Llvm 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+)

#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();
    }