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6 <title>Extending LLVM: Adding instructions, intrinsics, types, etc.</title>
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13 Extending LLVM: Adding instructions, intrinsics, types, etc.
17 <li><a href="#introduction">Introduction and Warning</a></li>
18 <li><a href="#intrinsic">Adding a new intrinsic function</a></li>
19 <li><a href="#instruction">Adding a new instruction</a></li>
20 <li><a href="#sdnode">Adding a new SelectionDAG node</a></li>
21 <li><a href="#type">Adding a new type</a>
23 <li><a href="#fund_type">Adding a new fundamental type</a></li>
24 <li><a href="#derived_type">Adding a new derived type</a></li>
28 <div class="doc_author">
29 <p>Written by <a href="http://misha.brukman.net">Misha Brukman</a>,
30 Brad Jones, Nate Begeman,
31 and <a href="http://nondot.org/sabre">Chris Lattner</a></p>
34 <!-- *********************************************************************** -->
36 <a name="introduction">Introduction and Warning</a>
38 <!-- *********************************************************************** -->
42 <p>During the course of using LLVM, you may wish to customize it for your
43 research project or for experimentation. At this point, you may realize that
44 you need to add something to LLVM, whether it be a new fundamental type, a new
45 intrinsic function, or a whole new instruction.</p>
47 <p>When you come to this realization, stop and think. Do you really need to
48 extend LLVM? Is it a new fundamental capability that LLVM does not support at
49 its current incarnation or can it be synthesized from already pre-existing LLVM
50 elements? If you are not sure, ask on the <a
51 href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM-dev</a> list. The
52 reason is that extending LLVM will get involved as you need to update all the
53 different passes that you intend to use with your extension, and there are
54 <em>many</em> LLVM analyses and transformations, so it may be quite a bit of
57 <p>Adding an <a href="#intrinsic">intrinsic function</a> is far easier than
58 adding an instruction, and is transparent to optimization passes. If your added
59 functionality can be expressed as a
60 function call, an intrinsic function is the method of choice for LLVM
63 <p>Before you invest a significant amount of effort into a non-trivial
64 extension, <span class="doc_warning">ask on the list</span> if what you are
65 looking to do can be done with already-existing infrastructure, or if maybe
66 someone else is already working on it. You will save yourself a lot of time and
67 effort by doing so.</p>
71 <!-- *********************************************************************** -->
73 <a name="intrinsic">Adding a new intrinsic function</a>
75 <!-- *********************************************************************** -->
79 <p>Adding a new intrinsic function to LLVM is much easier than adding a new
80 instruction. Almost all extensions to LLVM should start as an intrinsic
81 function and then be turned into an instruction if warranted.</p>
84 <li><tt>llvm/docs/LangRef.html</tt>:
85 Document the intrinsic. Decide whether it is code generator specific and
86 what the restrictions are. Talk to other people about it so that you are
87 sure it's a good idea.</li>
89 <li><tt>llvm/include/llvm/Intrinsics*.td</tt>:
90 Add an entry for your intrinsic. Describe its memory access characteristics
91 for optimization (this controls whether it will be DCE'd, CSE'd, etc). Note
92 that any intrinsic using the <tt>llvm_int_ty</tt> type for an argument will
93 be deemed by <tt>tblgen</tt> as overloaded and the corresponding suffix
94 will be required on the intrinsic's name.</li>
96 <li><tt>llvm/lib/Analysis/ConstantFolding.cpp</tt>: If it is possible to
97 constant fold your intrinsic, add support to it in the
98 <tt>canConstantFoldCallTo</tt> and <tt>ConstantFoldCall</tt> functions.</li>
100 <li><tt>llvm/test/Regression/*</tt>: Add test cases for your test cases to the
104 <p>Once the intrinsic has been added to the system, you must add code generator
105 support for it. Generally you must do the following steps:</p>
109 <dt>Add support to the .td file for the target(s) of your choice in
110 <tt>lib/Target/*/*.td</tt>.</dt>
112 <dd>This is usually a matter of adding a pattern to the .td file that matches
113 the intrinsic, though it may obviously require adding the instructions you
114 want to generate as well. There are lots of examples in the PowerPC and X86
115 backend to follow.</dd>
120 <!-- *********************************************************************** -->
122 <a name="sdnode">Adding a new SelectionDAG node</a>
124 <!-- *********************************************************************** -->
128 <p>As with intrinsics, adding a new SelectionDAG node to LLVM is much easier
129 than adding a new instruction. New nodes are often added to help represent
130 instructions common to many targets. These nodes often map to an LLVM
131 instruction (add, sub) or intrinsic (byteswap, population count). In other
132 cases, new nodes have been added to allow many targets to perform a common task
133 (converting between floating point and integer representation) or capture more
134 complicated behavior in a single node (rotate).</p>
137 <li><tt>include/llvm/CodeGen/ISDOpcodes.h</tt>:
138 Add an enum value for the new SelectionDAG node.</li>
139 <li><tt>lib/CodeGen/SelectionDAG/SelectionDAG.cpp</tt>:
140 Add code to print the node to <tt>getOperationName</tt>. If your new node
141 can be evaluated at compile time when given constant arguments (such as an
142 add of a constant with another constant), find the <tt>getNode</tt> method
143 that takes the appropriate number of arguments, and add a case for your node
144 to the switch statement that performs constant folding for nodes that take
145 the same number of arguments as your new node.</li>
146 <li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
147 Add code to <a href="CodeGenerator.html#selectiondag_legalize">legalize,
148 promote, and expand</a> the node as necessary. At a minimum, you will need
149 to add a case statement for your node in <tt>LegalizeOp</tt> which calls
150 LegalizeOp on the node's operands, and returns a new node if any of the
151 operands changed as a result of being legalized. It is likely that not all
152 targets supported by the SelectionDAG framework will natively support the
153 new node. In this case, you must also add code in your node's case
154 statement in <tt>LegalizeOp</tt> to Expand your node into simpler, legal
155 operations. The case for <tt>ISD::UREM</tt> for expanding a remainder into
156 a divide, multiply, and a subtract is a good example.</li>
157 <li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
158 If targets may support the new node being added only at certain sizes, you
159 will also need to add code to your node's case statement in
160 <tt>LegalizeOp</tt> to Promote your node's operands to a larger size, and
161 perform the correct operation. You will also need to add code to
162 <tt>PromoteOp</tt> to do this as well. For a good example, see
164 which promotes its operand to a wider size, performs the byteswap, and then
165 shifts the correct bytes right to emulate the narrower byteswap in the
167 <li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
168 Add a case for your node in <tt>ExpandOp</tt> to teach the legalizer how to
169 perform the action represented by the new node on a value that has been
170 split into high and low halves. This case will be used to support your
171 node with a 64 bit operand on a 32 bit target.</li>
172 <li><tt>lib/CodeGen/SelectionDAG/DAGCombiner.cpp</tt>:
173 If your node can be combined with itself, or other existing nodes in a
174 peephole-like fashion, add a visit function for it, and call that function
175 from <tt></tt>. There are several good examples for simple combines you
176 can do; <tt>visitFABS</tt> and <tt>visitSRL</tt> are good starting places.
178 <li><tt>lib/Target/PowerPC/PPCISelLowering.cpp</tt>:
179 Each target has an implementation of the <tt>TargetLowering</tt> class,
180 usually in its own file (although some targets include it in the same
181 file as the DAGToDAGISel). The default behavior for a target is to
182 assume that your new node is legal for all types that are legal for
183 that target. If this target does not natively support your node, then
184 tell the target to either Promote it (if it is supported at a larger
185 type) or Expand it. This will cause the code you wrote in
186 <tt>LegalizeOp</tt> above to decompose your new node into other legal
187 nodes for this target.</li>
188 <li><tt>lib/Target/TargetSelectionDAG.td</tt>:
189 Most current targets supported by LLVM generate code using the DAGToDAG
190 method, where SelectionDAG nodes are pattern matched to target-specific
191 nodes, which represent individual instructions. In order for the targets
192 to match an instruction to your new node, you must add a def for that node
193 to the list in this file, with the appropriate type constraints. Look at
194 <tt>add</tt>, <tt>bswap</tt>, and <tt>fadd</tt> for examples.</li>
195 <li><tt>lib/Target/PowerPC/PPCInstrInfo.td</tt>:
196 Each target has a tablegen file that describes the target's instruction
197 set. For targets that use the DAGToDAG instruction selection framework,
198 add a pattern for your new node that uses one or more target nodes.
199 Documentation for this is a bit sparse right now, but there are several
200 decent examples. See the patterns for <tt>rotl</tt> in
201 <tt>PPCInstrInfo.td</tt>.</li>
202 <li>TODO: document complex patterns.</li>
203 <li><tt>llvm/test/Regression/CodeGen/*</tt>: Add test cases for your new node
204 to the test suite. <tt>llvm/test/Regression/CodeGen/X86/bswap.ll</tt> is
210 <!-- *********************************************************************** -->
212 <a name="instruction">Adding a new instruction</a>
214 <!-- *********************************************************************** -->
218 <p><span class="doc_warning">WARNING: adding instructions changes the bitcode
219 format, and it will take some effort to maintain compatibility with
220 the previous version.</span> Only add an instruction if it is absolutely
225 <li><tt>llvm/include/llvm/Instruction.def</tt>:
226 add a number for your instruction and an enum name</li>
228 <li><tt>llvm/include/llvm/Instructions.h</tt>:
229 add a definition for the class that will represent your instruction</li>
231 <li><tt>llvm/include/llvm/Support/InstVisitor.h</tt>:
232 add a prototype for a visitor to your new instruction type</li>
234 <li><tt>llvm/lib/AsmParser/Lexer.l</tt>:
235 add a new token to parse your instruction from assembly text file</li>
237 <li><tt>llvm/lib/AsmParser/llvmAsmParser.y</tt>:
238 add the grammar on how your instruction can be read and what it will
239 construct as a result</li>
241 <li><tt>llvm/lib/Bitcode/Reader/Reader.cpp</tt>:
242 add a case for your instruction and how it will be parsed from bitcode</li>
244 <li><tt>llvm/lib/VMCore/Instruction.cpp</tt>:
245 add a case for how your instruction will be printed out to assembly</li>
247 <li><tt>llvm/lib/VMCore/Instructions.cpp</tt>:
248 implement the class you defined in
249 <tt>llvm/include/llvm/Instructions.h</tt></li>
251 <li>Test your instruction</li>
253 <li><tt>llvm/lib/Target/*</tt>:
254 Add support for your instruction to code generators, or add a lowering
257 <li><tt>llvm/test/Regression/*</tt>: add your test cases to the test suite.</li>
261 <p>Also, you need to implement (or modify) any analyses or passes that you want
262 to understand this new instruction.</p>
267 <!-- *********************************************************************** -->
269 <a name="type">Adding a new type</a>
271 <!-- *********************************************************************** -->
275 <p><span class="doc_warning">WARNING: adding new types changes the bitcode
276 format, and will break compatibility with currently-existing LLVM
277 installations.</span> Only add new types if it is absolutely necessary.</p>
279 <!-- ======================================================================= -->
281 <a name="fund_type">Adding a fundamental type</a>
288 <li><tt>llvm/include/llvm/Type.h</tt>:
289 add enum for the new type; add static <tt>Type*</tt> for this type</li>
291 <li><tt>llvm/lib/VMCore/Type.cpp</tt>:
292 add mapping from <tt>TypeID</tt> => <tt>Type*</tt>;
293 initialize the static <tt>Type*</tt></li>
295 <li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
296 add ability to parse in the type from text assembly</li>
298 <li><tt>llvm/lib/AsmReader/llvmAsmParser.y</tt>:
299 add a token for that type</li>
305 <!-- ======================================================================= -->
307 <a name="derived_type">Adding a derived type</a>
313 <li><tt>llvm/include/llvm/Type.h</tt>:
314 add enum for the new type; add a forward declaration of the type
317 <li><tt>llvm/include/llvm/DerivedTypes.h</tt>:
318 add new class to represent new class in the hierarchy; add forward
319 declaration to the TypeMap value type</li>
321 <li><tt>llvm/lib/VMCore/Type.cpp</tt>:
322 add support for derived type to:
323 <div class="doc_code">
325 std::string getTypeDescription(const Type &Ty,
326 std::vector<const Type*> &TypeStack)
327 bool TypesEqual(const Type *Ty, const Type *Ty2,
328 std::map<const Type*, const Type*> & EqTypes)
331 add necessary member functions for type, and factory methods</li>
333 <li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
334 add ability to parse in the type from text assembly</li>
336 <li><tt>llvm/lib/BitCode/Writer/Writer.cpp</tt>:
337 modify <tt>void BitcodeWriter::outputType(const Type *T)</tt> to serialize
340 <li><tt>llvm/lib/BitCode/Reader/Reader.cpp</tt>:
341 modify <tt>const Type *BitcodeReader::ParseType()</tt> to read your data
344 <li><tt>llvm/lib/VMCore/AsmWriter.cpp</tt>:
346 <div class="doc_code">
348 void calcTypeName(const Type *Ty,
349 std::vector<const Type*> &TypeStack,
350 std::map<const Type*,std::string> &TypeNames,
351 std::string & Result)
354 to output the new derived type
364 <!-- *********************************************************************** -->
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