> For instance, NaN-tagging prevents (or largely complicates) optimizations relying on stack allocations. The stack uses high memory addresses that do not fit in 48 bits unless encoded relative to the location of the stack segment.

Er, what? The paper says they tested on a Xeon CPU, so x86-64, running Linux. On traditional x86-64, all pointers fit in 48 bits, period. Stack memory is no exception. More recently the architecture was extended to allow 56-bit pointers, but my impression is that Linux (like other OSes) keeps them disabled by default in userspace. According to the documentation [1]:

> Not all user space is ready to handle wide addresses. [..] To mitigate this, we are not going to allocate virtual address space above 47-bit by default.

So how would the stack end up above 47 bits? Is the documentation out of date?

[1] https://docs.kernel.org/arch/x86/x86_64/5level-paging.html