clang-bad-optimization.txt

Clang failed to vectorize the following code:

milovidov@milovidov-desktop:~/work/ClickHouse$ cat xors.cpp
#include <cstddef>
#include <vector>

__attribute__((__noinline__))
void xor10arrays(
    const char8_t * arr0,
    const char8_t * arr1,
    const char8_t * arr2,
    const char8_t * arr3,
    const char8_t * arr4,
    const char8_t * arr5,
    const char8_t * arr6,
    const char8_t * arr7,
    const char8_t * arr8,
    const char8_t * arr9,
    char8_t * res,
    size_t size)
{
    for (size_t i = 0; i < size; ++i)
    {
        res[i] = (((((((((
               arr0[i]
            != arr1[i])
            != arr2[i])
            != arr3[i])
            != arr4[i])
            != arr5[i])
            != arr6[i])
            != arr7[i])
            != arr8[i])
            != arr9[i]);
    }
}


int main(int, char **)
{
    size_t size = 65536;
    std::vector<char8_t> data(size * 11);

    size_t res = 0;
    for (size_t i = 0; i < 10000; ++i)
    {
        xor10arrays(
            &data[0],
            &data[size],
            &data[size * 2],
            &data[size * 3],
            &data[size * 4],
            &data[size * 5],
            &data[size * 6],
            &data[size * 7],
            &data[size * 8],
            &data[size * 9],
            &data[size * 10],
            size);

        res += data[size * 10];
    }

    return res;
}
milovidov@milovidov-desktop:~/work/ClickHouse$ g++-9 -std=c++2a -O3 -g -msse4.2 xors.cpp && time ./a.out 

real    0m0,591s
user    0m0,586s
sys     0m0,004s
milovidov@milovidov-desktop:~/work/ClickHouse$ clang++-10 -std=c++2a -O3 -g -msse4.2 xors.cpp && time ./a.out 

real    0m2,403s
user    0m2,402s
sys     0m0,000s