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Re: [avr-gcc-list] Use of variables and programming in avr-gcc


From: Robert Rozman
Subject: Re: [avr-gcc-list] Use of variables and programming in avr-gcc
Date: Mon, 19 Mar 2001 09:55:18 +0100

Hello,

thanks Jorg for explanation. Carried by those advices I got an idea to keep
all global variables in struct, hoping that Z will point to them with
displacement and I'll save quite some bytes cause global variables are
accessed frequently.

But then I discovered that Z register is loaded again and again prior each
access through Z with displacement. This behaviour makes this approach non
oseful, but I still think that maybe somehow different I can get compiler to
optimize away those redundant loading of Z register.

I've used following program:
typedef struct {
 char day;
 char hour;
 char min;
 char sec;
  } time_struct;

time_struct global ;

time_struct *time = &global;

char day;
char hour;
char min;
char sec;


int main(void)
{

day = 5;
hour= 9;
min = 10;
sec =10;

time->day = 5;
time->hour= 9;
time->min = 10;
time->sec =10;


    for (;;) {}            /* loop forever */
-----------------------------------------------
and I got:
  44:test1.c       **** day = 5;
  75                 .stabn 68,0,44,.LM2-main
  76                .LM2:
  77 0008 95E0        ldi r25,lo8(5)
  78 000a 9093 0000   sts day,r25
  45:test1.c       **** hour= 9;
  79                 .stabn 68,0,45,.LM3-main
  80                .LM3:
  81 000e 29E0        ldi r18,lo8(9)
  82 0010 2093 0000   sts hour,r18
  46:test1.c       **** min = 10;
  83                 .stabn 68,0,46,.LM4-main
  84                .LM4:
  85 0014 8AE0        ldi r24,lo8(10)
  86 0016 8093 0000   sts min,r24
  47:test1.c       **** sec =10;
  87                 .stabn 68,0,47,.LM5-main
  88                .LM5:
  89 001a 8093 0000   sts sec,r24
  48:test1.c       ****
  49:test1.c       **** time->day = 5;
  90                 .stabn 68,0,49,.LM6-main
  91                .LM6:
  92 001e E091 0000   lds r30,time
  93 0022 F091 0000   lds r31,(time)+1
  94 0026 9083        st Z,r25
  50:test1.c       **** time->hour= 9;
  95                 .stabn 68,0,50,.LM7-main
  96                .LM7:
  97 0028 E091 0000   lds r30,time
  98 002c F091 0000   lds r31,(time)+1
  99 0030 2183        std Z+1,r18
  51:test1.c       **** time->min = 10;
 100                 .stabn 68,0,51,.LM8-main
 101                .LM8:
 102 0032 E091 0000   lds r30,time
 103 0036 F091 0000   lds r31,(time)+1
 104 003a 8283        std Z+2,r24
  52:test1.c       **** time->sec =10;
 105                 .stabn 68,0,52,.LM9-main
 106                .LM9:
 107 003c E091 0000   lds r30,time
 108 0040 F091 0000   lds r31,(time)+1
 109 0044 8383        std Z+3,r24
--------------------------------------

It can be seen that all but first loading register Z are redundant. If
compiler would optimize them away one could save a lot of bytes of coding
space when globals are frequently accessed.

So I kindly ask for advice, what I'm doing wrong in definition of struct or
pointer. I surely don't believe that avr-gcc is not capable of optimizing
such redundants away (in example I used -Os, but also -O3 doesn't change
things).

Thanks in advance,

Robert Rozman
}

-----Izvorno sporońćilo-----
Od: Robert Rozman <address@hidden>
Za: address@hidden <address@hidden>
Datum: 1. marec 2001 12:06
Zadeva: [avr-gcc-list] Use of variables and programming in avr-gcc


>Hello,
>
>I'm relatively newbie to avr-gcc. I've not found any reference/advice on
>how to declare variables and how to use them in avr-gcc. Ther is only
>one Application note on atmel site about efficient programming in C, but
>it is for one of commercial compilers.
>
>I've several question and I'd kindly ask if someone from more
>experienced guys is willing to organize his experience in few sentences
>of advice to novice users. Among other I have few specific questions:
>- do recomendations from mentioned application note apply also for
>avr-gcc ? if not, what should be done differently ?
>- what declaration keywords are really implemented and what is their
>effect (const, register, volatile, ...) and how exactly to use them ?
>- is it better to keep SRAM variables in structures - as APP note
>recomends) or not ?
>
>I'd kindly ask for answers in code space and also from speed point of
>view.
>
>Thanks in advance and special thanks to all guys working on avr-gcc,
>
>Robert Rozman
>
>
>
>_______________________________________________
>avr-gcc-list mailing list
>address@hidden
>http://avr.jpk.co.nz/mailman/listinfo/avr-gcc-list




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