崔添翼 § 翼若垂天之云

（本文是 http://llvm.org/releases/2.9/docs/LangRef.html 的阅读笔记，前作为《LLVM笔记(0)：在一切开始之前》。）

• 一句话总结： LLVM is a Static Single Assignment (SSA) based representation that provides type safety, low-level operations, flexibility, and the capability of representing ‘all’ high-level languages cleanly. It is the common code representation used throughout all phases of the LLVM compilation strategy.
• LLVM语言有三种形式：in-memory compiler IR, on-disk bitcode representation, human readable assembly language representation。三种表现形式是等价的。
• LLVM语言的目标是：一方面 light-weight and low-level，另一方面expressive, typed, and extensible。
• “universal IR”
• Identifiers
• global: @开头
• local: %开头
• ‘;’开始注释
• High Level Structure
• Module
• llvm linker to combine
• global values: functions and global variables
• linkage types
• private, linker_private, linker_private_weak, linker_private_weak_def_auto
• internal, available_externally, linkonce, weak, common, appending, extern_weak
• linkonce_odr, weak_odr
• externally visible (default)
• Calling Conventions
• ccc: the C calling convention
• fastcc: the fast calling convention
• coldcc: the cold calling convention
• Visibility styles (for global variables and functions):
• default, hidden, protected
• Named types
• e.g. %mytype = type { %mytype*, i32 }
• Global variables
• many aspects can be controlled
• Functions
• definition and declaration
• definition contains a list of basic blocks, Control Flow Graph for the function
• each basic block optionally starts with a label, contains a list of instructions, and ends with a terminator instruction
• first basic block is special: 1. immediately executed on entrance; 2. not allowed to have predecessors
• Aliases
• Named Metadata: a collection of metadata
• Parameter Attributes
• zeroext, signext, inreg, byval, sret, noalias, nocapture, nest
• Garbage Collector Names
• Function Attributes
• …
• Module-Level Inline Assembly
• Data Layout
• target datalayout = “layout specification”
• Pointer Aliasing Rules
• memory access must be through pointers
• a pointer value is based on another pointer value according to the rules
• Volatile Memory Accesses