It wasn't an efficiency issue (for a change:), it was my other windmill, that of elegance, 'once and once only', duplcated code is two chances to get wrong etc.

Realise that there i no way to avoid (in assembler terms), loading the two values, comparing them, branching if condition not satisfied, do the assignment.

But avoiding the need for the programmer to type the terms twice each as well as the potential for invoking the rvalue expression twice when it could be a volatile function or one with side effects appealled to me.