When to use XOR R32 , R64 in Silvermont?

In 64bit mode, still use xor r32, r32, because writing a 32-bit reg zeros the upper 32. xor r64, r64 is a waste of a byte, because it needs a REX prefix. Even worse than that, Silvermont only recognizes xor r32,r32 as dep-breaking, not 64-bit operand-size.
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