Re: Unoptimal code.

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Dec 10, 2013 at 4:03 PM, Umesh Kalappa <umesh.kalappa0@gmail.com> wrote:
> Hi All,
>
> Below is the patterns defined  for the  mov and add  instruction
> .
> [(set (match_operand:HI 0 "general_mov_operand" "=r,rRA")
>         (match_operand:HI 1 "general_mov_operand" "rRAi,ri"))]
>   ""
>          {
>
>         }
> )
>
> (define_insn "addhi3"
>   [(set (match_operand:HI 0 "register_operand" "=Ar")
>         (plus:HI (match_operand:HI 1 "register_operand" "%0")
>                  (match_operand:HI 2 "general_mov_operand" "Ar")))]
>   ""
>   "add\t%0, (%2)"
> )
>
> The problem we stuck with is that the compiler emit unoptimal code for
> the below testcase with -O0 option
>
> int a,b;
>
> int func()
> {
>    return a=b;
> }
>
> .s file
>
>  ld      BC, (a)
>  ld      WA, (b)
>  add     WA, BC
>  ld      (a), WA
>  ret
>
> the compiler try to load a and b to the register BC and WA
> respectively in the expand_assignment and add them , then store back
> the result to a.
>
> But  if you see the addhi3 definition ,it states that i'm allowed to
> emit instruction like
>
> add WA,(a)
>
> where second operand can be register indirect addressing .
>
> I can write peephole pattern to optimize  the emitted code like
>
> .s file
>
>  ld      WA, (b)
>  add     WA, (a)
>  ld      (a), WA
>  ret
>
>
> the reason for the unoptimal  code is that  the code is expanded to
> load  the memory contents  to the registers  and then update the add
> operands accordingly. I don't want this to happen .
>
> I will be glad ,if somebody from the group  share their experience or
> through some insights  how i can achieve this .

Use -On with n > 0.

Richard.

> Thanks
> ~Umesh