Re: [avr-gcc-list] Register usage

[E. Weddington]

On 21 Nov 2002 at 9:30, Tvrtko A. Ursulin wrote:

> 
> Something I don't understand... I have a small application which uses
> some simple functions. Only a few local variables in each function. I
> am pretty sure that there are many free registers in each function
> scope. But nevertheless stack is used to save some register upon
> function entry.
> 
> At the same time, registers like r2-r14 are not used at all. (Both in
> caller function, and in called function). Why does avr-gcc behave like
> that? If there are some fixed calling conventions, why can't he
> preserve some registers in other unused registers instead of
> pushing/poping to stack? It would be both faster and preserve stack
> space.
> 
> Here is an example:
> 
> void lcd_reset(void)
> {
>  3d8:   1f 93           push    r17
>         lcd_enable();
>  3da:   15 b7           in      r17, 0x35       ; 53
>  3dc:   10 6c           ori     r17, 0xC0       ; 192
>  3de:   15 bf           out     0x35, r17       ; 53
>         schedule_timeout(15*HZ/1000);
>  3e0:   8a e3           ldi     r24, 0x3A       ; 58
>  3e2:   90 e0           ldi     r25, 0x00       ; 0
>  3e4:   62 d1           rcall   .+708           ; 0x6aa
>         lcd_write_ir_nw(IR_FUNCTION_SET|0x10);
>  3e6:   10 e3           ldi     r17, 0x30       ; 48
>  3e8:   10 93 00 80     sts     0x8000, r17
>         schedule_timeout(5*HZ/1000);
>  3ec:   83 e1           ldi     r24, 0x13       ; 19
>  3ee:   90 e0           ldi     r25, 0x00       ; 0
>  3f0:   5c d1           rcall   .+696           ; 0x6aa
>         lcd_write_ir_nw(IR_FUNCTION_SET|0x10);
>  3f2:   10 93 00 80     sts     0x8000, r17
>         schedule_timeout(1*HZ/1000);
>  3f6:   83 e0           ldi     r24, 0x03       ; 3
>  3f8:   90 e0           ldi     r25, 0x00       ; 0
>  3fa:   57 d1           rcall   .+686           ; 0x6aa
>         lcd_write_ir_nw(IR_FUNCTION_SET|0x10);
>  3fc:   10 93 00 80     sts     0x8000, r17
> }
>  400:   1f 91           pop     r17
>  402:   08 95           ret
> 
> As you can se, r17 is pushed to stack because it is used inside the
> function. But I think r0 (__tmp_reg__) could be used without stacking
> instead. No?
> 
> Maybe it is needed, the function lcd_reset calls is schedule_timeout:
> 
> jiffie_t schedule_timeout(jiffie_t timeout)
> {
>         cli();
>  6aa:   f8 94           cli
> 
>         struct task     *pct = (struct task *)current;
>  6ac:   a0 91 62 00     lds     r26, 0x0062
>  6b0:   b0 91 63 00     lds     r27, 0x0063
>         pct->sleep_start = jiffies;
>  6b4:   60 91 86 00     lds     r22, 0x0086
>  6b8:   70 91 87 00     lds     r23, 0x0087
>  6bc:   16 96           adiw    r26, 0x06       ; 6
>  6be:   6d 93           st      X+, r22
>  6c0:   7c 93           st      X, r23
>  6c2:   17 97           sbiw    r26, 0x07       ; 7
>         pct->sleep = timeout;
>  6c4:   14 96           adiw    r26, 0x04       ; 4
>  6c6:   8d 93           st      X+, r24
>  6c8:   9c 93           st      X, r25
>  6ca:   15 97           sbiw    r26, 0x05       ; 5
>         pct->state &= ~TASK_READY;
>  6cc:   2c 91           ld      r18, X
>  6ce:   2e 7f           andi    r18, 0xFE       ; 254
>  6d0:   2c 93           st      X, r18
> 
>         _schedule();
>  6d2:   4f d0           rcall   .+158           ; 0x772
> 
>         return current->sleep;
>  6d4:   a0 91 62 00     lds     r26, 0x0062
>  6d8:   b0 91 63 00     lds     r27, 0x0063
>  6dc:   14 96           adiw    r26, 0x04       ; 4
>  6de:   8d 91           ld      r24, X+
>  6e0:   9c 91           ld      r25, X
>  6e2:   15 97           sbiw    r26, 0x05       ; 5
> }
>  6e4:   08 95           ret
> 
> r17 is not used at all.... that is also true for _schedule called from
> schedule_timeout. However, function which calls lcd_reset in the first
> place does use r17, but not in the scope of calling lcd_reset. Only
> later in the function there is a loop which uses r17 internally...
> 
> 
> Maybe I am talking rubish, and expecting too much of our beloved
> compiler, all over this post but I don't know any better... :)
> 
> Thanks for every comment!
> 

See:

http://savannah.nongnu.org/download/avr-libc/doc/avr-libc-user-
manual/FAQ.html#faq_reg_usage

Eric
avr-gcc-list at http://avr1.org