LLVM Learning Skill

This skill provides curated learning paths and resources for mastering LLVM, Clang, and compiler development.

Learning Paths

Beginner Path

1. Start with Kaleidoscope: Official LLVM tutorial building a simple language
2. Understand LLVM IR: Learn the intermediate representation
3. Write Simple Passes: Transform IR with custom passes
4. Explore Clang: Understand C/C++ frontend

Intermediate Path

1. Deep Dive into Optimization: Study built-in optimization passes
2. Backend Development: Target code generation
3. Analysis Frameworks: Pointer analysis, dataflow
4. Clang Tooling: LibTooling, AST matchers

Advanced Path

1. MLIR: Multi-level IR for domain-specific compilation
2. JIT Compilation: ORC JIT framework
3. Security Features: Sanitizers, CFI implementation
4. Contribute to LLVM: Patches, reviews, community

Official Resources

Documentation

• LLVM Docs: https://llvm.org/docs
• Clang Docs: https://clang.llvm.org/docs
• LLVM Language Reference: Complete IR specification
• Programmer's Manual: Core APIs and usage

Tutorials

• Kaleidoscope: https://llvm.org/docs/tutorial/MyFirstLanguageFrontend/index.html
• Writing an LLVM Pass: https://llvm.org/docs/WritingAnLLVMPass.html
• Building LLVM: https://llvm.org/docs/GettingStarted.html

Community Resources

Books and Guides

• Learn LLVM 12 by Kai Nacke: Comprehensive practical guide
• LLVM Techniques, Tips, and Best Practices: Advanced patterns
• Getting Started with LLVM Core Libraries: Foundation concepts
• LLVM Essentials: Quick start guide

Video Resources

• LLVM Developer Meetings: Recorded talks on YouTube
• LLVM Weekly: Newsletter with latest developments
• EuroLLVM/US LLVM: Conference recordings

GitHub Learning Projects

# Highly recommended starter projects
banach-space/llvm-tutor     # Comprehensive pass examples
hunterzju/llvm-tutorial     # Step-by-step tutorials
jauhien/iron-kaleidoscope   # Kaleidoscope in Rust
bigconvience/llvm-ir-in-action  # IR examples

LLVM IR Essentials

Basic Structure

; Module level
@global_var = global i32 42

; Function definition
define i32 @add(i32 %a, i32 %b) {
entry:
    %sum = add i32 %a, %b
    ret i32 %sum
}

; External declaration
declare i32 @printf(i8*, ...)

Common Instructions

; Arithmetic
%result = add i32 %a, %b
%result = sub i32 %a, %b
%result = mul i32 %a, %b

; Memory
%ptr = alloca i32
store i32 %value, i32* %ptr
%loaded = load i32, i32* %ptr

; Control flow
br i1 %cond, label %then, label %else
br label %next

; Function calls
%retval = call i32 @function(i32 %arg)

Type System

; Integer types: i1, i8, i16, i32, i64, i128
; Floating point: half, float, double
; Pointers: i32*, [10 x i32]*, { i32, i64 }*
; Arrays: [10 x i32]
; Structs: { i32, i64, i8* }
; Vectors: <4 x i32>

Writing LLVM Passes

New Pass Manager (Recommended)

#include "llvm/Passes/PassPlugin.h"
#include "llvm/Passes/PassBuilder.h"

struct MyPass : public llvm::PassInfoMixin<MyPass> {
    llvm::PreservedAnalyses run(llvm::Function &F,
                                 llvm::FunctionAnalysisManager &FAM) {
        for (auto &BB : F) {
            for (auto &I : BB) {
                llvm::errs() << I << "\n";
            }
        }
        return llvm::PreservedAnalyses::all();
    }
};

// Plugin registration
extern "C" LLVM_ATTRIBUTE_WEAK ::llvm::PassPluginLibraryInfo
llvmGetPassPluginInfo() {
    return {LLVM_PLUGIN_API_VERSION, "MyPass", LLVM_VERSION_STRING,
            [](llvm::PassBuilder &PB) {
                PB.registerPipelineParsingCallback(
                    [](llvm::StringRef Name, llvm::FunctionPassManager &FPM,
                       llvm::ArrayRef<llvm::PassBuilder::PipelineElement>) {
                        if (Name == "my-pass") {
                            FPM.addPass(MyPass());
                            return true;
                        }
                        return false;
                    });
            }};
}

Running Custom Passes

# Build as shared library
clang++ -shared -fPIC -o MyPass.so MyPass.cpp `llvm-config --cxxflags --ldflags`

# Run with opt
opt -load-pass-plugin=./MyPass.so -passes="my-pass" input.ll -o output.ll

Clang Development

AST Exploration

# Dump AST
clang -Xclang -ast-dump -fsyntax-only source.cpp

# AST as JSON
clang -Xclang -ast-dump=json -fsyntax-only source.cpp

LibTooling Basics

#include "clang/Tooling/Tooling.h"
#include "clang/ASTMatchers/ASTMatchers.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"

using namespace clang::ast_matchers;

// Match all function declarations
auto matcher = functionDecl().bind("func");

class Callback : public MatchFinder::MatchCallback {
    void run(const MatchFinder::MatchResult &Result) override {
        if (auto *FD = Result.Nodes.getNodeAs<FunctionDecl>("func")) {
            llvm::outs() << "Found: " << FD->getName() << "\n";
        }
    }
};

AST Matchers Reference

// Common matchers
functionDecl()               // Match function declarations
varDecl()                    // Match variable declarations
binaryOperator()             // Match binary operators
callExpr()                   // Match function calls
ifStmt()                     // Match if statements

// Narrowing matchers
hasName("foo")               // Match by name
hasType(asString("int"))     // Match by type
isPrivate()                  // Match private members

// Traversal matchers
hasDescendant(...)           // Match in subtree
hasAncestor(...)             // Match in parent tree
hasBody(...)                 // Match function body

Development Tools

Essential Commands

# Compile to LLVM IR
clang -S -emit-llvm source.c -o source.ll

# Optimize IR
opt -O2 source.ll -o optimized.ll

# Disassemble bitcode
llvm-dis source.bc -o source.ll

# Assemble to bitcode
llvm-as source.ll -o source.bc

# Generate native code
llc source.ll -o source.s

# View CFG
opt -passes=view-cfg source.ll

Debugging LLVM IR

# With debug info
clang -g -S -emit-llvm source.c

# Debug IR transformation
opt -debug-pass=Structure -O2 source.ll

# Verify IR validity
opt -passes=verify source.ll

Common Analysis APIs

// Dominator Tree
#include "llvm/Analysis/Dominators.h"
DominatorTree &DT = FAM.getResult<DominatorTreeAnalysis>(F);
if (DT.dominates(BB1, BB2)) { /* BB1 dominates BB2 */ }

// Loop Analysis
#include "llvm/Analysis/LoopInfo.h"
LoopInfo &LI = FAM.getResult<LoopAnalysis>(F);
for (Loop *L : LI) { /* process loops */ }

// Alias Analysis
#include "llvm/Analysis/AliasAnalysis.h"
AAResults &AA = FAM.getResult<AAManager>(F);
AliasResult R = AA.alias(Ptr1, Ptr2);

// Call Graph
#include "llvm/Analysis/CallGraph.h"
CallGraph &CG = MAM.getResult<CallGraphAnalysis>(M);

Practice Projects

Beginner

1. Instruction counter pass
2. Function call logger
3. Dead code finder

Intermediate

1. Simple constant propagation
2. Branch probability estimator
3. Memory access pattern analyzer

Advanced

1. Custom optimization pass
2. Security vulnerability detector
3. Domain-specific language frontend

Community

Getting Help

• LLVM Discourse: https://discourse.llvm.org
• LLVM Discord: Real-time chat
• LLVM Mailing Lists: llvm-dev, cfe-dev

Contributing

• Phabricator: Code reviews (being migrated to GitHub)
• GitHub: https://github.com/llvm/llvm-project
• Bug Tracker: https://github.com/llvm/llvm-project/issues

Resources Index

For a comprehensive list of tutorials, books, and example projects, see the LLVM Tutorial and Clang Tutorial sections in the main README.md.