Re: [avr-llvm-devel] Interrupt handling

[Weddington, E. <Eri...@at...>]

Hi Borja,

>From my view, "interrupt" and "signal" are mutually exclusive. They both
define an ISR, where "interrupt" allows nested interrupts and "signal"
disallows nested interrupts.

I don't know if we can error out if a user defines both simultaneously.

"naked" does something completely different than interrupt|signal. One
can have a naked function without it being an ISR.

Can one have a "naked" ISR? I suppose if one is writing the ISR in
assembly... Another reason to have interrupt|signal is to put that
function into the IVT. So, yes, the combination is valid, but naked
should have priority, and the prologue/epilogue should not be generated
for that ISR, and it is up to the user to write.

Technically, the RET/RETI is usually a part of the epilogue. Naked
functions do not have prologues/epilogues, hence, they do not have a
RET/RETI, and the user has to provide those.

Hope that answers any questions. Again, my only suggestion would be to
error out on simultaneous "interrupt" and "signal" attributes.

Eric Weddington

> -----Original Message-----
> From: Borja Ferrer [mailto:bor...@gm...]
> Sent: Monday, March 05, 2012 1:58 PM
> To: avr...@li...
> Subject: [avr-llvm-devel] Interrupt handling
> 
> I'm currently implementing support for interrupt handling. But before
going
> any further into implementing it in the backend (the frontend part is
nearly
> done) I want to get things clear. Eric this one is probably for you
because
> it's about avr-libc but anyways, here we go:
> 
> There are 2 function attributes to add:
> 
> 1) signal (ISR macro): normal interrupt handler. Every register used
inside
> the handler gets saved in the stack, including special code to save
SREG. So
> we have to mark all registers to be callee saved.
> 2) interrupt (ISR_NOBLOCK): same as above but adding a sei instruction
at the
> very top of function entry.
> 
> Now, the naked attribute is already implemented and i don't need to
handle
> anything with it. It avoids any prologue/epilogue code to be inserted.
BUT by
> default the ret/reti instruction gets emitted, and avr-libc says that
it
> should be done by the programmer so I have to handle that, right?
> This doesn't matter if it's an ISR or a normal func.
> 
> The ISR_ALIASOF macro works correctly, so no more work is needed there
(using
> NOBLOCK or NAKED with ALIASOF doesn't make sense at all, right?)
> 
> Now i want to talk about combining attributes:
> Truth table format: |signal | interrupt | naked|
> 
> 1) 0 0 0 -> nothing
> 
> 2) 0 0 1 -> no prologue/epilogue and no ret/reti (used in any
arbitrary
> function)(so as i said before, don't emit ret/reti?)
> 
> 3) 0 1 0 -> emit sei, and save all used regs inside the function plus
SREG.
> 
> 4) 0 1 1 -> naked wins, do case 1.
> 
> 5) 1 0 0 -> normal ISR macro, do what i said when explaining the
signal
> attrib.
> 
> 6) 1 0 1 -> naked wins again.
> 
> 7) 1 1 0 -> interrupt wins, do case 3.
> 
> 8) 1 1 1 -> naked wins.
> 
> So this is what I understand I have to do, please let me know if I'm
wrong on
> anything I've mentioned.
>