Extending LLVM: Adding instructions, intrinsics, types, etc.

Introduction and Warning

During the course of using LLVM, you may wish to customize it for your researchproject or for experimentation. At this point, you may realize that you need toadd something to LLVM, whether it be a new fundamental type, a new intrinsicfunction, or a whole new instruction.

When you come to this realization, stop and think. Do you really need to extendLLVM? Is it a new fundamental capability that LLVM does not support at itscurrent incarnation or can it be synthesized from already pre-existing LLVMelements? If you are not sure, ask on the LLVM-dev list. The reason is thatextending LLVM will get involved as you need to update all the different passesthat you intend to use with your extension, and there are many LLVM analysesand transformations, so it may be quite a bit of work.

Adding an intrinsic function is far easier than adding aninstruction, and is transparent to optimization passes. If your addedfunctionality can be expressed as a function call, an intrinsic function is themethod of choice for LLVM extension.

Before you invest a significant amount of effort into a non-trivial extension,ask on the list if what you are looking to do can be done withalready-existing infrastructure, or if maybe someone else is already working onit. You will save yourself a lot of time and effort by doing so.

Adding a new intrinsic function

Adding a new intrinsic function to LLVM is much easier than adding a newinstruction. Almost all extensions to LLVM should start as an intrinsicfunction and then be turned into an instruction if warranted.

• llvm/docs/LangRef.html:

Document the intrinsic. Decide whether it is code generator specific andwhat the restrictions are. Talk to other people about it so that you aresure it’s a good idea.

• llvm/include/llvm/IR/Intrinsics*.td:

Add an entry for your intrinsic. Describe its memory accesscharacteristics for optimization (this controls whether it will beDCE’d, CSE’d, etc). If any arguments need to be immediates, thesemust be indicated with the ImmArg property. Note that any intrinsicusing one of the llvm_any*_ty types for an argument or returntype will be deemed by tblgen as overloaded and thecorresponding suffix will be required on the intrinsic’s name.

• llvm/lib/Analysis/ConstantFolding.cpp:

If it is possible to constant fold your intrinsic, add support to it in thecanConstantFoldCallTo and ConstantFoldCall functions.

• llvm/test/*:

Add test cases for your test cases to the test suite

Once the intrinsic has been added to the system, you must add code generatorsupport for it. Generally you must do the following steps:

Add support to the .td file for the target(s) of your choice inlib/Target//.td.

This is usually a matter of adding a pattern to the .td file that matches theintrinsic, though it may obviously require adding the instructions you want togenerate as well. There are lots of examples in the PowerPC and X86 backendto follow.

Adding a new SelectionDAG node

As with intrinsics, adding a new SelectionDAG node to LLVM is much easier thanadding a new instruction. New nodes are often added to help representinstructions common to many targets. These nodes often map to an LLVMinstruction (add, sub) or intrinsic (byteswap, population count). In othercases, new nodes have been added to allow many targets to perform a common task(converting between floating point and integer representation) or capture morecomplicated behavior in a single node (rotate).

• include/llvm/CodeGen/ISDOpcodes.h:

Add an enum value for the new SelectionDAG node.

• lib/CodeGen/SelectionDAG/SelectionDAG.cpp:

  • Add code to print the node to getOperationName. If your new node can be
  • evaluated at compile time when given constant arguments (such as an add of aconstant with another constant), find the getNode method that takes theappropriate number of arguments, and add a case for your node to the switchstatement that performs constant folding for nodes that take the same numberof arguments as your new node.

• lib/CodeGen/SelectionDAG/LegalizeDAG.cpp:

Add code to legalize, promote, and expand the node as necessary. At aminimum, you will need to add a case statement for your node inLegalizeOp which calls LegalizeOp on the node’s operands, and returns anew node if any of the operands changed as a result of being legalized. Itis likely that not all targets supported by the SelectionDAG framework willnatively support the new node. In this case, you must also add code in yournode’s case statement in LegalizeOp to Expand your node into simpler,legal operations. The case for ISD::UREM for expanding a remainder intoa divide, multiply, and a subtract is a good example.

• lib/CodeGen/SelectionDAG/LegalizeDAG.cpp:

  • If targets may support the new node being added only at certain sizes, you
  • will also need to add code to your node’s case statement in LegalizeOpto Promote your node’s operands to a larger size, and perform the correctoperation. You will also need to add code to PromoteOp to do this aswell. For a good example, see ISD::BSWAP, which promotes its operand toa wider size, performs the byteswap, and then shifts the correct bytes rightto emulate the narrower byteswap in the wider type.

• lib/CodeGen/SelectionDAG/LegalizeDAG.cpp:

Add a case for your node in ExpandOp to teach the legalizer how toperform the action represented by the new node on a value that has been splitinto high and low halves. This case will be used to support your node with a64 bit operand on a 32 bit target.

• lib/CodeGen/SelectionDAG/DAGCombiner.cpp:

If your node can be combined with itself, or other existing nodes in apeephole-like fashion, add a visit function for it, and call that functionfrom. There are several good examples for simple combines you can do;visitFABS and visitSRL are good starting places.

• lib/Target/PowerPC/PPCISelLowering.cpp:

Each target has an implementation of the TargetLowering class, usually inits own file (although some targets include it in the same file as theDAGToDAGISel). The default behavior for a target is to assume that your newnode is legal for all types that are legal for that target. If this targetdoes not natively support your node, then tell the target to either Promoteit (if it is supported at a larger type) or Expand it. This will cause thecode you wrote in LegalizeOp above to decompose your new node into otherlegal nodes for this target.

• lib/Target/TargetSelectionDAG.td:

Most current targets supported by LLVM generate code using the DAGToDAGmethod, where SelectionDAG nodes are pattern matched to target-specificnodes, which represent individual instructions. In order for the targets tomatch an instruction to your new node, you must add a def for that node tothe list in this file, with the appropriate type constraints. Look atadd, bswap, and fadd for examples.

• lib/Target/PowerPC/PPCInstrInfo.td:

Each target has a tablegen file that describes the target’s instruction set.For targets that use the DAGToDAG instruction selection framework, add apattern for your new node that uses one or more target nodes. Documentationfor this is a bit sparse right now, but there are several decent examples.See the patterns for rotl in PPCInstrInfo.td.

• TODO: document complex patterns.

• llvm/test/CodeGen/*:

Add test cases for your new node to the test suite.llvm/test/CodeGen/X86/bswap.ll is a good example.

Adding a new instruction

Warning

Adding instructions changes the bitcode format, and it will take some effortto maintain compatibility with the previous version. Only add an instructionif it is absolutely necessary.

• llvm/include/llvm/IR/Instruction.def:

add a number for your instruction and an enum name

• llvm/include/llvm/IR/Instructions.h:

add a definition for the class that will represent your instruction

• llvm/include/llvm/IR/InstVisitor.h:

add a prototype for a visitor to your new instruction type

• llvm/lib/AsmParser/LLLexer.cpp:

add a new token to parse your instruction from assembly text file

• llvm/lib/AsmParser/LLParser.cpp:

add the grammar on how your instruction can be read and what it willconstruct as a result

• llvm/lib/Bitcode/Reader/BitcodeReader.cpp:

add a case for your instruction and how it will be parsed from bitcode

• llvm/lib/Bitcode/Writer/BitcodeWriter.cpp:

add a case for your instruction and how it will be parsed from bitcode

• llvm/lib/IR/Instruction.cpp:

add a case for how your instruction will be printed out to assembly

• llvm/lib/IR/Instructions.cpp:

implement the class you defined in llvm/include/llvm/Instructions.h

• Test your instruction

• llvm/lib/Target/*:

add support for your instruction to code generators, or add a lowering pass.

• llvm/test/*:

add your test cases to the test suite.

Also, you need to implement (or modify) any analyses or passes that you want tounderstand this new instruction.

Adding a new type

Warning

Adding new types changes the bitcode format, and will break compatibility withcurrently-existing LLVM installations. Only add new types if it is absolutelynecessary.

Adding a fundamental type

• llvm/include/llvm/IR/Type.h:

add enum for the new type; add static Type* for this type

• llvm/lib/IR/Type.cpp and llvm/lib/IR/ValueTypes.cpp:

add mapping from TypeID => Type; initialize the static Type

• llvm/llvm/llvm-c/Core.cpp:

add enum LLVMTypeKind and modifyLLVMTypeKind LLVMGetTypeKind(LLVMTypeRef Ty) for the new type

• llvm/lib/AsmParser/LLLexer.cpp:

add ability to parse in the type from text assembly

• llvm/lib/AsmParser/LLParser.cpp:

add a token for that type

• llvm/lib/Bitcode/Writer/BitcodeWriter.cpp:

modify static void WriteTypeTable(const ValueEnumerator &VE,BitstreamWriter &Stream) to serialize your type

• llvm/lib/Bitcode/Reader/BitcodeReader.cpp:

modify bool BitcodeReader::ParseTypeType() to read your data type

• include/llvm/Bitcode/LLVMBitCodes.h:

add enum TypeCodes for the new type

Adding a derived type

• llvm/include/llvm/IR/Type.h:

add enum for the new type; add a forward declaration of the type also

• llvm/include/llvm/IR/DerivedTypes.h:

add new class to represent new class in the hierarchy; add forwarddeclaration to the TypeMap value type

• llvm/lib/IR/Type.cpp and llvm/lib/IR/ValueTypes.cpp:

add support for derived type, notably enum TypeID and is, get methods.

• llvm/llvm/llvm-c/Core.cpp:

add enum LLVMTypeKind and modifyLLVMTypeKind LLVMGetTypeKind(LLVMTypeRef Ty) for the new type

• llvm/lib/AsmParser/LLLexer.cpp:

modify lltok::Kind LLLexer::LexIdentifier() to add ability toparse in the type from text assembly

• llvm/lib/Bitcode/Writer/BitcodeWriter.cpp:

modify static void WriteTypeTable(const ValueEnumerator &VE,BitstreamWriter &Stream) to serialize your type

• llvm/lib/Bitcode/Reader/BitcodeReader.cpp:

modify bool BitcodeReader::ParseTypeType() to read your data type

• include/llvm/Bitcode/LLVMBitCodes.h:

add enum TypeCodes for the new type

• llvm/lib/IR/AsmWriter.cpp:

modify void TypePrinting::print(Type *Ty, raw_ostream &OS)to output the new derived type