https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81611
--- Comment #15 from Alexandre Oliva <aoliva at gcc dot gnu.org> --- As we create_mem_ref within ivopts, create_mem_ref_raw requires a valid_mem_ref_p, which in memory_address_addr_space_p calls targetm.addr_space.legitimate_address_p, and that's avr_addr_space_legitimate_address_p, that calls avr_legitimate_address_p, that rejects negative offsets to a REG. That's where avr loses, compared with x86 and arm. Because of this rejected address offset, we end up creating _16 = str_11 + 65535. That eventually gets simplified to _16 = str_6 (in forwprop, no less), but the str_11 = str_6 + 1 stmt remains because of the use in the phi node, and it appears before the MEM_REF. Now, the reason we validate the address is that create_mem_ref calls create_mem_ref_raw with verify enabled. Even though the letter might end up creating a machine-independent MEM_REF, we end up constraining it according to machine-dependent addressing limitations anyway.
