source: doc/theses/thierry_delisle_PhD/code/bitbench/select.cpp@ 2aa25cc2

#include "../utils.hpp"

void consume(int i, int j) __attribute__((noinline));
void consume(int i, int j) {
        asm volatile("":: "rm" (i), "rm" (i) );
}

static inline unsigned rand_bit_sw(unsigned rnum, size_t mask) {
        unsigned bit = mask ? rnum % __builtin_popcountl(mask) : 0;
        uint64_t v = mask;   // Input value to find position with rank r.
        unsigned int r = bit + 1;// Input: bit's desired rank [1-64].
        unsigned int s;      // Output: Resulting position of bit with rank r [1-64]
        uint64_t a, b, c, d; // Intermediate temporaries for bit count.
        unsigned int t;      // Bit count temporary.

        // Do a normal parallel bit count for a 64-bit integer,
        // but store all intermediate steps.
        a =  v - ((v >> 1) & ~0UL/3);
        b = (a & ~0UL/5) + ((a >> 2) & ~0UL/5);
        c = (b + (b >> 4)) & ~0UL/0x11;
        d = (c + (c >> 8)) & ~0UL/0x101;


        t = (d >> 32) + (d >> 48);
        // Now do branchless select!
        s  = 64;
        s -= ((t - r) & 256) >> 3; r -= (t & ((t - r) >> 8));
        t  = (d >> (s - 16)) & 0xff;
        s -= ((t - r) & 256) >> 4; r -= (t & ((t - r) >> 8));
        t  = (c >> (s - 8)) & 0xf;
        s -= ((t - r) & 256) >> 5; r -= (t & ((t - r) >> 8));
        t  = (b >> (s - 4)) & 0x7;
        s -= ((t - r) & 256) >> 6; r -= (t & ((t - r) >> 8));
        t  = (a >> (s - 2)) & 0x3;
        s -= ((t - r) & 256) >> 7; r -= (t & ((t - r) >> 8));
        t  = (v >> (s - 1)) & 0x1;
        s -= ((t - r) & 256) >> 8;
        return s - 1;
}

static inline unsigned rand_bit_hw(unsigned rnum, size_t mask) {
        unsigned bit = mask ? rnum % __builtin_popcountl(mask) : 0;
        uint64_t picked = _pdep_u64(1ul << bit, mask);
        return picked ? __builtin_ctzl(picked) : 0;
}

struct TLS {
        Random rng = { 6 };
} tls;

const unsigned numLists = 64;

static inline void blind() {
        int i = tls.rng.next() % numLists;
        int j = tls.rng.next() % numLists;

        consume(i, j);
}

std::atomic_size_t list_mask[7];
static inline void bitmask_sw() {
        unsigned i, j;
        {
                // Pick two lists at random
                unsigned num = ((numLists - 1) >> 6) + 1;

                unsigned ri = tls.rng.next();
                unsigned rj = tls.rng.next();

                unsigned wdxi = (ri >> 6u) % num;
                unsigned wdxj = (rj >> 6u) % num;

                size_t maski = list_mask[wdxi].load(std::memory_order_relaxed);
                size_t maskj = list_mask[wdxj].load(std::memory_order_relaxed);

                unsigned bi = rand_bit_sw(ri, maski);
                unsigned bj = rand_bit_sw(rj, maskj);

                i = bi | (wdxi << 6);
                j = bj | (wdxj << 6);
        }

        consume(i, j);
}

static inline void bitmask_hw() {
        #if !defined(__BMI2__)
                #warning NO bmi2 for pdep rand_bit
                return;
        #endif
        unsigned i, j;
        {
                // Pick two lists at random
                unsigned num = ((numLists - 1) >> 6) + 1;

                unsigned ri = tls.rng.next();
                unsigned rj = tls.rng.next();

                unsigned wdxi = (ri >> 6u) % num;
                unsigned wdxj = (rj >> 6u) % num;

                size_t maski = list_mask[wdxi].load(std::memory_order_relaxed);
                size_t maskj = list_mask[wdxj].load(std::memory_order_relaxed);

                unsigned bi = rand_bit_hw(ri, maski);
                unsigned bj = rand_bit_hw(rj, maskj);

                i = bi | (wdxi << 6);
                j = bj | (wdxj << 6);
        }

        consume(i, j);
}

struct {
        const unsigned mask = 7;
        const unsigned depth = 3;
        const uint64_t indexes = 0x0706050403020100;
        uint64_t masks( unsigned node ) {
                return 0xff00ffff00ff;
        }
} snzm;
static inline void sparsemask() {
        #if !defined(__BMI2__)
                #warning NO bmi2 for sparse mask
                return;
        #endif
        unsigned i, j;
        {
                // Pick two random number
                unsigned ri = tls.rng.next();
                unsigned rj = tls.rng.next();

                // Pick two nodes from it
                unsigned wdxi = ri & snzm.mask;
                unsigned wdxj = rj & snzm.mask;

                // Get the masks from the nodes
                size_t maski = snzm.masks(wdxi);
                size_t maskj = snzm.masks(wdxj);

                uint64_t idxsi = _pext_u64(snzm.indexes, maski);
                uint64_t idxsj = _pext_u64(snzm.indexes, maskj);

                auto pi = __builtin_popcountll(maski);
                auto pj = __builtin_popcountll(maskj);

                ri = pi ? ri & ((pi >> 3) - 1) : 0;
                rj = pj ? rj & ((pj >> 3) - 1) : 0;

                unsigned bi = (idxsi >> (ri << 3)) & 0xff;
                unsigned bj = (idxsj >> (rj << 3)) & 0xff;

                i = (bi << snzm.depth) | wdxi;
                j = (bj << snzm.depth) | wdxj;
        }

        consume(i, j);
}

template<typename T>
void benchmark( T func, const std::string & name ) {
        std::cout << "Starting " << name << std::endl;
        auto before = Clock::now();
        const int N = 250'000'000;
        for(int i = 0; i < N; i++) {
                func();
        }
        auto after = Clock::now();
        duration_t durr = after - before;
        double duration = durr.count();
        std::cout << "Duration(s) : " << duration << std::endl;
        std::cout << "Ops/sec     : " << uint64_t(N / duration) << std::endl;
        std::cout << "ns/Op       : " << double(duration * 1'000'000'000.0 / N) << std::endl;
        std::cout << std::endl;
}

int main() {
        std::cout.imbue(std::locale(""));

        benchmark(blind, "Blind guess");
        benchmark(bitmask_sw, "Dense bitmask");
        benchmark(bitmask_hw, "Dense bitmask with Parallel Deposit");
        benchmark(sparsemask, "Parallel Extract bitmask");
}