The LLVM Target-Independent Code Generator

1. Introduction
   - Required components in the code generator
   - The high-level design of the code generator
   - Using TableGen for target description
2. Target description classes
   - The `TargetMachine` class
   - The `TargetData` class
   - The `MRegisterInfo` class
   - The `TargetInstrInfo` class
   - The `TargetFrameInfo` class
   - The `TargetJITInfo` class
3. Machine code description classes
4. Target-independent code generation algorithms
5. Target description implementations
   - The X86 backend

Written by Chris Lattner

Warning: This is a work in progress.

Introduction

The LLVM target-independent code generator is a framework that provides a suite of reusable components for translating the LLVM internal representation to the machine code for a specified target -- either in assembly form (suitable for a static compiler) or in binary machine code format (usable for a JIT compiler). The LLVM target-independent code generator consists of four main components:

1. Abstract target description interfaces which capture improtant properties about various aspects of the machine independently of how they will be used. These interfaces are defined in `include/llvm/Target/`.
2. Classes used to represent the machine code being generator for a target. These classes are intended to be abstract enough to represent the machine code for any target machine. These classes are defined in `include/llvm/CodeGen/`.
3. Target-independent algorithms used to implement various phases of native code generation (register allocation, scheduling, stack frame representation, etc). This code lives in `lib/CodeGen/`.
4. Implementations of the abstract target description interfaces for particular targets. These machine descriptions make use of the components provided by LLVM, and can optionally provide custom target-specific passes, to build complete code generators for a specific target. Target descriptions live in `lib/Target/`.

Depending on which part of the code generator you are interested in working on, different pieces of this will be useful to you. In any case, you should be familiar with the target description and machine code representation classes. If you want to add a backend for a new target, you will need implement the targe description classes for your new target and understand the LLVM code representation. If you are interested in implementing a new code generation algorithm, it should only depend on the target-description and machine code representation classes, ensuring that it is portable.

Required components in the code generator

The two pieces of the LLVM code generator are the high-level interface to the code generator and the set of reusable components that can be used to build target-specific backends. The two most important interfaces (`TargetMachine` and `TargetData` classes) are the only ones that are required to be defined for a backend to fit into the LLVM system, but the others must be defined if the reusable code generator components are going to be used.

This design has two important implications. The first is that LLVM can support completely non-traditional code generation targets. For example, the C backend does not require register allocation, instruction selection, or any of the other standard components provided by the system. As such, it only implements these two interfaces, and does its own thing. Another example of a code generator like this is a (purely hypothetical) backend that converts LLVM to the GCC RTL form and uses GCC to emit machine code for a target.

The other implication of this design is that it is possible to design and implement radically different code generators in the LLVM system that do not make use of any of the built-in components. Doing so is not recommended at all, but could be required for radically different targets that do not fit into the LLVM machine description model: programmable FPGAs for example.

Important Note: For historical reasons, the LLVM SparcV9 code generator uses almost entirely different code paths than described in this document. For this reason, there are some deprecated interfaces (such as `TargetRegInfo` and `TargetSchedInfo`), which are only used by the V9 backend and should not be used by any other targets. Also, all code in the `lib/Target/SparcV9` directory and subdirectories should be considered deprecated, and should not be used as the basis for future code generator work. The SparcV9 backend is slowly being merged into the rest of the target-independent code generators, but this is a low-priority process with no predictable completion date.

The high-level design of the code generator

The LLVM target-indendent code generator is designed to support efficient and quality code generation for standard register-based microprocessors. Code generation in this model is divided into the following stages:

1. Instruction Selection - Determining a efficient implementation of the input LLVM code in the target instruction set. This stage produces the initial code for the program in the target instruction set the makes use of virtual registers in SSA form and physical registers that represent any required register assignments due to target constraints or calling conventions.
2. SSA-based Machine Code Optimizations - This (optional) stage consists of a series of machine-code optimizations that operate on the SSA-form produced by the instruction selector. Optimizations like modulo-scheduling, normal scheduling, or peephole optimization work here.
3. Register Allocation - The target code is transformed from an infinite virtual register file in SSA form to the concrete register file used by the target. This phase introduces spill code and eliminates all virtual register references from the program.
4. Prolog/Epilog Code Insertion - Once the machine code has been generated for the function and the amount of stack space required is known (used for LLVM alloca's and spill slots), the prolog and epilog code for the function can be inserted and "abstract stack location references" can be eliminated. This stage is responsible for implementing optimizations like frame-pointer elimination and stack packing.
5. Late Machine Code Optimizations - Optimizations that operate on "final" machine code can go here, such as spill code scheduling and peephole optimizations.
6. Code Emission - The final stage actually outputs the machine code for the current function, either in the target assembler format or in machine code.

The code generator is based on the assumption that the instruction selector will use an optimal pattern matching selector to create high-quality sequences of native code. Alternative code generator designs based on pattern expansion and aggressive iterative peephole optimization are much slower. This design is designed to permit efficient compilation (important for JIT environments) and aggressive optimization (used when generate code offline) by allowing components of varying levels of sophisication to be used for any step of compilation.

In addition to these stages, target implementations can insert arbitrary target-specific passes into the flow. For example, the X86 target uses a special pass to handle the 80x87 floating point stack architecture. Other targets with unusual requirements can be supported with custom passes as needed.

Using TableGen for target description

The target description classes require a detailed description of the target architecture. These target descriptions often have a large amount of common information (e.g., an add instruction is almost identical to a sub instruction). In order to allow the maximum amount of commonality to be factored out, the LLVM code generator uses the TableGen tool to describe big chunks of the target machine, which allows the use of domain- and target-specific abstractions to reduce the amount of repetition.

Target description classes

The LLVM target description classes (which are located in the `include/llvm/Target` directory) provide an abstract description of the target machine, independent of any particular client. These classes are designed to capture the abstract properties of the target (such as what instruction and registers it has), and do not incorporate any particular pieces of code generation algorithms (these interfaces do not take interference graphs as inputs or other algorithm-specific data structures).

All of the target description classes (except the `TargetData` class) are designed to be subclassed by the concrete target implementation, and have virtual methods implemented. To get to these implementations, `TargetMachine` class provides accessors that should be implemented by the target.

The `TargetMachine` class

The `TargetMachine` class provides virtual methods that are used to access the target-specific implementations of the various target description classes (with the `getInstrInfo`, `getRegisterInfo`, `getFrameInfo`, ... methods). This class is designed to be subclassed by a concrete target implementation (e.g., `X86TargetMachine`) which implements the various virtual methods. The only required target description class is the `TargetData` class, but if the code generator components are to be used, the other interfaces should be implemented as well.

The `TargetData` class

The `TargetData` class is the only required target description class, and it is the only class that is not extensible (it cannot be derived from). It specifies information about how the target lays out memory for structures, the alignment requirements for various data types, the size of pointers in the target, and whether the target is little- or big-endian.

The `MRegisterInfo` class

The `MRegisterInfo` class (which will eventually be renamed to `TargetRegisterInfo`) is used to describe the register file of the target and any interactions between the registers.

Registers in the code generator are represented in the code generator by unsigned numbers. Physical registers (those that actually exist in the target description) are unique small numbers, and virtual registers are generally large.

Each register in the processor description has an associated `MRegisterDesc` entry, which provides a textual name for the register (used for assembly output and debugging dumps), a set of aliases (used to indicate that one register overlaps with another), and some flag bits.

In addition to the per-register description, the `MRegisterInfo` class exposes a set of processor specific register classes (instances of the `TargetRegisterClass` class). Each register class contains sets of registers that have the same properties (for example, they are all 32-bit integer registers). Each SSA virtual register created by the instruction selector has an associated register class. When the register allocator runs, it replaces virtual registers with a physical register in the set.

The target-specific implementations of these classes is auto-generated from a TableGen description of the register file.

The `TargetInstrInfo` class
The `TargetFrameInfo` class
The `TargetJITInfo` class
Machine code description classes