It's really unfortunate that all of the mainstream OSes run userspace in the lower portions of the address space.
Setting the most-significant 13 bits (really, setting the second to 12th bits and at least one of the following bits) of an IEEE-754 float will result in a NaN bit pattern. That means that any pointer to the top 2 petabytes of a 64-bit address space, if cast to an IEEE-754 double will be NaN. This means the NaN-boxing used in Safari and Firefox's JavaScript engines, LuaJIT, etc. would be no-ops. (Safari and Firefox use different mechanisms, but they'd become the same if moved to the top of the address space.)
It's not enough of a performance difference to re-jigger everything in mainstream OSes, but I imagine if someone were to come up with a unikernel/exokernel OS specifically for JITing some dynamic language, there's some performance to be had by having all of the dynamic language objects in the upper 2 petabytes of the address space.
Setting the most-significant 13 bits (really, setting the second to 12th bits and at least one of the following bits) of an IEEE-754 float will result in a NaN bit pattern. That means that any pointer to the top 2 petabytes of a 64-bit address space, if cast to an IEEE-754 double will be NaN. This means the NaN-boxing used in Safari and Firefox's JavaScript engines, LuaJIT, etc. would be no-ops. (Safari and Firefox use different mechanisms, but they'd become the same if moved to the top of the address space.)
It's not enough of a performance difference to re-jigger everything in mainstream OSes, but I imagine if someone were to come up with a unikernel/exokernel OS specifically for JITing some dynamic language, there's some performance to be had by having all of the dynamic language objects in the upper 2 petabytes of the address space.