Re: [avr-gcc-list] interrupt optimization

[Paulo Marques]

David VanHorn wrote:
> What's the way to go in this situation? I thought of using a naked ISR and
> adding the necessary push by inline asm but what if I later change my code
> and the compiler use different registers? I'm afraid to end up with broken
> code.
>  
> [...]
> Second, it dosen't matter what the compiler is doing, an ASM coded int 
> needs to save SREG, and any regs you modify during the int, and that's all.

Not really. This is a very common problem and the compiler could be 
smarter about it.

Basically the function should be compiled as:

  push just the registers needed to decrement "sampling"
  decrement sampling
  if not zero goto exit:
        push the extra registers needed in the function
        call the function
        pop the extra registers
  exit:
  pop just the registers needed to decrement "sampling"

The problem is that the compiler pushes all the registers regardless of 
the code path. This is understandable, but it _could_ be better.

One thing that might work (but I never tried it) is to code the sampling 
decrement in assembly in a "naked" interrupt handler and mark the other 
function as "interrupt" so that it pushes everything.

Then, to call the function, just do in assembly:
        in __tmp_reg__, SREG
        push __tmp_reg__
        call _not_inline_function_

The __tmp_reg__ (SREG) push is made to match the IRET instruction from 
the now "interrupt" function.

This is still a little inneficient as some registers will be pushed 
twice, but is only inneficient when the function is actually called and 
not in all interrupt calls.

I hope this helps,

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com

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