1 ============================================================ 2 Extending LLVM: Adding instructions, intrinsics, types, etc. 3 ============================================================ 4 5 Introduction and Warning 6 ======================== 7 8 9 During the course of using LLVM, you may wish to customize it for your research 10 project or for experimentation. At this point, you may realize that you need to 11 add something to LLVM, whether it be a new fundamental type, a new intrinsic 12 function, or a whole new instruction. 13 14 When you come to this realization, stop and think. Do you really need to extend 15 LLVM? Is it a new fundamental capability that LLVM does not support at its 16 current incarnation or can it be synthesized from already pre-existing LLVM 17 elements? If you are not sure, ask on the `LLVM-dev 18 <http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev>`_ list. The reason is that 19 extending LLVM will get involved as you need to update all the different passes 20 that you intend to use with your extension, and there are ``many`` LLVM analyses 21 and transformations, so it may be quite a bit of work. 22 23 Adding an `intrinsic function`_ is far easier than adding an 24 instruction, and is transparent to optimization passes. If your added 25 functionality can be expressed as a function call, an intrinsic function is the 26 method of choice for LLVM extension. 27 28 Before you invest a significant amount of effort into a non-trivial extension, 29 **ask on the list** if what you are looking to do can be done with 30 already-existing infrastructure, or if maybe someone else is already working on 31 it. You will save yourself a lot of time and effort by doing so. 32 33 .. _intrinsic function: 34 35 Adding a new intrinsic function 36 =============================== 37 38 Adding a new intrinsic function to LLVM is much easier than adding a new 39 instruction. Almost all extensions to LLVM should start as an intrinsic 40 function and then be turned into an instruction if warranted. 41 42 #. ``llvm/docs/LangRef.html``: 43 44 Document the intrinsic. Decide whether it is code generator specific and 45 what the restrictions are. Talk to other people about it so that you are 46 sure it's a good idea. 47 48 #. ``llvm/include/llvm/Intrinsics*.td``: 49 50 Add an entry for your intrinsic. Describe its memory access characteristics 51 for optimization (this controls whether it will be DCE'd, CSE'd, etc). Note 52 that any intrinsic using the ``llvm_int_ty`` type for an argument will 53 be deemed by ``tblgen`` as overloaded and the corresponding suffix will 54 be required on the intrinsic's name. 55 56 #. ``llvm/lib/Analysis/ConstantFolding.cpp``: 57 58 If it is possible to constant fold your intrinsic, add support to it in the 59 ``canConstantFoldCallTo`` and ``ConstantFoldCall`` functions. 60 61 #. ``llvm/test/Regression/*``: 62 63 Add test cases for your test cases to the test suite 64 65 Once the intrinsic has been added to the system, you must add code generator 66 support for it. Generally you must do the following steps: 67 68 Add support to the .td file for the target(s) of your choice in 69 ``lib/Target/*/*.td``. 70 71 This is usually a matter of adding a pattern to the .td file that matches the 72 intrinsic, though it may obviously require adding the instructions you want to 73 generate as well. There are lots of examples in the PowerPC and X86 backend 74 to follow. 75 76 Adding a new SelectionDAG node 77 ============================== 78 79 As with intrinsics, adding a new SelectionDAG node to LLVM is much easier than 80 adding a new instruction. New nodes are often added to help represent 81 instructions common to many targets. These nodes often map to an LLVM 82 instruction (add, sub) or intrinsic (byteswap, population count). In other 83 cases, new nodes have been added to allow many targets to perform a common task 84 (converting between floating point and integer representation) or capture more 85 complicated behavior in a single node (rotate). 86 87 #. ``include/llvm/CodeGen/ISDOpcodes.h``: 88 89 Add an enum value for the new SelectionDAG node. 90 91 #. ``lib/CodeGen/SelectionDAG/SelectionDAG.cpp``: 92 93 Add code to print the node to ``getOperationName``. If your new node can be 94 evaluated at compile time when given constant arguments (such as an add of a 95 constant with another constant), find the ``getNode`` method that takes the 96 appropriate number of arguments, and add a case for your node to the switch 97 statement that performs constant folding for nodes that take the same number 98 of arguments as your new node. 99 100 #. ``lib/CodeGen/SelectionDAG/LegalizeDAG.cpp``: 101 102 Add code to `legalize, promote, and expand 103 <CodeGenerator.html#selectiondag_legalize>`_ the node as necessary. At a 104 minimum, you will need to add a case statement for your node in 105 ``LegalizeOp`` which calls LegalizeOp on the node's operands, and returns a 106 new node if any of the operands changed as a result of being legalized. It 107 is likely that not all targets supported by the SelectionDAG framework will 108 natively support the new node. In this case, you must also add code in your 109 node's case statement in ``LegalizeOp`` to Expand your node into simpler, 110 legal operations. The case for ``ISD::UREM`` for expanding a remainder into 111 a divide, multiply, and a subtract is a good example. 112 113 #. ``lib/CodeGen/SelectionDAG/LegalizeDAG.cpp``: 114 115 If targets may support the new node being added only at certain sizes, you 116 will also need to add code to your node's case statement in ``LegalizeOp`` 117 to Promote your node's operands to a larger size, and perform the correct 118 operation. You will also need to add code to ``PromoteOp`` to do this as 119 well. For a good example, see ``ISD::BSWAP``, which promotes its operand to 120 a wider size, performs the byteswap, and then shifts the correct bytes right 121 to emulate the narrower byteswap in the wider type. 122 123 #. ``lib/CodeGen/SelectionDAG/LegalizeDAG.cpp``: 124 125 Add a case for your node in ``ExpandOp`` to teach the legalizer how to 126 perform the action represented by the new node on a value that has been split 127 into high and low halves. This case will be used to support your node with a 128 64 bit operand on a 32 bit target. 129 130 #. ``lib/CodeGen/SelectionDAG/DAGCombiner.cpp``: 131 132 If your node can be combined with itself, or other existing nodes in a 133 peephole-like fashion, add a visit function for it, and call that function 134 from. There are several good examples for simple combines you can do; 135 ``visitFABS`` and ``visitSRL`` are good starting places. 136 137 #. ``lib/Target/PowerPC/PPCISelLowering.cpp``: 138 139 Each target has an implementation of the ``TargetLowering`` class, usually in 140 its own file (although some targets include it in the same file as the 141 DAGToDAGISel). The default behavior for a target is to assume that your new 142 node is legal for all types that are legal for that target. If this target 143 does not natively support your node, then tell the target to either Promote 144 it (if it is supported at a larger type) or Expand it. This will cause the 145 code you wrote in ``LegalizeOp`` above to decompose your new node into other 146 legal nodes for this target. 147 148 #. ``lib/Target/TargetSelectionDAG.td``: 149 150 Most current targets supported by LLVM generate code using the DAGToDAG 151 method, where SelectionDAG nodes are pattern matched to target-specific 152 nodes, which represent individual instructions. In order for the targets to 153 match an instruction to your new node, you must add a def for that node to 154 the list in this file, with the appropriate type constraints. Look at 155 ``add``, ``bswap``, and ``fadd`` for examples. 156 157 #. ``lib/Target/PowerPC/PPCInstrInfo.td``: 158 159 Each target has a tablegen file that describes the target's instruction set. 160 For targets that use the DAGToDAG instruction selection framework, add a 161 pattern for your new node that uses one or more target nodes. Documentation 162 for this is a bit sparse right now, but there are several decent examples. 163 See the patterns for ``rotl`` in ``PPCInstrInfo.td``. 164 165 #. TODO: document complex patterns. 166 167 #. ``llvm/test/Regression/CodeGen/*``: 168 169 Add test cases for your new node to the test suite. 170 ``llvm/test/Regression/CodeGen/X86/bswap.ll`` is a good example. 171 172 Adding a new instruction 173 ======================== 174 175 .. warning:: 176 177 Adding instructions changes the bitcode format, and it will take some effort 178 to maintain compatibility with the previous version. Only add an instruction 179 if it is absolutely necessary. 180 181 #. ``llvm/include/llvm/Instruction.def``: 182 183 add a number for your instruction and an enum name 184 185 #. ``llvm/include/llvm/Instructions.h``: 186 187 add a definition for the class that will represent your instruction 188 189 #. ``llvm/include/llvm/Support/InstVisitor.h``: 190 191 add a prototype for a visitor to your new instruction type 192 193 #. ``llvm/lib/AsmParser/Lexer.l``: 194 195 add a new token to parse your instruction from assembly text file 196 197 #. ``llvm/lib/AsmParser/llvmAsmParser.y``: 198 199 add the grammar on how your instruction can be read and what it will 200 construct as a result 201 202 #. ``llvm/lib/Bitcode/Reader/Reader.cpp``: 203 204 add a case for your instruction and how it will be parsed from bitcode 205 206 #. ``llvm/lib/VMCore/Instruction.cpp``: 207 208 add a case for how your instruction will be printed out to assembly 209 210 #. ``llvm/lib/VMCore/Instructions.cpp``: 211 212 implement the class you defined in ``llvm/include/llvm/Instructions.h`` 213 214 #. Test your instruction 215 216 #. ``llvm/lib/Target/*``: 217 218 add support for your instruction to code generators, or add a lowering pass. 219 220 #. ``llvm/test/Regression/*``: 221 222 add your test cases to the test suite. 223 224 Also, you need to implement (or modify) any analyses or passes that you want to 225 understand this new instruction. 226 227 Adding a new type 228 ================= 229 230 .. warning:: 231 232 Adding new types changes the bitcode format, and will break compatibility with 233 currently-existing LLVM installations. Only add new types if it is absolutely 234 necessary. 235 236 Adding a fundamental type 237 ------------------------- 238 239 #. ``llvm/include/llvm/Type.h``: 240 241 add enum for the new type; add static ``Type*`` for this type 242 243 #. ``llvm/lib/VMCore/Type.cpp``: 244 245 add mapping from ``TypeID`` => ``Type*``; initialize the static ``Type*`` 246 247 #. ``llvm/lib/AsmReader/Lexer.l``: 248 249 add ability to parse in the type from text assembly 250 251 #. ``llvm/lib/AsmReader/llvmAsmParser.y``: 252 253 add a token for that type 254 255 Adding a derived type 256 --------------------- 257 258 #. ``llvm/include/llvm/Type.h``: 259 260 add enum for the new type; add a forward declaration of the type also 261 262 #. ``llvm/include/llvm/DerivedTypes.h``: 263 264 add new class to represent new class in the hierarchy; add forward 265 declaration to the TypeMap value type 266 267 #. ``llvm/lib/VMCore/Type.cpp``: 268 269 add support for derived type to: 270 271 .. code-block:: c++ 272 273 std::string getTypeDescription(const Type &Ty, 274 std::vector<const Type*> &TypeStack) 275 bool TypesEqual(const Type *Ty, const Type *Ty2, 276 std::map<const Type*, const Type*> &EqTypes) 277 278 add necessary member functions for type, and factory methods 279 280 #. ``llvm/lib/AsmReader/Lexer.l``: 281 282 add ability to parse in the type from text assembly 283 284 #. ``llvm/lib/BitCode/Writer/Writer.cpp``: 285 286 modify ``void BitcodeWriter::outputType(const Type *T)`` to serialize your 287 type 288 289 #. ``llvm/lib/BitCode/Reader/Reader.cpp``: 290 291 modify ``const Type *BitcodeReader::ParseType()`` to read your data type 292 293 #. ``llvm/lib/VMCore/AsmWriter.cpp``: 294 295 modify 296 297 .. code-block:: c++ 298 299 void calcTypeName(const Type *Ty, 300 std::vector<const Type*> &TypeStack, 301 std::map<const Type*,std::string> &TypeNames, 302 std::string &Result) 303 304 to output the new derived type 305
