15 — Performance Fundamentals

Core Idea

C++ gives you control over performance, but you must measure. Guessing is unreliable.

Measure First

Use profilers before optimizing.

Tools:

• perf on Linux,
• Instruments on macOS,
• Visual Studio Profiler on Windows,
• valgrind/callgrind,
• compiler reports,
• benchmarks with Google Benchmark.

Big-O Still Matters

An O(n log n) algorithm usually beats O(n^2) at scale.

But constant factors, memory layout, and cache behavior also matter.

Cache Locality

Contiguous memory is fast.

std::vector<User> users;

Usually better than:

std::vector<std::unique_ptr<User>> users;

Unless you need polymorphism, stable addresses, or independent ownership.

Allocation Costs

Heap allocation is relatively expensive.

Avoid unnecessary allocation in hot loops.

Bad:

for (int i = 0; i < 10; ++i) {
    auto p = std::make_unique<Item>();
}

Maybe better:

std::vector<Item> items;
items.reserve(10);

Avoid Unnecessary Copies

Use references for large read-only objects.

void process(const std::vector<int>& values);

Use move semantics when transferring ownership.

class Holder {
    std::vector<int> data_;
public:
    void set(std::vector<int> data) {
        this->data_ = std::move(data);
    }
};

reserve

If you know the size, reserve capacity.

std::vector<int> v;
v.reserve(1000);

Branching and Layout

Hot code can be affected by:

• branch prediction,
• cache misses,
• false sharing,
• memory alignment,
• virtual calls,
• allocation patterns.

Do not optimize these blindly. Measure.

Compiler Optimization

Common flags:

-O0  # no optimization, debugging
-O2  # good production default
-O3  # more aggressive, not always better
-g   # debug symbols

Performance Mindset

Good performance often comes from:

• better algorithms,
• better data layout,
• fewer allocations,
• fewer copies,
• simpler ownership,
• batching work,
• using the right container.

Quick Self-Test

You should be able to answer:

• Why is vector cache-friendly?
• Why can heap allocation hurt hot paths?
• What does reserve do?
• Why should you profile before optimizing?
• What is false sharing?