[Bug sanitizer/85230] asan: false positives in kernel on allocas

[chefmax at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85230

--- Comment #14 from chefmax at gcc dot gnu.org ---
(In reply to Jakub Jelinek from comment #13)
> or introduce a new virtual pseudo register that vregs pass would map directly to dynamic_offset.

Yeah, that's what I though about (LLVM does pretty the same thing). But (new_sp
+ virtual_stack_dynamic_rtx - stack_pointer_rtx) seems like an appropriate
solution too. I'll cover the testing for both approaches.