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Message-ID: <4D832F02.6080508@gmail.com>
Date: Fri, 18 Mar 2011 10:08:00 -0000
From: "WANG.Jiong" <wong.kwongyuan@gmail.com>
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To: "Paulo J. Matos" <pocmatos@gmail.com>
CC: gcc@gcc.gnu.org
Subject: Re: avr compilation
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This may related with subreg regmove finding
Suggest specifiy -fdump-rtl-regmove to see what happen after this pass
Maybe avr need a target dependent regmove pass to handle this


Best,
Jiong

On 03/18/2011 04:47 PM, Paulo J. Matos wrote:
> Hi all,
>
> I am looking at the avr backend in order to try to sort some things 
> out on my own backend.
>
> One of the tests I am doing is by compiling the following:
> int x = 0x1010;
> int y = 0x0101;
>
> int add(void)
> {
>   return x+y;
> }
>
> It compiles to (in gcc-4.3.5_avr with -Os)
> add:
> /* prologue: function */
> /* frame size = 0 */
>         lds r18,y
>         lds r19,(y)+1
>         lds r24,x
>         lds r25,(x)+1
>         add r18,r24
>         adc r19,r25
>         mov r24,r18
>         mov r25,r19
> /* epilogue start */
>         ret
>
> I don't know much avr assembler so bear with me but I would expect 
> this to be written:
> add:
> /* prologue: function */
> /* frame size = 0 */
>         lds r18,y
>         lds r19,(y)+1
>         lds r24,x
>         lds r25,(x)+1
>         add r24,r18
>         adc r25,r19
> /* epilogue start */
>         ret
>
> By inverting the add arguments we save two mov instructions.
>
> If it can be written like this any ideas on why GCC is avoiding it?
>
> Cheers,
>
> -- 
> PMatos
>