Re: [avr-gcc-list] Re: [avr-libc-dev] [bug #29774] prologue/epilogue stack pointer manipulation not interrupt safe in XMega

[Matthew]

On 6/8/2010 10:31 AM, Bob Paddock wrote:
> > There are several places that handle SP adjustment:
> >
> > avr.c: output_movhi, expand_prologue, expand_epilogue
> > avr.md: movhi_sp_r_irq_off, movhi_sp_r_irq_on (both generated in
> > epilogue/prologue)
> > libgcc.S: __prologue_saves__, __epilogue_restores__
> >
> > For changes in libgcc.S to take effect, appropriate builtin-defines must
> > be provided so that the right multilib variants will be generated.
> >      
> When I was patching my libgcc.a for the avrxmega7 lib, the only place
> that I saw, that turned off the IRQs were the epilogue/prologue.
> Is there some reason they don't need turned off in the other contexts?
>   Seems like such a problem would have shown up long before now?
>    

So here's what I have found so far. When executing SEI, one more 
instruction can execute before the first interrupt is serviced. When 
enabling interrupts by restoring the status register, interrupts can 
occur immediately. When writing to SPL, interrupts are automatically 
disabled until the next memory write (up to 4 cycles).

Therefore, the prologue and epilogue do not need to disable interrupts 
at all. A write to SPL followed by a write to SPH should be interrupt 
safe. I have not tested this yet.