Mingw-w64 is an advancement of the original mingw.org project, created to support the GCC compiler on Windows systems. It has forked it in 2007 in order to provide support for 64 bits and new APIs. It has since then gained widespread use and distribution.
The development and community are very active and welcoming with new contributors every month and simple installers.
You can also look at the full list of versions.
Mingw-w64 interacts a lot with other projects in order to help everyone move forward. Contributions have been going to and coming from these projects:
6.0 released but can't get new binaries with <filesystem> includable
Compiler optimizations are different between SSE2 and x87 instructions at the -O0 and -O2 levels. I gave you the cause, it is a rounding error introduced by a floating point coprocessor at a place beyond the IEEE significant digit area due to probably floating point constants and loop optimizations. There is no bug, except in the code, by using a transfinite variant that is not increasing monotonically due to the floating point rounding error. Even then, the optimizer has optimized out certain things...
mingw-w64-headers: Bump __MINGW64_VERSION_MAJOR for new release