Re: [Sdcc-user] Code generated for small loop

[Maarten B. <sou...@ds...>]

Hello Vincent,

The reason for this behaviour is that SDCC turns polling and then clearing
a bit/sbit into an atomic action (using JBC). This is very useful when the
bit is a semaphore or is set inside an interrupt handler.

Maarten

> Hi.
>
> I'm looking at asm generated for the following loop (8051, a CYCFX2LP
> actually) with sdcc 3.1.0#7066:
>
> __sbit __at 0x98+1 TI;
> void main() {
>   while (!TI);
>   TI = 0;
> }
>
> I get this:
>
> 00101$:
> 	jbc     _TI,00115$
> 	sjmp    00101$
> 00115$:
>
> Wouldn't it be more efficient, for such bit polling loop, to use instead:
>
> 00101$:
> 	jnb     _TI,00101$
> 	clr     _TI
>
> The first version polls _TI every 7 cycles.
> The second version polls _TI every 4 cycles.
> If _TI is set on loop entry, the first version exits after 4 cycles while
> the
> second exits after 6 cycles. I believe it's sane to assume this is an
> unlikely
> event, so it shouldn't matter too much.
>
> Regards,
> --
> Vincent Pelletier