From build log, e.g. : g++ -I. -I. -I/usr/lib64/ruby/1.8/x86_64-linux -I. -DHAVE_SYS_TIME_H -DHAVE_SIGNAL_H -I/usr/local/include/fxscintilla -I/usr/local/include/fox-1.6 -I/usr/include/fox-1.6 -I/usr/include/fxscintilla -fPIC -march=native -O2 -g0 -fno-strict-aliasing -fPIC -O0 -I./include -DWITH_FXSCINTILLA -DHAVE_FOX_1_6 -c scintilla_wrap.cpp So, as you see the last -O0 overwrites my -O2
Fixed in fxruby-1.6.20-r1