Loop-Carried Dependency: Single vs Split Accumulator

Question 19 / 51 • Correct so far: 0 (0 answered)

Snippet A

Single Sum

double sumArray(const std::vector<double>& data) {
    double sum = 0.0;
    for (double x : data)
        sum += x;
    return sum;
}

double result = sumArray(DATA);
Snippet B

Split Sum

double sumArray(const std::vector<double>& data) {
    double s0 = 0.0, s1 = 0.0, s2 = 0.0, s3 = 0.0;
    const std::size_t n = data.size();
    std::size_t i = 0;
    for (; i + 3 < n; i += 4) {
        s0 += data[i];
        s1 += data[i + 1];
        s2 += data[i + 2];
        s3 += data[i + 3];
    }
    for (; i < n; ++i) s0 += data[i];
    return s0 + s1 + s2 + s3;
}

double result = sumArray(DATA);
Shared test data (shared-setup) 
constexpr int kDataSize = 65536;

static std::vector<double> makeData() {
    std::vector<double> v(kDataSize);
    for (int i = 0; i < kDataSize; ++i)
        v[i] = 1.0 + static_cast<double>(i) * 0.000001;
    return v;
}

static const std::vector<double> DATA = makeData();

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