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

1. Introduction and Warning
2. Adding a new intrinsic function
3. Adding a new instruction
4. Adding a new type
   1. Adding a new fundamental type
   2. Adding a new derived type

Written by Misha Brukman

Introduction and Warning

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

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

Adding an intrinsic function is easier than adding an instruction, and is transparent to optimization passes which treat it as an unanalyzable function. If your added functionality can be expressed as a function call, an intrinsic function is the method 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 with already-existing infrastructure, or if maybe someone else is already working on it. You will save yourself a lot of time and effort by doing so.

Finally, these are my notes, and since my extensions are not complete, I may be missing steps. If you find some omissions, please let me know directly or post on LLVM-dev.

Adding a new intrinsic function

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

1. `llvm/docs/LangRef.html`: Document the intrinsic. Decide whether it is code generator specific and what the restrictions are. Talk to other people about it so that you are sure it's a good idea.
2. `llvm/include/llvm/Intrinsics.h`: add an enum in the `llvm::Intrinsic` namespace
3. `llvm/lib/VMCore/IntrinsicLowering.cpp`: implement the lowering for this intrinsic
4. `llvm/lib/VMCore/Verifier.cpp`: Add code to check the invariants of the intrinsic are respected.
5. `llvm/lib/VMCore/Function.cpp (`Function::getIntrinsicID()`)`: Identify the new intrinsic function, returning the enum for the intrinsic that you added.
6. `llvm/lib/Analysis/BasicAliasAnalysis.cpp`: If the new intrinsic does not access memory, or does not write to memory, add it to the relevant list of functions.
7. `llvm/lib/Transforms/Utils/Local.cpp`: If it is possible to constant propagate your intrinsic, add support to it in the `canConstantFoldCallTo` and `ConstantFoldCall` functions.
8. Test your intrinsic
9. `llvm/test/Regression/*`: add your test cases to the test suite.

If this intrinsic requires code generator support (ie, it cannot be lowered). You should also add support to the code generator in question.

Adding a new instruction

WARNING: adding instructions changes the bytecode format, and it will take some effort to maintain compatibility with the previous version. Only add an instruction if it is absolutely necessary.

1. `llvm/include/llvm/Instruction.def`: add a number for your instruction and an enum name
2. `llvm/include/llvm/i*.h`: add a definition for the class that will represent your instruction
3. `llvm/include/llvm/Support/InstVisitor.h`: add a prototype for a visitor to your new instruction type
4. `llvm/lib/AsmParser/Lexer.l`: add a new token to parse your instruction from assembly text file
5. `llvm/lib/AsmParser/llvmAsmParser.y`: add the grammar on how your instruction can be read and what it will construct as a result
6. `llvm/lib/Bytecode/Reader/InstructionReader.cpp`: add a case for your instruction and how it will be parsed from bytecode
7. `llvm/lib/VMCore/Instruction.cpp`: add a case for how your instruction will be printed out to assembly
8. `llvm/lib/VMCore/i*.cpp`: implement the class you defined in `llvm/include/llvm/i*.h`

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

Adding a new type

WARNING: adding new types changes the bytecode format, and will break compatibility with currently-existing LLVM installations. Only add new types if it is absolutely necessary.

Adding a fundamental type

1. `llvm/include/llvm/Type.def`: add enum for the type
2. `llvm/include/llvm/Type.h`: add ID number for the new type; add static `Type*` for this type
3. `llvm/lib/VMCore/Type.cpp`: add mapping from `TypeID` => `Type*`; initialize the static `Type*`
4. `llvm/lib/AsmReader/Lexer.l`: add ability to parse in the type from text assembly
5. `llvm/lib/AsmReader/llvmAsmParser.y`: add a token for that type

Adding a derived type

TODO