Question

GCC’s vector extensions offer a nice, reasonably portable way of accessing some SIMD instructions on different hardware architectures without resorting to hardware specific intrinsics (or auto-vectorization).

A real use case, is calculating a simple additive checksum. The one thing that isn’t clear is how to safely load data into a vector.

typedef char v16qi __attribute__ ((vector_size(16)));

static uint8_t checksum(uint8_t *buf, size_t size)
{
    assert(size%16 == 0);
    uint8_t sum = 0;

    vec16qi vec = {0};
    for (size_t i=0; i<(size/16); i++)
    {
        // XXX: Yuck! Is there a better way?
        vec += *((v16qi*) buf+i*16);
    }

    // Sum up the vector
    sum = vec[0] + vec[1] + vec[2] + vec[3] + vec[4] + vec[5] + vec[6] + vec[7] + vec[8] + vec[9] + vec[10] + vec[11] + vec[12] + vec[13] + vec[14] + vec[15];

    return sum;
}

Casting a pointer to the vector type appears to work, but I’m worried this might explode in a horrible fashion if SIMD hardware expects the vector types to be correctly aligned.

The only other option I’ve thought of is use a temp vector and explicitly load the values (via either a memcpy or element-wise assignment), but in testing this counteract most of speedup gained use of SIMD instructions. Ideally I’d imagine this would be something like a generic __builtin_load() function, but none seems to exist.

What’s a safer way of loading data into a vector risking alignment issues?

Answer 1

You could use an initializer to load the values, i.e. do

const vec16qi e = { buf[0], buf[1], ... , buf[15] }

and hope that GCC turns this into a SSE load instruction. I’d verify that with a dissassembler, though ;-). Also, for better performance, you try to make buf 16-byte aligned, and inform that compiler via an aligned attribute. If you can guarantee that the input buffer will be aligned, process it bytewise until you’ve reached a 16-byte boundard.