LLVM每日谈之四十七 LLVM后端文档解析

去年在HelloLLVM杭州站的活动中，我做了一个分享： LLVM后端简介 。主要是从LLVM后端的几大环节上展开，简单介绍了LLVM后端。在新的芯片和指令集大火的今天，为LLVM添加一个新的后端，也成了大家比较关注的热点。

LLVM官方提供了两个关于后端的官方文档：

1、 The LLVM Target-Independent Code Generator

2、 Writing an LLVM Backend

阅读2之前要求阅读1。将这两个文档参照起来读，基本上可以获取新建后端所需要的各类信息。这两个文档从三个层面解析了后端的架构，将后端从代码组织、执行环节和建立步骤都讲的很清楚。我们在这里就一起看看这三个层面：

1、后端的六大部分

• Target description classes : abstract target description interfaces (代码地址：include/llvm/Target/)
• Marchine code description classes : classes used to repesent the code being generated for a target (代码地址：include/llvm/CodeGen/)
• The "MC" Layer : use to represent and process code at the raw machine code level
• Target-independent code generation algorithms  (代码地址：lib/CodeGen)
• Implementations of the abstract description interfaces for particular targets  (代码地址： lib/Target)
• The target-independent JIT components  (代码地址：lib/ExecutionEngine/JIT)

2、后端的七大环节

• Instruction Selection
• Scheduling and Formation
• SSA-based Machine Code Optimizations
• Register Allocation
• Prolog/Epilog Code Insertion
• Late Machine Code Optimizations
• Code Emission

3、建立新后端的七大步骤

• Create a subclass of the TargetMachine class that describes characteristics of your target machine.
• Describe the register set of the target.
• Describe the instruction set of the target.
• Describe the selection and conversion of the LLVM IR from a Directed Acyclic Graph (DAG) representation of instructions to native target-specific instructions.
• Write code for an assembly printer that converts LLVM IR to a GAS format for your target machine.
• Optionally, add support for subtargets.
• Optionally, add JIT support and create a machine code emitter (subclass of TargetJITInfo) that is used to emit binary code directly into memory.

这三个层面是从不同的维度去审视LLVM的后端，每个维度自成体系，不同维度之间还有着对应的关系。后续也会写文章分析其中的关系。