i386.md comparision pattern confusion / questions

[John Wehle]

I'm slightly confused by the following pattern from i386.md:

  (define_insn "cmpsi_1"
    [(set (cc0)
	  (compare (match_operand:SI 0 "nonimmediate_operand" "mr,r")
		   (match_operand:SI 1 "general_operand" "ri,mr")))]
    "GET_CODE (operands[0]) != MEM || GET_CODE (operands[1]) != MEM"
    "*
  {
    if (CONSTANT_P (operands[0]) || GET_CODE (operands[1]) == MEM)
      {
        cc_status.flags |= CC_REVERSED;
        return AS2 (cmp%L0,%0,%1);
      }
    return AS2 (cmp%L0,%1,%0);
  }")

What I'm confused about is:

  1) When will CONSTANT_P (operands[0]) ever be true?  The predicate
     requires a nonimmediate operand, doesn't this imply that operand
     0 can't be a constant?  I'm assuming here that CONSTANT_P is
     only true if operand 0 is a constant.

  2) Why is it neccessary to reverse the comparison if operand 1
     refers to memory?  My impression from the Intel Instruction Set
     reference is that cmp will accept a memory operand for either
     operand 0 or operand 1 when the other operand is a register.

 -- John
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