Memory Layout: Array of Structs vs Struct of Arrays

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Snippet A

Aos Sum

float sumX(const std::vector<Particle>& particles) {
    float total = 0.0f;
    for (const auto& p : particles)
        total += p.x;
    return total;
}

float result = sumX(AOS);
Snippet B

Soa Sum

float sumX(const ParticlesSoA& particles) {
    float total = 0.0f;
    for (float v : particles.x)
        total += v;
    return total;
}

float result = sumX(SOA);
Shared test data (shared-setup) 
constexpr int kCount = 1 << 20;

// Particle is padded to exactly 64 bytes (one cache line).
struct Particle {
    float x;
    float y, z;
    float padding[13]; // total: 16 floats = 64 bytes
};

static std::vector<Particle> makeAoS() {
    std::vector<Particle> v(kCount);
    for (int i = 0; i < kCount; ++i) {
        v[i] = {};
        v[i].x = i;
        v[i].y = i * 2.0f;
        v[i].z = i * 3.0f;
    }
    return v;
}

struct ParticlesSoA {
    std::vector<float> x, y, z;
};

static ParticlesSoA makeSoA() {
    ParticlesSoA p{ std::vector<float>(kCount), std::vector<float>(kCount), std::vector<float>(kCount) };
    for (int i = 0; i < kCount; ++i) {
        p.x[i] = i;
        p.y[i] = i * 2.0f;
        p.z[i] = i * 3.0f;
    }
    return p;
};

static const std::vector<Particle> AOS = makeAoS();
static const ParticlesSoA SOA = makeSoA();

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