llvm dev - May 2015 - [LLVMdev] Memory Allocation Optimized away with new by not with ::operator new

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 Fayard
Founder & Consultant - Inside Loop
Tel: +33 (0)6 01 44 06 93 <tel:+33%206%2001%2044%2006%2093>
Web: www.insideloop.io <http://www.insideloop.io/>
Twitter: @insideloop_io

====
#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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Didn't we just have this discussion on cfe-dev? (or is there something
different I've missed?)

On Mon, May 4, 2015 at 2:31 AM, François Fayard <fayard at insideloop.io>
wrote:
> 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, 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 Fayard
> Founder & Consultant - Inside Loop
> Tel: +33 (0)6 01 44 06 93 <+33%206%2001%2044%2006%2093>
> Web: www.insideloop.io
> Twitter: @insideloop_io
>
> ====>
> #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;
> }
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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I’ve missed it. I did a quick search and the only thread I found is back from
2013. Could you please provide me with a link to the thread?

François Fayard
Founder & Consultant - Inside Loop
Tel: +33 (0)6 01 44 06 93 <tel:+33%206%2001%2044%2006%2093>
Web: www.insideloop.io <http://www.insideloop.io/>
Twitter: @insideloop_io
> On 06 May 2015, at 07:21, David Blaikie <dblaikie at gmail.com>
wrote:
> 
> Didn't we just have this discussion on cfe-dev? (or is there something
different I've missed?)
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