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

[Dennis Luehring]

look at the "newly" added __builtin_operator_new/delete (by Richard Smith)

discussed missing heap optimization for vector and std::string
http://clang-developers.42468.n3.nabble.com/missing-optimization-opportunity-for-const-std-vector-compared-to-std-array-td4034587.html#none

resulting patches to clang/libc++ to use the optimization
http://reviews.llvm.org/rL210137
http://reviews.llvm.org/rL210211

or is it now in c++14 explicitly allowed?



Am 04.05.2015 um 12:37 schrieb 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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