CXXFLAGS modification of Qt pro file? [duplicate]

You were very close. What you want is:

QMAKE_CXXFLAGS += -O1

If you would like to apply flags to just the release build, then you can use this:

QMAKE_CXXFLAGS_RELEASE += -O1

You also probably want to change your condition to be a little more flexible. In summary, something like this:

*-g++* {    QMAKE_CXXFLAGS += -O1}

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