> So does it really matter if the upper 32bits are empty from a performance standpoint? Yes! main() would push %eax instead of %rax, effectively doubling your

> So does it really matter if the upper 32bits are empty from a performance standpoint? Yes! main() would push %eax instead of %rax, effectively doubling your cache size (in words). > I don't see how this will work without recompiling every library The x86_64 abi uses registers for the arguments, so fopen() would receive file in %rdi, and mode in %rsi, but the caller only used %edi and %esi. When fopen() calls malloc() our libc knows we want 32-bit pointers, so it uses mmap(MAP_32BIT) so the pointer has the top bits clear and thus when fopen() returns the pointer in %rax, main() can still store it in a 32-bit pointer reading %eax. Structs are certainly a problem. If main() tries to look e.g. at struct utsname, it would need to know (in this case) that these are "long" pointers rather than short ones. This pain would need to be handled by main() (for example, using wrappers of some kind), but it wouldn't require building or maintaining two sets of the libraries.