Derive more alias information from named address space

[Bingfeng Mei]

Hi,
I am trying to implement named address space for our target. 

In alias.c,  I found the following piece of code several times. 

  /* If we have MEMs refering to different address spaces (which can
     potentially overlap), we cannot easily tell from the addresses
     whether the references overlap.  */
  if (MEM_ADDR_SPACE (mem) != MEM_ADDR_SPACE (x))
    return 1;

I think we can do better with the existing target hook:

- Target Hook: bool TARGET_ADDR_SPACE_SUBSET_P (addr_space_t superset, 
addr_space_t subset)

If A is not subset of B and B is not subset of A, we can conclude
they are either disjoint or overlapped. According to standard draft 
(section 3.1.3),

"For any two address spaces, either the address spaces must be
disjoint, they must be equivalent, or one must be a subset of
the other. Other forms of overlapping are not permitted."

Therefore, A & B could only be disjoint, i.e., not aliased to each other.
We should be able to write: 

  if (MEM_ADDR_SPACE (mem) != MEM_ADDR_SPACE (x))
  {
    if (!targetm.addr_space.subset_p (MEM_ADDR_SPACE (mem), MEM_ADDR_SPACE (x))
       && !targetm.addr_space.subset_p (MEM_ADDR_SPACE (x), MEM_ADDR_SPACE 
(mem)))
      return 0;
    else
      return 1;
  }

Is this correct?

Thanks,
Bingfeng Mei