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Re: [avr-gcc-list] Not naked but "topless" functions?

From: Larry Barello
Subject: Re: [avr-gcc-list] Not naked but "topless" functions?
Date: Fri, 11 Oct 2002 09:02:46 -0700

There is nothing wrong with this prolog.  The CLI clears the interrupt
enable flag.  When the OUT instruction restores the flag by restoring SREG,
that doesn't take affect until AFTER the next instructions (read the chip
data sheets carefully), so the update of the stack will be complete.

If an interrupt handler modifies and fails to restore *any* register upon
return, then all code will break regardless of how you write it.   Since
writing the stack is a two-instruction operation you *must* protect it with
a CLI/SEI sequence.  GCC just does it in a very clever way with restoring
the interrupt flag along with the SREG.

IMHO the "naked" function is simply broken code.  Unfortunately there is no
"No Return" attribute implemented for GCC (well, there is, but it has never
made it into the GCC source) which is what you really want.  I too wrote an
RTOS (www.barello.net/avrx) and ran into the same sorts of trouble with
stack usage.  Fortunately, all subroutines called from the top level "naked"
routine will set up a proper stack frame.  Unfortunately, they will also
save, on the stack,  all those un-used registers from the top level "naked"
function.  So, in the end, "naked" doesn't do anything useful.

I still use the "naked" attribute in my RTOS, but I have the restriction
that you can't have more than 8 bytes of automatic variables in the top
level routine (GCC allocates registers, then stack frame variables).
Fortunately, GCC is so fantastic about register recycling, I can often get
10-12 bytes worth of automatics before the compiler needs to resort to the
frame.  Unfortunately, I have to inspect the resulting assembly all the time
to make sure modifying the code didn't introduce frame variables which, of
course, would break.


----- Original Message -----
From: "Tvrtko A. Ursulin" <address@hidden>

> Hello everyone!
> More questions...
> I need to write functions that dont push/pop registers on stack. But if I
> __attribute__((naked)) then compiler omits code that adjusts stack pointer
> also. That is ok if my function uses less than available registers, but if
> not... trouble.
> My functions should adjust stack like this:
>  14e:   cd b7           in      r28, 0x3d       ; 61
>  150:   de b7           in      r29, 0x3e       ; 62
>  152:   25 97           sbiw    r28, 0x05       ; 5
>  154:   0f b6           in      r0, 0x3f        ; 63
>  156:   f8 94           cli
>  158:   de bf           out     0x3e, r29       ; 62
>  15a:   0f be           out     0x3f, r0        ; 63
>  15c:   cd bf           out     0x3d, r28       ; 61
> This is taken from function which uses 5 stack variables, hence sbiw
> Btw, what does in/cli/out of SREG in default function prolog do? If it is
> supposed to ensure no interrupt occurs while manipulating SP, why "out
> 0x3f,r0" (enable interrupts right?) is not the last instruction?
> Thanks!
> avr-gcc-list at http://avr1.org

avr-gcc-list at http://avr1.org

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