I have a (likely silly) question for anyone well versed in low level CPU stuff. §20.7 says there's a stack engine that optimizes manipulation of the stack pointer. Does this only apply to the dedicated hardware register (ie %rsp) or to other registers as well?

(Potentially related, assuming it's of benefit are modern compilers smart enough to repurpose %rsp (is this even allowed?) if I use a block of memory as a stack inside a hot loop?)