Re: [avr-gcc-list] code generation

[J.C. Wren]

It's a lack of understanding the keyword 'volatile'.  'volatile' means that it 
may be changed between reference by an external influence.  If it doesn't 
update it, it wouldn't be 'volatile'.

        --John

On Monday 27 October 2003 09:56 am, Torsten Hahn wrote:
> Hi, compiling a code like this in an interrupt handler
>
> ------------------------
> extern volatile uint16_t cnt;
>
> if(!cnt--) {
>       asm("nop");
> }
>
> -------------------------
>
> gives this code:
>
> (avr-gcc (GCC) 3.3.1), with -O2
>
> ....
> 0 push    r24
> 1 push    r25
> 2 lds     r24, 0x0113
> 3 lds     r25, 0x0114
> 4 sbiw    r24, 0x01       ; 1
> 5 sts     0x0114, r25
> 6 sts     0x0113, r24
> 7 lds     r24, 0x0113
> 8 lds     r25, 0x0114
> 9 or      r24, r25
> 10 brne    .+2             ; 0x5d6
> 11 nop
> ....
>
> I think the code in lines 5, 6, 7 and 8 is a bit inefficient. Why
> storing the regs to sram and loading them back again ? How can i help
> the compiler to do a better job there ?
>
> thx,
> Torsten.
>
>
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