On Thu, 29 Jul 2021, Florian Weimer via Gcc wrote: > On GNU/Linux, SEGFS is used to implement the thread pointer, to avoid > dedicating a general-purpose register to it. At address zero with the > SEGFS prefix, the offset itself is stored so that userspace can read it > without having to call into the kernel. So the SEGFS null pointer is a > valid address, and so are some bytes after it (depending on TCB layout, > some of which is specified by the ABI or is part of the de-facto ABI > used by GCC).
That suggests that we need a target hook to describe null pointer properties for a given address space. In an address space where null pointers are valid to dereference, there should be no diagnostics for arithmetic on / dereferencing them - and more generally, -fno-delete-null-pointer-checks should be in effect for pointers to such an address space (so I don't think this is just a warning issue, you can probably get wrong code from null pointer check deletion in such an address space). -- Joseph S. Myers jos...@codesourcery.com
