Re: [PATCH-1, rs6000] Add a new type of CC mode - CCBCD for bcd insns [PR100736, PR114732]

[HAO CHEN GUI <guihaoc@linux.ibm.com>]

Hi Kewen,

在 2024/5/29 13:26, Kewen.Lin 写道:
> I can understand re-using "unordered" and "eq" will save some efforts than
> doing with unspecs, but they are actually RTL codes instead of bits on the
> specific hardware CR, a downside is that people who isn't aware of this
> design point can have some misunderstanding when reading/checking the code
> or dumping, from this perspective unspecs (with reasonable name) can be
> more meaningful.  Normally adopting RTL code is better since they have the
> chance to be considered (optimized) in generic pass/code, but it isn't the
> case here as we just use the code itself but not be with the same semantic
> (meaning).  Looking forward to others' opinions on this, if we want to adopt
> "unordered" and "eq" like what this patch does, I think we should at least
> emphasize such points in rs6000-modes.def.

Thanks so much for your comments. IMHO, the core is if we can re-define
"unordered" or "eq" for certain CC mode on a specific target. If we can't or
it's unsafe, we have to use the unspecs. In this case, I just want to define
the code "unordered" on CCBCD as testing if the bit 3 is set on this CR field.
Actually rs6000 already use "lt" code to test if bit 0 is set for vector
compare instructions. The following expand is an example.

(define_expand "vector_ae_<mode>_p"
  [(parallel
    [(set (reg:CC CR6_REGNO)
          (unspec:CC [(ne:CC (match_operand:VI 1 "vlogical_operand")
                             (match_operand:VI 2 "vlogical_operand"))]
           UNSPEC_PREDICATE))
     (set (match_dup 3)
          (ne:VI (match_dup 1)
                 (match_dup 2)))])
   (set (match_operand:SI 0 "register_operand" "=r")
        (lt:SI (reg:CC CR6_REGNO)
               (const_int 0)))
   (set (match_dup 0)
        (xor:SI (match_dup 0)
                (const_int 1)))]

I think the "lt" on CC just doesn't mean it compares if CC value is less than an
integer. It just tests the "lt" bit (bit 0) is set or not on this CC.

  Looking forward to your and Segher's further invaluable comments.

Thanks
Gui Haochen