[Bug rtl-optimization/52235] New: rtlanal: commutative_operand_precedence should prioritize regs

["Paulo.Matos at csr dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52235

             Bug #: 52235
           Summary: rtlanal: commutative_operand_precedence should
                    prioritize regs
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: Paulo.Matos@csr.com


I noticed the combine receives insns of the form:
(set (reg ...) (ior (mem (reg ...)) (reg ...)))

(in my specific case the reg inside mem was an argument pointer that later
would be eliminated in terms of a function pointer.

After an initial discussion in the mailing list:
http://www.mail-archive.com/gcc@gcc.gnu.org/msg62132.html
Richard pointed me to commutative_operand_precedence. Both him and I expected
to see the register come first.

Having the register come first is more intuitive and under my backend it
performs much better. The whole reason to look into this stemmed from the fact
that commutative operations like ior were not being correctly combined since
the backend rule looked like:
(define_insn "iorqi3"
  [(set (match_operand:QI 0 "register_operand" "=c")
        (ior:QI (match_operand:QI 1 "register_operand" "%0")
                (match_operand:QI 2 "general_operand" "cwmi")))
   (clobber (reg:CC RCC))]
  ""
  "or\\t%0,%f2")

Makes sense for operand1 to be a register_operand because it will be forced to
be the same as operand0. However, if we can't assume registers come first a lot
of rule duplication has to occur in order to achieve the same effect. I would
guess in this case I would need to have instead of the above:
(define_insn "iorqi3"
  [(set (match_operand:QI 0 "register_operand" "=c")
        (ior:QI (match_operand:QI 1 "register_operand" "0")
                (match_operand:QI 2 "general_operand" "cwmi")))
   (clobber (reg:CC RCC))]
  ""
  "or\\t%0,%f2")

(define_insn "iorqi3"
  [(set (match_operand:QI 0 "register_operand" "=c")
        (ior:QI (match_operand:QI 1 "general_operand" "cwmi")
                (match_operand:QI 2 "register_operand" "0")))
   (clobber (reg:CC RCC))]
  ""
  "or\\t%0,%f1")

The simple fix of making registers have priority over everything else works
like a charm. Patch follows.