Adding -O3 compiler option in Qt Creator

[stampede]

You can always check the command used to invoke the compiler, eg

g++ -c -O3 -g -frtti -fexceptions -mthreads -Wall -DUNICODE -DQT_LARGEFILE_SUPPO
RT -DQT_DLL -DQT_SQL_LIB -DQT_GUI_LIB -DQT_CORE_LIB -DQT_HAVE_MMX -DQT_HAVE_3DNO
W -DQT_HAVE_SSE -DQT_HAVE_MMXEXT -DQT_HAVE_SSE2 -DQT_THREAD_SUPPORT -I'../../../
../Qt/include/QtCore' -I'../../../../Qt/include/QtGui' -I'../../../../Qt/include
/QtSql' -I'../../../../Qt/include' -I'.' -I'../../../../Qt/include/ActiveQt' -I'
debug' -I'../../../../Qt/mkspecs/default' -o debug/main.o main.cpp

g++ -c -O3 -g -frtti -fexceptions -mthreads -Wall -DUNICODE -DQT_LARGEFILE_SUPPO
RT -DQT_DLL -DQT_SQL_LIB -DQT_GUI_LIB -DQT_CORE_LIB -DQT_HAVE_MMX -DQT_HAVE_3DNO
W -DQT_HAVE_SSE -DQT_HAVE_MMXEXT -DQT_HAVE_SSE2 -DQT_THREAD_SUPPORT -I'../../../
../Qt/include/QtCore' -I'../../../../Qt/include/QtGui' -I'../../../../Qt/include
/QtSql' -I'../../../../Qt/include' -I'.' -I'../../../../Qt/include/ActiveQt' -I'
debug' -I'../../../../Qt/mkspecs/default' -o debug/main.o main.cpp

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You can see the -O3 flag added to compiler options.
Btw. it worked with the -j switch, because -j is the make command option, whereas "-O3" is the compiler option.