putting operands in MEM even with register_operand predicate

["Sean D'Epagnier" <geckosenator at gmail dot com>]

Hi,

I have a rule in machine descriptor:

(define_insn "fract<FIXED2:mode><FIXED1:mode>2"
  [(set (match_operand:FIXED1 0 "register_operand" "")
        (fract_convert:FIXED1 (match_operand:FIXED2 1
"register_operand" "")))]
  ""
  "* return fract_out (insn, operands, 1, NULL);"
  [(set_attr "cc" "clobber")])

Basically it generates instructions for fixed point conversions, but I
found that in certain very unlikely cases, sometimes it calls
fract_out where one of the operands is in MEM and is not a register.
This caused my function to output incorrect assembly which could not
be assembled.

I found that I can fix this problem by adding "=r" and "r" as follows:

(define_insn "fract<FIXED2:mode><FIXED1:mode>2"
  [(set (match_operand:FIXED1 0 "register_operand" "=r")
        (fract_convert:FIXED1 (match_operand:FIXED2 1
"register_operand" "r")))]
  ""
  "* return fract_out (insn, operands, 1, NULL);"
  [(set_attr "cc" "clobber")])


So I'm trying to understand this since I thought register_operand was
supposed to make things be in a register... I'm sure this is easy to
explain.   Eventually I need to output instructions so one or the
other operands is allowed to be in MEM for efficiency reasons, but I
want to understand this first.

Thanks,
Sean