C++17 adds extensions for parallelism to the standard library (e.g. std::sort(std::execution::par_unseq, arr, arr + 1000), which will allow the sort to be done with multiple threads and with vector instructions).
I noticed that Microsoft's experimental implementation mentions that the VC++ compiler lacks support to do vectorization over here, which surprises me - I thought that modern C++ compilers are able to reason about the vectorizability of loops, but apparently the VC++ compiler/optimizer is unable to generate SIMD code even if explicitly told to do so. The seeming lack of automatic vectorization support contradicts the answers for this 2011 question on Quora, which suggests that compilers will do vectorization where possible.
Maybe, compilers will only vectorize very obvious cases such as a std::array<int, 4>, and no more than that, thus C++17's explicit parallelization would be useful.
Hence my question: Do current compilers automatically vectorize my code when not explicitly told to do so? (To make this question more concrete, let's narrow this down to Intel x86 CPUs with SIMD support, and the latest versions of GCC, Clang, MSVC, and ICC.)
As an extension: Do compilers for other languages do better automatic vectorization (maybe due to language design) (so that the C++ standards committee decides it necessary for explicit (C++17-style) vectorization)?