[Bug tree-optimization/54900] write introduction incorrect wrt the C11 memory model (2)

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54900

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |aldyh at gcc dot gnu.org,
                   |                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-10-11 11:59:14 UTC ---
This is ifcvt.c in action.
This is the if (!set_b && MEM_P (orig_x)) case where we already do some checks:
if (noce_mem_write_may_trap_or_fault_p (orig_x)) return FALSE; and
if (!noce_can_store_speculate_p (test_bb, orig_x)) return FALSE;
I'd say noce_can_store_speculate_p is buggy, it uses
          if (memory_modified_in_insn_p (mem, insn))
            return true;
but memory_modified_in_insn_p is pessimistic, it doesn't tell whether mem is
surely set, but whether it might be set.  I guess it would need to use
note_stores that would just do rtx_equal_p on the addresses or similarly prove
it is surely (and unconditionally) written.  So even note_stores might not be
the right thing, perhaps just looking at single_set SET_DEST.  And avoiding
inline asm, that doesn't have to store unconditionally.