There appears to be a related bug report, Bug 59659 – large zero-initialized std::array compile time excessive. It was considered “fixed” for 4.9.0, so I consider this testcase either a regression or an edgecase not covered by the patch. For what it’s worth, two of the bug report’s test cases1, 2 exhibit symptoms for me … Read more
You can use templates to unroll. See the disassembly for the sample Live on Godbolt But -funroll-loops has the same effect for this sample. Live On Coliru template <unsigned N> struct faux_unroll { template <typename F> static void call(F const& f) { f(); faux_unroll<N-1>::call(f); } }; template <> struct faux_unroll<0u> { template <typename F> static … Read more
Loop unrolling makes sense if you can break dependency chains. This gives a out of order or super-scalar CPU the possibility to schedule things better and thus run faster. A simple example: for (int i=0; i<n; i++) { sum += data[i]; } Here the dependency chain of the arguments is very short. If you get … Read more