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Re: [avr-chat] Re: Atmega128, JTAG and ADC (Jeff Epler)

From: Jeff Epler
Subject: Re: [avr-chat] Re: Atmega128, JTAG and ADC (Jeff Epler)
Date: Tue, 19 Jul 2005 09:57:16 -0500
User-agent: Mutt/1.4.1i

On Tue, Jul 19, 2005 at 08:42:52AM -0500, Dean Hall wrote:
> Jeff,
> The rest of your post was good research, but
> your example code isn't quite right.
> Look what happens in the non-skip-if-bit case:
> the code rjmps to p2, executes p2, returns
> and then rjmps to p1 (regardless), executes p1,
> returns.

Here's my thought process while "optimizing" that code:

The original code generated by gcc was
    sbrc r24, 3
    rjmp l1
    rcall p1
    rcall p2

First, rcall+ret is the same as rjmp, so turn the code into 
    sbrc r24, 3
    rjmp l1
    rjmp p1
    rjmp p2

This code has a jump to a jump, so turn the code into the final form
    sbrc r24, 3
    rjmp p2
    rjmp p1

I don't see how p2 could return to the 'rjmp p1' instruction; instead,
it will return to whatever address the 'ret' in the original code would
have returned to.

I haven't done much low-level coding for avr, but the 'call followed by
return' optimization is one I've used on other systems.  Is there
something I'm missing about avr that means this doesn't work?


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