[Bug target/108004] x-form logical operations with dot instructions are not emitted.

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108004

Segher Boessenkool <segher at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
         Resolution|INVALID                     |---
             Status|RESOLVED                    |SUSPENDED
   Last reconfirmed|                            |2023-01-09

--- Comment #7 from Segher Boessenkool <segher at gcc dot gnu.org> ---
It's not really invalid, but it won't happen any time soon.

The upper bits *are* defined for argument passing, in all our 64-bit ABIs:
for signed type (like here the value is passed sign-extended.  But the code
has "(a & b) > 0" which does the comparison as an int.  In combine we get

Trying 11 -> 14:
   11: r124:SI=r129:DI#4&r130:DI#4
      REG_DEAD r130:DI
      REG_DEAD r129:DI
   14: r125:CC=cmp(r124:SI,0)
      REG_DEAD r124:SI
Failed to match this instruction:
(set (reg:CC 125)
    (compare:CC (and:SI (subreg:SI (reg:DI 129) 4)
            (subreg:SI (reg:DI 130) 4))
        (const_int 0 [0])))

If we upgraded some stuff to DImode instead of SImode, sometimes we can
make better code, like we could here.  But in other cases the opposite is
true.  I think it is likely it helps more often than it would hurt, and we
can upgrade the mode only sometimes as well of course.

In any case, this is just a special case of a much more generic problem
(in all ports, not just rs6000!), that has been known for a very long time,
and no real progress has been made yet.  But it definitely should be doable.
To simplify the problem a lot it probably is okay to only consider upgrading
the mode of a pseudo everywhere (so not do it in some insns but not others),
and then assign a score to it.  Probably a higher score inside loops, that
is the case where we see this most / where we see it as a shortcoming most.

Where rs6000 is special here is that we have "w" and "d" (32-bit and 64-bit)
variants of many insns (but no smaller versions most of the time fwiw).