Which compiler do you use, and which options? The instructions are read-modify-write. A compiler with low optimisation will do 2 mov and one or at minimum. It may also add some address calculation. A good compiler should do it with CLR1 and SET1. These instructions take each 2 cycles to execute. If you push th "i" button in the debug window you can switch to assembler debug, and then you also see an assembler window.
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