Conditional Write: Two Control-Flow Styles

Question 7 / 51 • Correct so far: 0 (0 answered)

Snippet A

Branch Write

std::size_t filterAbove(const std::vector<int>& data,
                        std::vector<int>& out, int threshold) {
    std::size_t n = 0;
    for (int x : data) {
        if (x >= threshold) out[n++] = x;
    }
    return n;
}

std::size_t n = filterAbove(DATA, OUTPUT, kThreshold);
Snippet B

Branchless Write

std::size_t filterAbove(const std::vector<int>& data,
                        std::vector<int>& out, int threshold) {
    std::size_t n = 0;
    for (int x : data) {
        out[n] = x;
        n += (x >= threshold);
    }
    return n;
}

std::size_t n = filterAbove(DATA, OUTPUT, kThreshold);
Shared test data (shared-setup) 
constexpr int kDataSize  = 32768;
constexpr int kThreshold = 128;

static std::vector<int> makeRandomData() {
    std::mt19937 rng(42);
    std::uniform_int_distribution<int> dist(0, 255);
    std::vector<int> v(kDataSize);
    for (auto& x : v) x = dist(rng);
    return v;
}

static const std::vector<int> DATA = makeRandomData();
static std::vector<int> OUTPUT(kDataSize);

Which snippet is faster?

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