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5 <title>Extending LLVM: Adding instructions, intrinsics, types, etc.</title>
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10
11<div class="doc_title">
12 Extending LLVM: Adding instructions, intrinsics, types, etc.
13</div>
14
15<ol>
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="#sdnode">Adding a new SelectionDAG node</a></li>
20 <li><a href="#type">Adding a new type</a>
21 <ol>
22 <li><a href="#fund_type">Adding a new fundamental type</a></li>
23 <li><a href="#derived_type">Adding a new derived type</a></li>
24 </ol></li>
25</ol>
26
27<div class="doc_author">
28 <p>Written by <a href="http://misha.brukman.net">Misha Brukman</a>,
29 Brad Jones, Nate Begeman,
30 and <a href="http://nondot.org/sabre">Chris Lattner</a></p>
31</div>
32
33<!-- *********************************************************************** -->
34<div class="doc_section">
35 <a name="introduction">Introduction and Warning</a>
36</div>
37<!-- *********************************************************************** -->
38
39<div class="doc_text">
40
41<p>During the course of using LLVM, you may wish to customize it for your
42research project or for experimentation. At this point, you may realize that
43you need to add something to LLVM, whether it be a new fundamental type, a new
44intrinsic function, or a whole new instruction.</p>
45
46<p>When you come to this realization, stop and think. Do you really need to
47extend LLVM? Is it a new fundamental capability that LLVM does not support at
48its current incarnation or can it be synthesized from already pre-existing LLVM
49elements? If you are not sure, ask on the <a
50href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM-dev</a> list. The
51reason is that extending LLVM will get involved as you need to update all the
52different passes that you intend to use with your extension, and there are
53<em>many</em> LLVM analyses and transformations, so it may be quite a bit of
54work.</p>
55
56<p>Adding an <a href="#intrinsic">intrinsic function</a> is far easier than
57adding an instruction, and is transparent to optimization passes. If your added
58functionality can be expressed as a
59function call, an intrinsic function is the method of choice for LLVM
60extension.</p>
61
62<p>Before you invest a significant amount of effort into a non-trivial
63extension, <span class="doc_warning">ask on the list</span> if what you are
64looking to do can be done with already-existing infrastructure, or if maybe
65someone else is already working on it. You will save yourself a lot of time and
66effort by doing so.</p>
67
68</div>
69
70<!-- *********************************************************************** -->
71<div class="doc_section">
72 <a name="intrinsic">Adding a new intrinsic function</a>
73</div>
74<!-- *********************************************************************** -->
75
76<div class="doc_text">
77
78<p>Adding a new intrinsic function to LLVM is much easier than adding a new
79instruction. Almost all extensions to LLVM should start as an intrinsic
80function and then be turned into an instruction if warranted.</p>
81
82<ol>
83<li><tt>llvm/docs/LangRef.html</tt>:
84 Document the intrinsic. Decide whether it is code generator specific and
85 what the restrictions are. Talk to other people about it so that you are
86 sure it's a good idea.</li>
87
88<li><tt>llvm/include/llvm/Intrinsics*.td</tt>:
89 Add an entry for your intrinsic. Describe its memory access characteristics
90 for optimization (this controls whether it will be DCE'd, CSE'd, etc). Note
91 that any intrinsic using the <tt>llvm_int_ty</tt> type for an argument will
92 be deemed by <tt>tblgen</tt> as overloaded and the corresponding suffix
93 will be required on the intrinsic's name.</li>
94
95<li><tt>llvm/lib/Analysis/ConstantFolding.cpp</tt>: If it is possible to
96 constant fold your intrinsic, add support to it in the
97 <tt>canConstantFoldCallTo</tt> and <tt>ConstantFoldCall</tt> functions.</li>
98
99<li><tt>llvm/test/Regression/*</tt>: Add test cases for your test cases to the
100 test suite</li>
101</ol>
102
103<p>Once the intrinsic has been added to the system, you must add code generator
104support for it. Generally you must do the following steps:</p>
105
106<dl>
107<dt>Add support to the C backend in <tt>lib/Target/CBackend/</tt></dt>
108
109<dd>Depending on the intrinsic, there are a few ways to implement this. For
110most intrinsics, it makes sense to add code to lower your intrinsic in
111<tt>LowerIntrinsicCall</tt> in <tt>lib/CodeGen/IntrinsicLowering.cpp</tt>.
112Second, if it makes sense to lower the intrinsic to an expanded sequence of C
113code in all cases, just emit the expansion in <tt>visitCallInst</tt> in
114<tt>Writer.cpp</tt>. If the intrinsic has some way to express it with GCC
115(or any other compiler) extensions, it can be conditionally supported based on
116the compiler compiling the CBE output (see <tt>llvm.prefetch</tt> for an
117example).
118Third, if the intrinsic really has no way to be lowered, just have the code
119generator emit code that prints an error message and calls abort if executed.
120</dd>
121
122<dl>
123<dt>Add support to the .td file for the target(s) of your choice in
124 <tt>lib/Target/*/*.td</tt>.</dt>
125
126<dd>This is usually a matter of adding a pattern to the .td file that matches
127 the intrinsic, though it may obviously require adding the instructions you
128 want to generate as well. There are lots of examples in the PowerPC and X86
129 backend to follow.</dd>
130
131</div>
132
133<!-- *********************************************************************** -->
134<div class="doc_section">
135 <a name="sdnode">Adding a new SelectionDAG node</a>
136</div>
137<!-- *********************************************************************** -->
138
139<div class="doc_text">
140
141<p>As with intrinsics, adding a new SelectionDAG node to LLVM is much easier
142than adding a new instruction. New nodes are often added to help represent
143instructions common to many targets. These nodes often map to an LLVM
144instruction (add, sub) or intrinsic (byteswap, population count). In other
145cases, new nodes have been added to allow many targets to perform a common task
146(converting between floating point and integer representation) or capture more
147complicated behavior in a single node (rotate).</p>
148
149<ol>
150<li><tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt>:
151 Add an enum value for the new SelectionDAG node.</li>
152<li><tt>lib/CodeGen/SelectionDAG/SelectionDAG.cpp</tt>:
153 Add code to print the node to <tt>getOperationName</tt>. If your new node
154 can be evaluated at compile time when given constant arguments (such as an
155 add of a constant with another constant), find the <tt>getNode</tt> method
156 that takes the appropriate number of arguments, and add a case for your node
157 to the switch statement that performs constant folding for nodes that take
158 the same number of arguments as your new node.</li>
159<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
160 Add code to <a href="CodeGenerator.html#selectiondag_legalize">legalize,
161 promote, and expand</a> the node as necessary. At a minimum, you will need
162 to add a case statement for your node in <tt>LegalizeOp</tt> which calls
163 LegalizeOp on the node's operands, and returns a new node if any of the
164 operands changed as a result of being legalized. It is likely that not all
165 targets supported by the SelectionDAG framework will natively support the
166 new node. In this case, you must also add code in your node's case
167 statement in <tt>LegalizeOp</tt> to Expand your node into simpler, legal
168 operations. The case for <tt>ISD::UREM</tt> for expanding a remainder into
169 a divide, multiply, and a subtract is a good example.</li>
170<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
171 If targets may support the new node being added only at certain sizes, you
172 will also need to add code to your node's case statement in
173 <tt>LegalizeOp</tt> to Promote your node's operands to a larger size, and
174 perform the correct operation. You will also need to add code to
175 <tt>PromoteOp</tt> to do this as well. For a good example, see
176 <tt>ISD::BSWAP</tt>,
177 which promotes its operand to a wider size, performs the byteswap, and then
178 shifts the correct bytes right to emulate the narrower byteswap in the
179 wider type.</li>
180<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
181 Add a case for your node in <tt>ExpandOp</tt> to teach the legalizer how to
182 perform the action represented by the new node on a value that has been
183 split into high and low halves. This case will be used to support your
184 node with a 64 bit operand on a 32 bit target.</li>
185<li><tt>lib/CodeGen/SelectionDAG/DAGCombiner.cpp</tt>:
186 If your node can be combined with itself, or other existing nodes in a
187 peephole-like fashion, add a visit function for it, and call that function
188 from <tt></tt>. There are several good examples for simple combines you
189 can do; <tt>visitFABS</tt> and <tt>visitSRL</tt> are good starting places.
190 </li>
191<li><tt>lib/Target/PowerPC/PPCISelLowering.cpp</tt>:
192 Each target has an implementation of the <tt>TargetLowering</tt> class,
193 usually in its own file (although some targets include it in the same
194 file as the DAGToDAGISel). The default behavior for a target is to
195 assume that your new node is legal for all types that are legal for
196 that target. If this target does not natively support your node, then
197 tell the target to either Promote it (if it is supported at a larger
198 type) or Expand it. This will cause the code you wrote in
199 <tt>LegalizeOp</tt> above to decompose your new node into other legal
200 nodes for this target.</li>
201<li><tt>lib/Target/TargetSelectionDAG.td</tt>:
202 Most current targets supported by LLVM generate code using the DAGToDAG
203 method, where SelectionDAG nodes are pattern matched to target-specific
204 nodes, which represent individual instructions. In order for the targets
205 to match an instruction to your new node, you must add a def for that node
206 to the list in this file, with the appropriate type constraints. Look at
207 <tt>add</tt>, <tt>bswap</tt>, and <tt>fadd</tt> for examples.</li>
208<li><tt>lib/Target/PowerPC/PPCInstrInfo.td</tt>:
209 Each target has a tablegen file that describes the target's instruction
210 set. For targets that use the DAGToDAG instruction selection framework,
211 add a pattern for your new node that uses one or more target nodes.
212 Documentation for this is a bit sparse right now, but there are several
213 decent examples. See the patterns for <tt>rotl</tt> in
214 <tt>PPCInstrInfo.td</tt>.</li>
215<li>TODO: document complex patterns.</li>
216<li><tt>llvm/test/Regression/CodeGen/*</tt>: Add test cases for your new node
217 to the test suite. <tt>llvm/test/Regression/CodeGen/X86/bswap.ll</tt> is
218 a good example.</li>
219</ol>
220
221</div>
222
223<!-- *********************************************************************** -->
224<div class="doc_section">
225 <a name="instruction">Adding a new instruction</a>
226</div>
227<!-- *********************************************************************** -->
228
229<div class="doc_text">
230
231<p><span class="doc_warning">WARNING: adding instructions changes the bitcode
232format, and it will take some effort to maintain compatibility with
233the previous version.</span> Only add an instruction if it is absolutely
234necessary.</p>
235
236<ol>
237
238<li><tt>llvm/include/llvm/Instruction.def</tt>:
239 add a number for your instruction and an enum name</li>
240
241<li><tt>llvm/include/llvm/Instructions.h</tt>:
242 add a definition for the class that will represent your instruction</li>
243
244<li><tt>llvm/include/llvm/Support/InstVisitor.h</tt>:
245 add a prototype for a visitor to your new instruction type</li>
246
247<li><tt>llvm/lib/AsmParser/Lexer.l</tt>:
248 add a new token to parse your instruction from assembly text file</li>
249
250<li><tt>llvm/lib/AsmParser/llvmAsmParser.y</tt>:
251 add the grammar on how your instruction can be read and what it will
252 construct as a result</li>
253
254<li><tt>llvm/lib/Bitcode/Reader/Reader.cpp</tt>:
255 add a case for your instruction and how it will be parsed from bitcode</li>
256
257<li><tt>llvm/lib/VMCore/Instruction.cpp</tt>:
258 add a case for how your instruction will be printed out to assembly</li>
259
260<li><tt>llvm/lib/VMCore/Instructions.cpp</tt>:
261 implement the class you defined in
262 <tt>llvm/include/llvm/Instructions.h</tt></li>
263
264<li>Test your instruction</li>
265
266<li><tt>llvm/lib/Target/*</tt>:
267 Add support for your instruction to code generators, or add a lowering
268 pass.</li>
269
270<li><tt>llvm/test/Regression/*</tt>: add your test cases to the test suite.</li>
271
272</ol>
273
274<p>Also, you need to implement (or modify) any analyses or passes that you want
275to understand this new instruction.</p>
276
277</div>
278
279
280<!-- *********************************************************************** -->
281<div class="doc_section">
282 <a name="type">Adding a new type</a>
283</div>
284<!-- *********************************************************************** -->
285
286<div class="doc_text">
287
288<p><span class="doc_warning">WARNING: adding new types changes the bitcode
289format, and will break compatibility with currently-existing LLVM
290installations.</span> Only add new types if it is absolutely necessary.</p>
291
292</div>
293
294<!-- ======================================================================= -->
295<div class="doc_subsection">
296 <a name="fund_type">Adding a fundamental type</a>
297</div>
298
299<div class="doc_text">
300
301<ol>
302
303<li><tt>llvm/include/llvm/Type.h</tt>:
304 add enum for the new type; add static <tt>Type*</tt> for this type</li>
305
306<li><tt>llvm/lib/VMCore/Type.cpp</tt>:
307 add mapping from <tt>TypeID</tt> =&gt; <tt>Type*</tt>;
308 initialize the static <tt>Type*</tt></li>
309
310<li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
311 add ability to parse in the type from text assembly</li>
312
313<li><tt>llvm/lib/AsmReader/llvmAsmParser.y</tt>:
314 add a token for that type</li>
315
316</ol>
317
318</div>
319
320<!-- ======================================================================= -->
321<div class="doc_subsection">
322 <a name="derived_type">Adding a derived type</a>
323</div>
324
325<div class="doc_text">
326
327<ol>
328<li><tt>llvm/include/llvm/Type.h</tt>:
329 add enum for the new type; add a forward declaration of the type
330 also</li>
331
332<li><tt>llvm/include/llvm/DerivedTypes.h</tt>:
333 add new class to represent new class in the hierarchy; add forward
334 declaration to the TypeMap value type</li>
335
336<li><tt>llvm/lib/VMCore/Type.cpp</tt>:
337 add support for derived type to:
338<div class="doc_code">
339<pre>
340std::string getTypeDescription(const Type &amp;Ty,
341 std::vector&lt;const Type*&gt; &amp;TypeStack)
342bool TypesEqual(const Type *Ty, const Type *Ty2,
343 std::map&lt;const Type*, const Type*&gt; &amp; EqTypes)
344</pre>
345</div>
346 add necessary member functions for type, and factory methods</li>
347
348<li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
349 add ability to parse in the type from text assembly</li>
350
351<li><tt>llvm/lib/BitCode/Writer/Writer.cpp</tt>:
352 modify <tt>void BitcodeWriter::outputType(const Type *T)</tt> to serialize
353 your type</li>
354
355<li><tt>llvm/lib/BitCode/Reader/Reader.cpp</tt>:
356 modify <tt>const Type *BitcodeReader::ParseType()</tt> to read your data
357 type</li>
358
359<li><tt>llvm/lib/VMCore/AsmWriter.cpp</tt>:
360 modify
361<div class="doc_code">
362<pre>
363void calcTypeName(const Type *Ty,
364 std::vector&lt;const Type*&gt; &amp;TypeStack,
365 std::map&lt;const Type*,std::string&gt; &amp;TypeNames,
366 std::string &amp; Result)
367</pre>
368</div>
369 to output the new derived type
370</li>
371
372
373</ol>
374
375</div>
376
377<!-- *********************************************************************** -->
378
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