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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>
Misha Brukmana3ce4292004-04-06 03:53:49 +000017 <li><a href="#intrinsic">Adding a new intrinsic function</a></li>
Chris Lattner36365402004-04-09 19:24:20 +000018 <li><a href="#instruction">Adding a new instruction</a></li>
Nate Begeman099d76c2006-01-16 07:54:23 +000019 <li><a href="#sdnode">Adding a new SelectionDAG node</a></li>
Misha Brukmana3ce4292004-04-06 03:53:49 +000020 <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
Chris Lattner7911ce22004-05-23 21:07:27 +000027<div class="doc_author">
Chris Lattner5eb9f0d2005-05-11 03:53:53 +000028 <p>Written by <a href="http://misha.brukman.net">Misha Brukman</a>,
Chris Lattnerb5f6e252006-01-16 16:31:40 +000029 Brad Jones, Nate Begeman,
30 and <a href="http://nondot.org/sabre">Chris Lattner</a></p>
Misha Brukmana3ce4292004-04-06 03:53:49 +000031</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
Chris Lattner35dd5b02006-04-12 17:42:39 +000056<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
Misha Brukmanb3b28272004-04-06 04:17:51 +000059function call, an intrinsic function is the method of choice for LLVM
60extension.</p>
61
Misha Brukmana3ce4292004-04-06 03:53:49 +000062<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
Misha Brukmana3ce4292004-04-06 03:53:49 +000068</div>
69
70<!-- *********************************************************************** -->
71<div class="doc_section">
Chris Lattner36365402004-04-09 19:24:20 +000072 <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
Chris Lattner35dd5b02006-04-12 17:42:39 +000088<li><tt>llvm/include/llvm/Intrinsics*.td</tt>:
89 Add an entry for your intrinsic. Describe its memory access characteristics
Reid Spencerc8d06362007-04-01 07:44:52 +000090 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>
Chris Lattner0190fdb2004-04-10 06:56:53 +000094
Nate Begeman2f86c222006-01-14 01:27:10 +000095<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
Nate Begeman099d76c2006-01-16 07:54:23 +000099<li><tt>llvm/test/Regression/*</tt>: Add test cases for your test cases to the
100 test suite</li>
Chris Lattner36365402004-04-09 19:24:20 +0000101</ol>
102
Chris Lattner5eb9f0d2005-05-11 03:53:53 +0000103<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
Nate Begeman099d76c2006-01-16 07:54:23 +0000109<dd>Depending on the intrinsic, there are a few ways to implement this. For
Bill Wendling068da272007-09-22 10:07:00 +0000110 most intrinsics, it makes sense to add code to lower your intrinsic in
111 <tt>LowerIntrinsicCall</tt> in <tt>lib/CodeGen/IntrinsicLowering.cpp</tt>.
112 Second, if it makes sense to lower the intrinsic to an expanded sequence of
113 C code 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
116 on the compiler compiling the CBE output (see <tt>llvm.prefetch</tt> for an
117 example). Third, if the intrinsic really has no way to be lowered, just
118 have the code generator emit code that prints an error message and calls
119 abort if executed.</dd>
Chris Lattner5eb9f0d2005-05-11 03:53:53 +0000120
Chris Lattner35dd5b02006-04-12 17:42:39 +0000121<dt>Add support to the .td file for the target(s) of your choice in
122 <tt>lib/Target/*/*.td</tt>.</dt>
Chris Lattner5eb9f0d2005-05-11 03:53:53 +0000123
Chris Lattner35dd5b02006-04-12 17:42:39 +0000124<dd>This is usually a matter of adding a pattern to the .td file that matches
125 the intrinsic, though it may obviously require adding the instructions you
126 want to generate as well. There are lots of examples in the PowerPC and X86
127 backend to follow.</dd>
Bill Wendling068da272007-09-22 10:07:00 +0000128</dl>
Chris Lattner5eb9f0d2005-05-11 03:53:53 +0000129
Nate Begeman099d76c2006-01-16 07:54:23 +0000130</div>
Chris Lattner5eb9f0d2005-05-11 03:53:53 +0000131
Nate Begeman099d76c2006-01-16 07:54:23 +0000132<!-- *********************************************************************** -->
133<div class="doc_section">
134 <a name="sdnode">Adding a new SelectionDAG node</a>
135</div>
136<!-- *********************************************************************** -->
Chris Lattner5eb9f0d2005-05-11 03:53:53 +0000137
Nate Begeman099d76c2006-01-16 07:54:23 +0000138<div class="doc_text">
Chris Lattner5eb9f0d2005-05-11 03:53:53 +0000139
Nate Begeman099d76c2006-01-16 07:54:23 +0000140<p>As with intrinsics, adding a new SelectionDAG node to LLVM is much easier
141than adding a new instruction. New nodes are often added to help represent
142instructions common to many targets. These nodes often map to an LLVM
143instruction (add, sub) or intrinsic (byteswap, population count). In other
144cases, new nodes have been added to allow many targets to perform a common task
145(converting between floating point and integer representation) or capture more
146complicated behavior in a single node (rotate).</p>
Chris Lattner5eb9f0d2005-05-11 03:53:53 +0000147
Nate Begeman099d76c2006-01-16 07:54:23 +0000148<ol>
149<li><tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt>:
150 Add an enum value for the new SelectionDAG node.</li>
151<li><tt>lib/CodeGen/SelectionDAG/SelectionDAG.cpp</tt>:
152 Add code to print the node to <tt>getOperationName</tt>. If your new node
153 can be evaluated at compile time when given constant arguments (such as an
154 add of a constant with another constant), find the <tt>getNode</tt> method
155 that takes the appropriate number of arguments, and add a case for your node
156 to the switch statement that performs constant folding for nodes that take
157 the same number of arguments as your new node.</li>
158<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
159 Add code to <a href="CodeGenerator.html#selectiondag_legalize">legalize,
160 promote, and expand</a> the node as necessary. At a minimum, you will need
161 to add a case statement for your node in <tt>LegalizeOp</tt> which calls
162 LegalizeOp on the node's operands, and returns a new node if any of the
163 operands changed as a result of being legalized. It is likely that not all
164 targets supported by the SelectionDAG framework will natively support the
165 new node. In this case, you must also add code in your node's case
166 statement in <tt>LegalizeOp</tt> to Expand your node into simpler, legal
Chris Lattnerb5f6e252006-01-16 16:31:40 +0000167 operations. The case for <tt>ISD::UREM</tt> for expanding a remainder into
168 a divide, multiply, and a subtract is a good example.</li>
Nate Begeman099d76c2006-01-16 07:54:23 +0000169<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
170 If targets may support the new node being added only at certain sizes, you
171 will also need to add code to your node's case statement in
172 <tt>LegalizeOp</tt> to Promote your node's operands to a larger size, and
173 perform the correct operation. You will also need to add code to
Chris Lattnerb5f6e252006-01-16 16:31:40 +0000174 <tt>PromoteOp</tt> to do this as well. For a good example, see
175 <tt>ISD::BSWAP</tt>,
Nate Begeman099d76c2006-01-16 07:54:23 +0000176 which promotes its operand to a wider size, performs the byteswap, and then
177 shifts the correct bytes right to emulate the narrower byteswap in the
178 wider type.</li>
179<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
180 Add a case for your node in <tt>ExpandOp</tt> to teach the legalizer how to
181 perform the action represented by the new node on a value that has been
182 split into high and low halves. This case will be used to support your
183 node with a 64 bit operand on a 32 bit target.</li>
184<li><tt>lib/CodeGen/SelectionDAG/DAGCombiner.cpp</tt>:
185 If your node can be combined with itself, or other existing nodes in a
186 peephole-like fashion, add a visit function for it, and call that function
187 from <tt></tt>. There are several good examples for simple combines you
188 can do; <tt>visitFABS</tt> and <tt>visitSRL</tt> are good starting places.
189 </li>
190<li><tt>lib/Target/PowerPC/PPCISelLowering.cpp</tt>:
191 Each target has an implementation of the <tt>TargetLowering</tt> class,
192 usually in its own file (although some targets include it in the same
193 file as the DAGToDAGISel). The default behavior for a target is to
194 assume that your new node is legal for all types that are legal for
195 that target. If this target does not natively support your node, then
196 tell the target to either Promote it (if it is supported at a larger
197 type) or Expand it. This will cause the code you wrote in
198 <tt>LegalizeOp</tt> above to decompose your new node into other legal
199 nodes for this target.</li>
200<li><tt>lib/Target/TargetSelectionDAG.td</tt>:
201 Most current targets supported by LLVM generate code using the DAGToDAG
202 method, where SelectionDAG nodes are pattern matched to target-specific
203 nodes, which represent individual instructions. In order for the targets
204 to match an instruction to your new node, you must add a def for that node
205 to the list in this file, with the appropriate type constraints. Look at
206 <tt>add</tt>, <tt>bswap</tt>, and <tt>fadd</tt> for examples.</li>
207<li><tt>lib/Target/PowerPC/PPCInstrInfo.td</tt>:
208 Each target has a tablegen file that describes the target's instruction
209 set. For targets that use the DAGToDAG instruction selection framework,
210 add a pattern for your new node that uses one or more target nodes.
211 Documentation for this is a bit sparse right now, but there are several
212 decent examples. See the patterns for <tt>rotl</tt> in
213 <tt>PPCInstrInfo.td</tt>.</li>
214<li>TODO: document complex patterns.</li>
215<li><tt>llvm/test/Regression/CodeGen/*</tt>: Add test cases for your new node
216 to the test suite. <tt>llvm/test/Regression/CodeGen/X86/bswap.ll</tt> is
217 a good example.</li>
218</ol>
Chris Lattner36365402004-04-09 19:24:20 +0000219
220</div>
221
222<!-- *********************************************************************** -->
223<div class="doc_section">
Misha Brukmana3ce4292004-04-06 03:53:49 +0000224 <a name="instruction">Adding a new instruction</a>
225</div>
226<!-- *********************************************************************** -->
227
228<div class="doc_text">
229
Gabor Greif04367bf2007-07-06 22:07:22 +0000230<p><span class="doc_warning">WARNING: adding instructions changes the bitcode
Misha Brukmanb3b28272004-04-06 04:17:51 +0000231format, and it will take some effort to maintain compatibility with
232the previous version.</span> Only add an instruction if it is absolutely
Misha Brukmana3ce4292004-04-06 03:53:49 +0000233necessary.</p>
234
235<ol>
Misha Brukmanb3b28272004-04-06 04:17:51 +0000236
Misha Brukmana3ce4292004-04-06 03:53:49 +0000237<li><tt>llvm/include/llvm/Instruction.def</tt>:
238 add a number for your instruction and an enum name</li>
239
Misha Brukman47b14a42004-07-29 17:30:56 +0000240<li><tt>llvm/include/llvm/Instructions.h</tt>:
Misha Brukmana3ce4292004-04-06 03:53:49 +0000241 add a definition for the class that will represent your instruction</li>
242
243<li><tt>llvm/include/llvm/Support/InstVisitor.h</tt>:
244 add a prototype for a visitor to your new instruction type</li>
245
246<li><tt>llvm/lib/AsmParser/Lexer.l</tt>:
247 add a new token to parse your instruction from assembly text file</li>
248
249<li><tt>llvm/lib/AsmParser/llvmAsmParser.y</tt>:
250 add the grammar on how your instruction can be read and what it will
251 construct as a result</li>
252
Gabor Greif04367bf2007-07-06 22:07:22 +0000253<li><tt>llvm/lib/Bitcode/Reader/Reader.cpp</tt>:
254 add a case for your instruction and how it will be parsed from bitcode</li>
Misha Brukmana3ce4292004-04-06 03:53:49 +0000255
256<li><tt>llvm/lib/VMCore/Instruction.cpp</tt>:
257 add a case for how your instruction will be printed out to assembly</li>
258
Chris Lattner8f363212004-07-29 17:31:57 +0000259<li><tt>llvm/lib/VMCore/Instructions.cpp</tt>:
Misha Brukmane39cd632004-09-28 16:58:12 +0000260 implement the class you defined in
261 <tt>llvm/include/llvm/Instructions.h</tt></li>
Misha Brukmana3ce4292004-04-06 03:53:49 +0000262
Misha Brukmana4242282004-12-01 20:58:54 +0000263<li>Test your instruction</li>
264
265<li><tt>llvm/lib/Target/*</tt>:
266 Add support for your instruction to code generators, or add a lowering
267 pass.</li>
268
269<li><tt>llvm/test/Regression/*</tt>: add your test cases to the test suite.</li>
270
Misha Brukmana3ce4292004-04-06 03:53:49 +0000271</ol>
272
273<p>Also, you need to implement (or modify) any analyses or passes that you want
274to understand this new instruction.</p>
275
276</div>
277
Misha Brukmana3ce4292004-04-06 03:53:49 +0000278
279<!-- *********************************************************************** -->
280<div class="doc_section">
281 <a name="type">Adding a new type</a>
282</div>
283<!-- *********************************************************************** -->
284
285<div class="doc_text">
286
Gabor Greif04367bf2007-07-06 22:07:22 +0000287<p><span class="doc_warning">WARNING: adding new types changes the bitcode
Misha Brukmana3ce4292004-04-06 03:53:49 +0000288format, and will break compatibility with currently-existing LLVM
289installations.</span> Only add new types if it is absolutely necessary.</p>
290
291</div>
292
293<!-- ======================================================================= -->
294<div class="doc_subsection">
295 <a name="fund_type">Adding a fundamental type</a>
296</div>
297
298<div class="doc_text">
299
300<ol>
301
Misha Brukmana3ce4292004-04-06 03:53:49 +0000302<li><tt>llvm/include/llvm/Type.h</tt>:
Chris Lattner55f95012005-04-23 21:59:11 +0000303 add enum for the new type; add static <tt>Type*</tt> for this type</li>
Misha Brukmana3ce4292004-04-06 03:53:49 +0000304
305<li><tt>llvm/lib/VMCore/Type.cpp</tt>:
306 add mapping from <tt>TypeID</tt> =&gt; <tt>Type*</tt>;
307 initialize the static <tt>Type*</tt></li>
308
309<li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
310 add ability to parse in the type from text assembly</li>
311
312<li><tt>llvm/lib/AsmReader/llvmAsmParser.y</tt>:
313 add a token for that type</li>
314
315</ol>
316
317</div>
318
319<!-- ======================================================================= -->
320<div class="doc_subsection">
321 <a name="derived_type">Adding a derived type</a>
322</div>
323
324<div class="doc_text">
325
Chris Lattner8dad40c2004-08-12 19:06:24 +0000326<ol>
Chris Lattner8dad40c2004-08-12 19:06:24 +0000327<li><tt>llvm/include/llvm/Type.h</tt>:
Chris Lattner55f95012005-04-23 21:59:11 +0000328 add enum for the new type; add a forward declaration of the type
Misha Brukman7cc8a892004-08-12 19:58:43 +0000329 also</li>
Chris Lattner8dad40c2004-08-12 19:06:24 +0000330
Chris Lattner47746aa2005-11-13 02:09:55 +0000331<li><tt>llvm/include/llvm/DerivedTypes.h</tt>:
Chris Lattner8dad40c2004-08-12 19:06:24 +0000332 add new class to represent new class in the hierarchy; add forward
333 declaration to the TypeMap value type</li>
334
335<li><tt>llvm/lib/VMCore/Type.cpp</tt>:
336 add support for derived type to:
Misha Brukman7cc8a892004-08-12 19:58:43 +0000337<div class="doc_code">
338<pre>
339std::string getTypeDescription(const Type &amp;Ty,
340 std::vector&lt;const Type*&gt; &amp;TypeStack)
341bool TypesEqual(const Type *Ty, const Type *Ty2,
342 std::map&lt;const Type*, const Type*&gt; &amp; EqTypes)
343</pre>
344</div>
345 add necessary member functions for type, and factory methods</li>
Chris Lattner8dad40c2004-08-12 19:06:24 +0000346
347<li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
348 add ability to parse in the type from text assembly</li>
349
Gabor Greif04367bf2007-07-06 22:07:22 +0000350<li><tt>llvm/lib/BitCode/Writer/Writer.cpp</tt>:
351 modify <tt>void BitcodeWriter::outputType(const Type *T)</tt> to serialize
Misha Brukman7cc8a892004-08-12 19:58:43 +0000352 your type</li>
Chris Lattner8dad40c2004-08-12 19:06:24 +0000353
Gabor Greif04367bf2007-07-06 22:07:22 +0000354<li><tt>llvm/lib/BitCode/Reader/Reader.cpp</tt>:
355 modify <tt>const Type *BitcodeReader::ParseType()</tt> to read your data
Misha Brukman7cc8a892004-08-12 19:58:43 +0000356 type</li>
Chris Lattner8dad40c2004-08-12 19:06:24 +0000357
358<li><tt>llvm/lib/VMCore/AsmWriter.cpp</tt>:
Misha Brukman7cc8a892004-08-12 19:58:43 +0000359 modify
360<div class="doc_code">
361<pre>
362void calcTypeName(const Type *Ty,
363 std::vector&lt;const Type*&gt; &amp;TypeStack,
364 std::map&lt;const Type*,std::string&gt; &amp;TypeNames,
365 std::string &amp; Result)
366</pre>
367</div>
Chris Lattner8dad40c2004-08-12 19:06:24 +0000368 to output the new derived type
369</li>
370
371
372</ol>
Misha Brukmana3ce4292004-04-06 03:53:49 +0000373
374</div>
375
376<!-- *********************************************************************** -->
377
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