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17.23.1.3 Mode Iterator Examples

Here is an example from the MIPS port. It defines the following modes and attributes (among others): 

(define_mode_iterator GPR [SI (DI "TARGET_64BIT")])
(define_mode_attr d [(SI "") (DI "d")])

and uses the following template to define both subsi3 and subdi3: 

(define_insn "sub<mode>3"
  [(set (match_operand:GPR 0 "register_operand" "=d")
        (minus:GPR (match_operand:GPR 1 "register_operand" "d")
                   (match_operand:GPR 2 "register_operand" "d")))]
  ""
  "<d>subu\t%0,%1,%2"
  [(set_attr "type" "arith")
   (set_attr "mode" "<MODE>")])

This is exactly equivalent to: 

(define_insn "subsi3"
  [(set (match_operand:SI 0 "register_operand" "=d")
        (minus:SI (match_operand:SI 1 "register_operand" "d")
                  (match_operand:SI 2 "register_operand" "d")))]
  ""
  "subu\t%0,%1,%2"
  [(set_attr "type" "arith")
   (set_attr "mode" "SI")])

(define_insn "subdi3"
  [(set (match_operand:DI 0 "register_operand" "=d")
        (minus:DI (match_operand:DI 1 "register_operand" "d")
                  (match_operand:DI 2 "register_operand" "d")))]
  ""
  "dsubu\t%0,%1,%2"
  [(set_attr "type" "arith")
   (set_attr "mode" "DI")])