From: "WANG.Jiong" <wong.kwongyuan@gmail.com>
To: "Paulo J. Matos" <pocmatos@gmail.com>
Cc: gcc@gcc.gnu.org
Subject: Re: avr compilation
Date: Fri, 18 Mar 2011 10:08:00 -0000 [thread overview]
Message-ID: <4D832F02.6080508@gmail.com> (raw)
In-Reply-To: <ilv67b$au6$1@dough.gmane.org>
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
>
next prev parent reply other threads:[~2011-03-18 10:08 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-03-18 8:48 Paulo J. Matos
2011-03-18 10:08 ` WANG.Jiong [this message]
2011-03-18 10:15 ` Paulo J. Matos
2011-03-18 12:11 ` David Brown
2011-03-18 13:37 ` Paulo J. Matos
2011-03-18 13:26 ` Georg-Johann Lay
2011-03-18 13:40 ` Paulo J. Matos
2011-03-18 14:20 ` Ian Lance Taylor
2011-03-18 14:50 ` Paulo J. Matos
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