gcc backend beginner: only -O1 gives 'unrecongnizable insn' error

[Alex <cirrus75 at uol dot com dot br>]

    Hi,

    I am a beginner on gcc backend. I was performing some experiments 
on .md file using a completly new backend set of files written by me. I 
end up on something I really could not understand. Regarding addition, I 
put the following code on my .md file:

(define_expand "addhi3"
  [(set (match_operand:HI 0 "nonimmediate_operand" "")
        (plus:HI (match_operand 1 "general_operand" "")
                 (match_operand 2 "general_operand" "")
        )
    )
  ]
  ""
  ""
)

(define_insn "*addhi3_2"
    [(set (match_operand:HI 0 "nonimmediate_operand" "=r,r,d")
              (plus:HI (match_operand:HI 1 "register_operand" "r,r,0")
                       (match_operand:HI 2 "nonmemory_operand" "r,K,K")
              )
   )]
  ""
  "@
   add \\t%0, %1, %2
   add \\t%0, %1, #%c2
   add \\t%0, #%c2"
)

   Using this code, gcc gives the following error only when I use -O1 
(for -O0, -O2 it works OK!):

(insn 16 14 17 0 (set (reg:HI 12)
        (plus:HI (mem/i:HI (symbol_ref:HI ("b") <var_decl 0x7fdad4a0 
b>) [0 b+0 S2 A16])
            (mem/i:HI (symbol_ref:HI ("a") [flags 0x2] <var_decl 
0x7fdad430 a>) [0 a+0 S2 A16]))) -1 (nil)
    (nil))

   I understood that gcc would like to have a insn which accepts "reg = 
mem + mem", but I really can not understand why gcc works OK with -O2 
and with -O0, but not with -O1.

thank you!
Alex Prado