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Actually it's the opposite ;) All non-class types should be by value, especially on 64bits were a double fit in a register just fine. Otherwise you force the compiler to use pointer operations everytime you access your const ref, and there's a lot of optimisation it can't do because ti's a ref not a value. There's even a pretty strong case to be made to pass small structs by value. More info here in case you're interested : http://www.macieira.org/blog/2012/02/the-value-of-passing-by-value/ .
But yeah, no need to check all your code, just stop doing it, and fix whatever comes by. And, if you're not sure, Google it ;)