My note on LLVM

This is my note for This LLVM Talk. This is a very beginning level note.

Code

• LLVM monorepo: Now the single source of truth is now in github.
• Subdirectories:
  • Each one is a sub project.
  • The most famous one is llvm-project/clang (the “core”)(front-end for C/C++/Cuda, etc)(what is front-end)
• Building: default building is debug build, and very slow
  • More on /GettingStarted.html

Compilers

file.cpp -> Clang AST -> LLVM IR -> Selection DAG -> LLVM MIR -> Machine Code

• LLVM-IR

  • LLVM IR Hierarchy

    Module
        GlobalVariable
        Function
            BasicBlock
                Instruction
                    ICmpInst
                    BranchInst
    

  • assembly-like language always in SSA-from with infinite registers
    • What is SSA: Static Single-assignment form

      • In compiler design, static single assignment form (often abbreviated as SSA form or simply SSA) is a type of intermediate representation (IR) where each variable is assigned exactly once. SSA is used in most high-quality optimizing compilers for imperative languages, including LLVM, the GNU Compiler Collection, and many commercial compilers.

      • 

• Target and Code Generation

  • Target
    • target means architecture the code is generated for: x86, ARM, powerPC etc.
    • SubTarget
      • Module level
      • Can choose only generate subtarget
  • Machine IR (MIR)
    • They are target dependent
    • produced out of llc program
      • What is llc program

        • The llc command compiles LLVM source inputs into assembly language for a specified architecture. The assembly language output can then be passed through a native assembler and linker to generate a native executable.The choice of architecture for the output assembly code is automatically determined from the input file, unless the -march option is used to override the default.

  • Instruction Selection and Register Allocation
    • What is Instruction Selection

      • In computer science, instruction selection is the stage of a compiler backend that transforms its middle-level intermediate representation (IR) into a low-level IR.

    • We have 3 instruction selectors
      • SelectionDAG
      • FastISel
      • GlobalISel