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.
Reported to Microsoft: https://developercommunity.visualstudio.com/content/problem/801905/19151916-regression-constructor-call-to-stdcomplex.html
Even more reduced example: https://godbolt.org/z/47AJDU I.e., just a struct with four std::complex<double> does not get optimized away either when compiled in C++17 mode. I don't see any possible workaround, except storing `std::complex<double>[size]` as `double[2*size]` or trying to provide our own `Eigen::complex` type. Both sound like a lot of effort just for a workaround. We may document this as "known issue" (not sure if anyone actually looks here): http://eigen.tuxfamily.org/index.php?title=FAQ#Known_MSVC_issues
I've added an entry to the FAQ. We cannot do better, so closing.
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