[cfe-dev] Vectorizer and buffer overlaps

[Nicola Gigante]

Hello

I feel like this is a trivial question but I can’t find an answer.

I’m looking at the llvm vectorizer to learn how to better
take advantage of it.

I have this function:

void product(float *data1, float *data2, float *result, size_t size) {
    size_t i = 0;
    for(i = 0; i < size; i++) {
        result[i] = data1[i] * data2[i];
    }
}

It seems it correctly get vectorized with -O3, but I can see
the vector.memcheck block that looks for overlapping buffers.

How can I inform the compiler that I know the buffer won’t overlap?

Simulating a real case (kind of), I’ve tried something like this:
struct mass_tag;
struct acceleration_tag;
struct force_tag;

template<typename T, typename>
class wrapper {
    T _v;
public:
    wrapper(T v) : _v(v) { }
    operator T() const { return _v; }
};

using mass = wrapper<float, mass_tag>;
using acceleration = wrapper<float, acceleration_tag>;
using force = wrapper<float, force_tag>;

void product(mass *data1, acceleration *data2,
             force *result, size_t size) {
    size_t i = 0;
    for(i = 0; i < size; i++) {
        result[i] = data1[i] * data2[i];
    }
}

The concept here reminds something like Boost.Unit.
I thought that being the pointers of different type, 
type based alias analysis would say that they don’t overlap,
but I’m missing something since the memcheck block is still
there. I’m compiling with clang 3.4 with -O3 -fstrict-aliasing, isn’t it enough?

Even this version has the memcheck block:
void product(std::array<mass, 32> data1, std::array<acceleration, 32> data2, std::array<force, 32> &result) {
    size_t i = 0;
    for(i = 0; i < 32; i++) {
        result[i] = data1[i] * data2[i];
    }
}

Inputs arrays are copied (unrealistic code)... Why does it check for overlaps?
I feel like I’m missing something obvious.
So how do I inform the compiler that it doesn’t need che memcheck block?

Thank you very much,
Nicola