I compiled GMP 5.0.1 on an intel quad core Q9450 with WIndows XP 64bit on Msys / Mingw64 using: ./configure --build=core2-pc-mingw32 make make check My bug in t-perfpow.exe was fixed with this: http://gmplib.org:8000/gmp-5.0/raw-rev/794410151f5f but make check exits with a new error: http://www.hoegge.dk/mersenne/makecheck.txt Andreas Höglund.