NaplesPU LLVM Documentation

The main task of the backend is to generate nu+ assembly code from the LLVM IR obtained by the Clang frontend. However the LLVM framework provides also base classes that can be used to create an assembler and a disassembler. The nu+ backend is contained in the NuPlus folder under "compiler/lib/Target" directory. It contains several files, each implementing a specific class of the LLVM Framework. There are both tablegen and C++ classes.

Required reading

Before working on LLVM, you should be familiar with some things. In particular:

1. Basic Blocks
2. SSA (Static Single Assignment) form
3. AST (Abstract Syntax tree)
4. DAG Direct Acyclic Graph.

In addition to general aspects about compilers, it is recommended to read about:

1. LLVM architecture
2. LLVM Intermediate Representation
3. Tablegen

See the following textbook for other informations Getting Started with LLVM Core Libraries and LLVM Cookbook.

See also this article to have an overview of the main CodeGenerator phases.

Tablegen files

These files describe, through the Tablegen description language, the target architecture like the register file, the instruction set and the calling conventions. The classes defined in the following files are derived from one or more base classes which, forming an inheritance hierarchy. The list of base classes, including some internal documentation, is provided in the Target.td file or in the other files located in the compiler/include/llvm/Target/ folder.

The files implemented in the nu+ backend are:

• NuPlus.td, used to describe a target and to instruct LLVM about the names of the classes used for each of the framework component.
• NuPlusRegisterInfo.td, contains the definition of the nu+ register file.
• NuPlusCallingConv.td, used to define the calling conventions to use when a function is called.
• NuPlusInstrFormats.td and NuPlusInstrInfo.td, define the instructions supported by the nu+ architecture and the pattern LLVM must use to translate the LLVM IR in nu+ asm.

C++ classes implementation

• NuPlusFrameLowering
• NuPlusInstrInfo
• NuPlusISelDAGToDAG
• NuPlusTargetLowering
• NuPlusMachineFunctionInfo
• NuPlusMCInstLower
• NuPlusRegisterInfo
• NuPlusTargetMachine
• NuPlusSubtarget
• NuPlusTargetObjectFile

in MCTargetDesc

• NuPlusAsmBackend
• NuPlusELFObjectWriter
• NuPlusMCAsmInfo
• NuPlusMCCodeEmitter
• NuPlusMCTargetDesc
• NuPlusFixupKinds

in InstPrinter

• NuPlusInstPrinter

in Disassembler

• NuPlusDisassembler

in AsmParser

• NuPlusAsmParser