uasm encode xchg eax,eax as nop (0x90) in 64 mode

[ntysdd]

I believe this is a bug that's there also in MASM.

In 64 bit mode, xchg r32, r32 generally clear upper half of 64 bit registers. So xchg a 32 bit register with itself is not a nop.

Should be encoded as "87 c0"