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name: llvm-optimization description: Expertise in LLVM optimization passes, performance tuning, and code transformation techniques. Use this skill when implementing custom optimizations, analyzing pass behavior, improving generated code quality, or understanding LLVM's optimization pipeline.

LLVM Optimization Skill

This skill covers LLVM optimization infrastructure, pass development, and performance tuning techniques.

Optimization Pipeline Overview

Pipeline Stages

CopySource → Frontend → LLVM IR → Optimization Passes → CodeGen → Machine Code
                        ↓
                 [Transform Passes]
                 [Analysis Passes]

Optimization Levels

Copy# No optimization
clang -O0 source.c

# Basic optimization (most optimizations enabled)
clang -O1 source.c

# Full optimization (aggressive inlining, vectorization)
clang -O2 source.c

# Maximum optimization (may increase code size)
clang -O3 source.c

# Size optimization
clang -Os source.c  # Optimize for size
clang -Oz source.c  # Aggressive size optimization

Core Optimization Passes

Scalar Optimizations

• Constant Propagation: Replace variables with known constant values
• Dead Code Elimination (DCE): Remove unreachable or unused code
• Common Subexpression Elimination (CSE): Avoid redundant computations
• Instruction Combining: Merge multiple instructions into simpler forms
• Scalar Replacement of Aggregates (SROA): Break up aggregate allocations

Loop Optimizations

• Loop Invariant Code Motion (LICM): Hoist invariant computations
• Loop Unrolling: Duplicate loop body to reduce overhead
• Loop Vectorization: Convert scalar loops to vector operations
• Loop Fusion/Fission: Combine or split loops
• Induction Variable Simplification: Optimize loop counters

Interprocedural Optimizations

• Inlining: Replace call sites with function body
• Dead Argument Elimination: Remove unused function parameters
• Interprocedural Constant Propagation: Propagate constants across functions
• Link-Time Optimization (LTO): Whole-program optimization

Writing Custom Optimization Passes

New Pass Manager (LLVM 13+)

Copy#include "llvm/IR/PassManager.h"
#include "llvm/Passes/PassBuilder.h"
#include "llvm/Passes/PassPlugin.h"

struct MyOptimizationPass : public llvm::PassInfoMixin<MyOptimizationPass> {
    llvm::PreservedAnalyses run(llvm::Function &F,
                                 llvm::FunctionAnalysisManager &FAM) {
        bool Changed = false;
        
        for (auto &BB : F) {
            for (auto &I : BB) {
                // Implement optimization logic
                if (optimizeInstruction(I)) {
                    Changed = true;
                }
            }
        }
        
        if (Changed)
            return llvm::PreservedAnalyses::none();
        return llvm::PreservedAnalyses::all();
    }
    
private:
    bool optimizeInstruction(llvm::Instruction &I) {
        // Example: Replace add x, 0 with x
        if (auto *BinOp = llvm::dyn_cast<llvm::BinaryOperator>(&I)) {
            if (BinOp->getOpcode() == llvm::Instruction::Add) {
                if (auto *C = llvm::dyn_cast<llvm::ConstantInt>(BinOp->getOperand(1))) {
                    if (C->isZero()) {
                        I.replaceAllUsesWith(BinOp->getOperand(0));
                        return true;
                    }
                }
            }
        }
        return false;
    }
};

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

Analysis Dependencies

Copystruct MyAnalysis : public llvm::AnalysisInfoMixin<MyAnalysis> {
    using Result = MyAnalysisResult;
    
    Result run(llvm::Function &F, llvm::FunctionAnalysisManager &FAM) {
        // Compute analysis result
        return Result();
    }
    
    static llvm::AnalysisKey Key;
};

// Using analysis in a pass
llvm::PreservedAnalyses run(llvm::Function &F,
                             llvm::FunctionAnalysisManager &FAM) {
    auto &DT = FAM.getResult<llvm::DominatorTreeAnalysis>(F);
    auto &LI = FAM.getResult<llvm::LoopAnalysis>(F);
    auto &AA = FAM.getResult<llvm::AAManager>(F);
    
    // Use analysis results...
}

Instruction Patterns

Strength Reduction

Copy// Replace expensive operations with cheaper ones
// x * 2  →  x << 1
// x / 4  →  x >> 2
// x % 8  →  x & 7

bool reduceStrength(llvm::BinaryOperator *BO) {
    if (BO->getOpcode() == llvm::Instruction::Mul) {
        if (auto *C = llvm::dyn_cast<llvm::ConstantInt>(BO->getOperand(1))) {
            if (C->getValue().isPowerOf2()) {
                unsigned Shift = C->getValue().exactLogBase2();
                auto *Shl = llvm::BinaryOperator::CreateShl(
                    BO->getOperand(0),
                    llvm::ConstantInt::get(C->getType(), Shift));
                BO->replaceAllUsesWith(Shl);
                return true;
            }
        }
    }
    return false;
}

Algebraic Simplification

Copy// x + 0 → x
// x * 1 → x
// x * 0 → 0
// x - x → 0
// x | x → x
// x & 0 → 0

Dominator Tree Usage

Finding Optimization Opportunities

Copyvoid optimizeWithDominators(llvm::Function &F,
                             llvm::DominatorTree &DT) {
    // Use dominance for safe code motion
    for (auto &BB : F) {
        for (auto &I : BB) {
            if (auto *Load = llvm::dyn_cast<llvm::LoadInst>(&I)) {
                // Check if we can hoist this load
                if (canHoist(Load, DT)) {
                    hoistInstruction(Load, DT);
                }
            }
        }
    }
}

bool canHoist(llvm::Instruction *I, llvm::DominatorTree &DT) {
    llvm::BasicBlock *DefBB = I->getParent();
    
    // Check all uses are dominated
    for (auto *U : I->users()) {
        if (auto *UI = llvm::dyn_cast<llvm::Instruction>(U)) {
            if (!DT.dominates(DefBB, UI->getParent())) {
                return false;
            }
        }
    }
    return true;
}

Loop Optimization Techniques

Loop Analysis

Copyvoid analyzeLoops(llvm::Function &F, llvm::LoopInfo &LI) {
    for (auto *L : LI) {
        // Get loop trip count
        if (auto *TC = L->getTripCount()) {
            llvm::errs() << "Trip count: " << *TC << "\n";
        }
        
        // Check if loop is simple
        if (L->isLoopSimplifyForm()) {
            llvm::BasicBlock *Header = L->getHeader();
            llvm::BasicBlock *Latch = L->getLoopLatch();
            llvm::BasicBlock *Exit = L->getExitBlock();
        }
        
        // Get induction variables
        llvm::PHINode *IV = L->getCanonicalInductionVariable();
    }
}

Loop Unrolling

Copy// Manually trigger loop unrolling
#pragma unroll 4
for (int i = 0; i < N; i++) {
    // Loop body will be unrolled 4x
}

// LLVM unroll metadata
!llvm.loop.unroll.count = !{i32 4}

Vectorization

Auto-Vectorization Hints

Copy// Enable vectorization
#pragma clang loop vectorize(enable)
for (int i = 0; i < N; i++) {
    a[i] = b[i] + c[i];
}

// Specify vector width
#pragma clang loop vectorize_width(8)
for (int i = 0; i < N; i++) {
    a[i] = b[i] * c[i];
}

SLP Vectorization

Superword Level Parallelism - vectorize straight-line code:

Copy// Before SLP
a[0] = b[0] + c[0];
a[1] = b[1] + c[1];
a[2] = b[2] + c[2];
a[3] = b[3] + c[3];

// After SLP (conceptual)
<4 x float> tmp = load <4 x float> b
<4 x float> tmp2 = load <4 x float> c
<4 x float> result = fadd tmp, tmp2
store result to a

Debugging Optimizations

Viewing Pass Execution

Copy# Print passes being run
opt -debug-pass-manager input.ll -O2

# Print IR after each pass
opt -print-after-all input.ll -O2

# Print specific pass output
opt -print-after=instcombine input.ll -O2

# Statistics
opt -stats input.ll -O2

Optimization Remarks

Copy# Enable all optimization remarks
clang -Rpass=.* source.c

# Specific remarks
clang -Rpass=loop-vectorize source.c
clang -Rpass-missed=inline source.c
clang -Rpass-analysis=loop-vectorize source.c

Link-Time Optimization (LTO)

Enabling LTO

Copy# Full LTO
clang -flto source1.c source2.c -o program

# Thin LTO (faster, parallel)
clang -flto=thin source1.c source2.c -o program

LTO Benefits

• Whole-program dead code elimination
• Interprocedural constant propagation
• Cross-module inlining
• Better devirtualization

Correctness Verification

Alive2

Automatic verification of LLVM optimizations:

Copy# Verify transformation correctness
alive-tv before.ll after.ll

# Check specific optimization
opt -instcombine input.ll | alive-tv input.ll -

Resources

See Optimization section in README.md for specific commits and optimization-related projects.