[avr-libc-dev] Single page stack suggestion

[David Brown]

This is just an idea, inspired by my examination of assembly listing 
code recently.

When a stack frame is needed on the AVR, there is a substantial amount 
of code needed to safely decrement the stack pointer in the prologue 
(and increment it in the epilogue) - a total of 19 words overhead.  The 
reason for this is that the stack pointer must be adjusted as a 16-bit 
value.  But if the stack were always within one 256-byte page, which is 
probably the case in the majority of avr programs (certainly on small 
AVRs), then only the SP_L register needs to updated.  Then the prologue 
and epilogue would be reduced to 4 and 3 instructions respectively.

I don't know if such a feature would be worth having - stack frames are 
not often needed, and it would have to be enabled with a compiler flag 
rather than being used by default.


An alternative change (more suited to bigger AVRs) could be made if it 
is safe to assume that the stack is given a whole number of pages, plus 
some extra for interrupt stack space.  Then there would be no need to 
disable interrupts during the prologue and epilogue:

        in r28,__SP_L__
        in r29,__SP_H__
        sbiw r28,XX
        out __SP_H__,r29
        out __SP_L__,r28
        ...
        adiw r28,XX
        out __SP_L__,r28
        out __SP_H__,r29

The important thing to consider is what happens if an interrupt occurs 
in the middle of the change to the stack pointer registers.  In the 
prologue, either the SP_H register remains the same (and thus the write 
to SP is effectively atomic), or it has decreased.  For example, if the 
original stack pointer were at 0x1008 and the frame size is 16 bytes, 
the new stack pointer will be 0xfff8.  If the interrupt comes in the 
middle, the stack pointer will be 0xff08 - given the assumption of 
enough stack space (the complete page plus enough for the interrupt), 
this is perfectly safe.  In the epilogue, the order of the writes is 
reversed, so again the intermediary value is 0xff08.


mvh.,

David