[LLVMdev] Memory Allocation Optimized away with new by not with ::operator new

[François Fayard]

Hi,

I’ve made my own version of std::vector which is called il::Vector. Due to http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3664.html <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3664.html>, LLVM can optimise away memory allocation. Therefore, the following code optimise away all memory allocation for w resulting in a single allocation during the whole program (for v).

When using my own vector implementation, I realised that the allocation were not optimized away because I was using ::operator new. When I’ve switched back to new, the optimisation came back.

Is it expected or a bug from LLVM?

François

=====

#include <iostream>
#include <vector>


std::vector<double> f_val(std::size_t i, std::size_t n) {
    auto v = std::vector<double>(n);
    for (std::size_t k = 0; k < v.size(); ++k) {
        v[k] = static_cast<double>(i);
    }
    return v;
}

int main (int argc, char const *argv[])
{
    const auto n = std::size_t{10};
    const auto nb_loops = std::size_t{300000000};

    auto v = std::vector<double>( n, 0.0 );
    for (std::size_t i = 0; i < nb_loops; ++i) {
        auto w = f_val(i, n);
        for (std::size_t k = 0; k < v.size(); ++k) {
            v[k] += w[k];
        }
    }
    std::cout << v[0] << " " << v[n - 1] << std::endl;

    return 0;
}

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