> now the null pointer is required to be canonical
Yikes! This particular oddity seems annoying but sort of harmless in x86 real mode, but not necessarily in protected mode. Imagine code that wants to load a pointer into a register: it loads the offset into an ordinary register and the selector portion into a segment register. It’s permissible to load the 0 (null) selector, but loading garbage will fault immediately. So, if you allow non canonical NULL, then knowing that a pointer is either valid or NULL does not allow you to hoist a segment load above a condition that might mean you never actually dereference the pointer.
(I have plenty of experience with low-level OS code in all kinds of nasty x86 modes but, thankfully, not so much experience writing ordinary C code targeting protected mode. It sometimes boggles my mind that anyone ever got decent performance with anything involving far data pointers. Segment loads are slow, and there are not a lot of segment registers to go around.)
In real mode assembly days, ES and sometimes DS were just another base register that you could use in a loop. Given the dearth of addressing modes it was quite nice to assume that large arrays started at xxxx0h and therefore that the offset part of the far pointer was zero.
Yikes! This particular oddity seems annoying but sort of harmless in x86 real mode, but not necessarily in protected mode. Imagine code that wants to load a pointer into a register: it loads the offset into an ordinary register and the selector portion into a segment register. It’s permissible to load the 0 (null) selector, but loading garbage will fault immediately. So, if you allow non canonical NULL, then knowing that a pointer is either valid or NULL does not allow you to hoist a segment load above a condition that might mean you never actually dereference the pointer.
(I have plenty of experience with low-level OS code in all kinds of nasty x86 modes but, thankfully, not so much experience writing ordinary C code targeting protected mode. It sometimes boggles my mind that anyone ever got decent performance with anything involving far data pointers. Segment loads are slow, and there are not a lot of segment registers to go around.)