## Submitted by Christoph Hertzberg @chhtz Assigned to **Nobody** **[Link to original bugzilla bug (#1765)](https://eigen.tuxfamily.org/bz/show_bug.cgi?id=1765)** **Version**: 3.3 (current stable) **Operating system**: Windows ## Description Reported via mail: > In some of our code using Eigen, we got a 50% increase in runtime > when updating from C++14 to C++17. > We are using Eigen 3.3.7 and MSVC 19.23. > > Below is a small example, which gives different assembly on Godbolt.org, > when compiled with > * MSVC 19.22 /std:c++14 /O2 /Ob2 /GS- > * MSVC 19.22 /std:c++17 /O2 /Ob2 /GS- > > It seems that with C++17, the constructor call is not optimized so the > runtime is much larger. Do you know what is going on? > > #include <Eigen/Core> > int main() > { > for (size_t i = 0; i < 0.3e9; ++i) > { > Eigen::Matrix2cd R; > } > return 0; > } I confirm this, even with /DNDEBUG, but it seems to be a regression from MSVC 19.15 to 19.16 (the constructor of std::complex does not get optimized away): https://godbolt.org/z/7WDacA Possible workaround: Add more force-inlines at some places. ### Blocking #1608