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LLVM IR Tutorial: From Beginner to Expert

Table of Contents​

  1. Introduction
  2. Basic Concepts
    • What is LLVM IR?
    • Why Use LLVM IR?
  3. Installation and Setup
    • Install LLVM
    • First Steps with clang
  4. LLVM IR Basics
    • LLVM IR Structure
    • LLVM Types
    • Basic Operations
  5. Intermediate Concepts
    • Functions in LLVM IR
    • Memory Management
    • Control Flow
  6. Advanced Topics
    • Optimization Techniques
    • Extending LLVM
  7. LLVM Passes and Transformations
  8. Conclusion

Introduction​

LLVM Intermediate Representation (LLVM IR) is a low-level programming language similar to assembly, but with a higher-level abstraction. It serves as a universal format for representing code during the compilation process, from high-level languages (like C, C++, Rust) down to machine code.

Basic Concepts​

What is LLVM IR?​

LLVM IR is a strongly-typed, language-agnostic, assembly-like language. It is designed to be an intermediate step in the process of compiling source code to machine code.

Why Use LLVM IR?​

  • Portability: LLVM IR can be generated from any high-level language and then compiled to machine code for different architectures.
  • Optimization: LLVM provides powerful optimizations at both compile-time and run-time.
  • Modularity: You can write custom passes and analyses for IR, enabling custom optimizations.

Installation and Setup​

Install LLVM​

To work with LLVM IR, you need to install the LLVM suite. The easiest way to install LLVM is through package managers.

For Ubuntu:

sudo apt-get update
sudo apt-get install llvm clang

For MacOS:

brew install llvm

First Steps with clang​

Once LLVM is installed, you can use clang to generate LLVM IR from C or C++ code.

Example:

clang -S -emit-llvm hello.c -o hello.ll

This command generates an LLVM IR file hello.ll from the hello.c source file.

LLVM IR Basics​

LLVM IR Structure​

LLVM IR consists of a series of instructions organized into functions. Each function contains basic blocks, which are sequences of instructions with one entry point and one exit point.

Example of a simple LLVM IR:

define i32 @main() {
entry:A
  %0 = alloca i32, align 4
  store i32 0, i32* %0, align 4
  ret i32 0
}

LLVM Types​

LLVM IR is strongly typed. Every instruction operates on values of a specific type, and all types must be explicitly declared.

  • i32: 32-bit integer
  • i8*: Pointer to an 8-bit integer (equivalent to char* in C)

Basic Operations​

LLVM IR supports a variety of basic operations, such as arithmetic and logical operations, memory manipulation, and control flow.

%1 = add i32 %a, %b    ; Add two integers
%2 = mul i32 %x, %y    ; Multiply two integers

Intermediate Concepts​

Functions in LLVM IR​

Functions in LLVM IR are defined using the define keyword, and their arguments and return types are specified explicitly.

Example:

define i32 @add(i32 %a, i32 %b) {
entry:
  %result = add i32 %a, %b
  ret i32 %result
}

Memory Management​

Memory management in LLVM IR is performed using instructions like alloca (allocate memory on the stack) and load/store for reading and writing values to memory.

%ptr = alloca i32  ; Allocate memory for an i32
store i32 42, i32* %ptr  ; Store 42 into the allocated memory
%val = load i32, i32* %ptr  ; Load the value from memory

Control Flow​

Control flow in LLVM IR is managed through labels and branching instructions like br and ret.

br label %condition   ; Unconditional branch
br i1 %cond, label %true, label %false  ; Conditional branch
ret i32 %result       ; Return from function

Advanced Topics​

Optimization Techniques​

LLVM provides several optimization techniques that you can apply to the IR code. Some common ones are constant folding, dead code elimination, and loop unrolling.

  • Use the opt tool to apply optimization passes:
opt -O3 <input.ll> -o <optimized.ll>

Extending LLVM​

You can write custom passes and transformations in LLVM to extend its functionality. A pass operates on LLVM IR and can either analyze or transform it.

Creating a pass involves:

  1. Writing the pass in C++.
  2. Registering the pass with LLVM.
  3. Applying it to IR code using opt.

LLVM Passes and Transformations​

LLVM provides several built-in passes for optimizations, such as:

  • Dead Code Elimination: Removes code that is not used.
  • Constant Propagation: Replaces variables that have constant values with the constants themselves.

Example of applying a pass:

opt -mem2reg <input.ll> -o <output.ll>

The mem2reg pass promotes memory to register, simplifying the IR.

Conclusion​

LLVM IR is a flexible and powerful intermediate representation that bridges the gap between high-level languages and machine code. Its modularity allows developers to extend and customize it to suit specific needs, making it a vital tool in the world of compiler development.