Visual Studio Optimization flag

How could add optimization flag -O3 for device code?
Is it enabled by default?

Yes , the default level is -O3 , see NVCC :: CUDA Toolkit Documentation .
You can specify optimization levels in Property Page | CUDA C/C++ | Command Line | Additional Option | ‘-Xptxas -Ox’

Thank you very much.
How could remove any -Ox optimization?

‘-Xptxas -O0’ turns off optimization .

Thank you again.