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

[Ian Lance Taylor]

Alex <cirrus75@uol.com.br> writes:

>     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" "")
>         )
>     )
>   ]
>   ""
>   ""
> )

If you need a register, use a predicate of register_operand for one of
the operands.  Or use force_reg in the define_expand body.


>    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.

gcc is a complex tool.  There are any number of reasons why this might
happen.  If you really want to know, -da will give you a lot of dump
files to look over.

Ian