
67 href="http://misha.brukman.net/contact.html">directly</a> or post on <a
68 href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM-dev</a>.</p>
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73 <div class="doc_section">
74 <a name="intrinsic">Adding a new intrinsic function</a>
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78 <div class="doc_text">
80 <p>Adding a new intrinsic function to LLVM is much easier than adding a new
81 instruction. Almost all extensions to LLVM should start as an intrinsic
82 function and then be turned into an instruction if warranted.</p>
85 <li><tt>llvm/docs/LangRef.html</tt>:
86 Document the intrinsic. Decide whether it is code generator specific and
87 what the restrictions are. Talk to other people about it so that you are
88 sure it's a good idea.</li>
90 <li><tt>llvm/include/llvm/Intrinsics.h</tt>:
91 add an enum in the <tt>llvm::Intrinsic</tt> namespace</li>
93 <li><tt>llvm/lib/CodeGen/IntrinsicLowering.cpp</tt>:
94 implement the lowering for this intrinsic</li>
96 <li><tt>llvm/lib/VMCore/Verifier.cpp</tt>:
97 Add code to check the invariants of the intrinsic are respected.</li>
99 <li><tt>llvm/lib/VMCore/Function.cpp (<tt>Function::getIntrinsicID()</tt>)</tt>:
100 Identify the new intrinsic function, returning the enum for the intrinsic
103 <li><tt>llvm/lib/Analysis/BasicAliasAnalysis.cpp</tt>: If the new intrinsic does
104 not access memory or does not write to memory, add it to the relevant list
107 <li><tt>llvm/lib/Transforms/Utils/Local.cpp</tt>: If it is possible to constant
108 propagate your intrinsic, add support to it in the
109 <tt>canConstantFoldCallTo</tt> and <tt>ConstantFoldCall</tt> functions.</li>
111 <li>Test your intrinsic</li>
112 <li><tt>llvm/test/Regression/*</tt>: add your test cases to the test suite.</li>
115 <p>If this intrinsic requires code generator support (ie, it cannot be lowered).
116 You should also add support to the code generator in question.</p>
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121 <div class="doc_section">
122 <a name="instruction">Adding a new instruction</a>
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126 <div class="doc_text">
128 <p><span class="doc_warning">WARNING: adding instructions changes the bytecode
129 format, and it will take some effort to maintain compatibility with
130 the previous version.</span> Only add an instruction if it is absolutely
135 <li><tt>llvm/include/llvm/Instruction.def</tt>:
136 add a number for your instruction and an enum name</li>
138 <li><tt>llvm/include/llvm/i*.h</tt>:
139 add a definition for the class that will represent your instruction</li>
141 <li><tt>llvm/include/llvm/Support/InstVisitor.h</tt>:
142 add a prototype for a visitor to your new instruction type</li>
144 <li><tt>llvm/lib/AsmParser/Lexer.l</tt>:
145 add a new token to parse your instruction from assembly text file</li>
147 <li><tt>llvm/lib/AsmParser/llvmAsmParser.y</tt>:
148 add the grammar on how your instruction can be read and what it will
149 construct as a result</li>
151 <li><tt>llvm/lib/Bytecode/Reader/InstructionReader.cpp</tt>:
152 add a case for your instruction and how it will be parsed from bytecode</li>
154 <li><tt>llvm/lib/VMCore/Instruction.cpp</tt>:
155 add a case for how your instruction will be printed out to assembly</li>
157 <li><tt>llvm/lib/VMCore/i*.cpp</tt>:
158 implement the class you defined in <tt>llvm/include/llvm/i*.h</tt></li>
162 <p>Also, you need to implement (or modify) any analyses or passes that you want
163 to understand this new instruction.</p>
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169 <div class="doc_section">
170 <a name="type">Adding a new type</a>
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174 <div class="doc_text">
176 <p><span class="doc_warning">WARNING: adding new types changes the bytecode
177 format, and will break compatibility with currently-existing LLVM
178 installations.</span> Only add new types if it is absolutely necessary.</p>
182 <!-- ======================================================================= -->
183 <div class="doc_subsection">
184 <a name="fund_type">Adding a fundamental type</a>
187 <div class="doc_text">
191 <li><tt>llvm/include/llvm/Type.def</tt>:
192 add enum for the type</li>
194 <li><tt>llvm/include/llvm/Type.h</tt>:
195 add ID number for the new type; add static <tt>Type*</tt> for this type</li>
197 <li><tt>llvm/lib/VMCore/Type.cpp</tt>:
198 add mapping from <tt>TypeID</tt> => <tt>Type*</tt>;
199 initialize the static <tt>Type*</tt></li>
201 <li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
202 add ability to parse in the type from text assembly</li>
204 <li><tt>llvm/lib/AsmReader/llvmAsmParser.y</tt>:
205 add a token for that type</li>
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212 <div class="doc_subsection">
213 <a name="derived_type">Adding a derived type</a>
216 <div class="doc_text">
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231 <a href="http://misha.brukman.net">Misha Brukman</a><br>
232 <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
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