[avr-gcc-list] New function attribute idea

[Tvrtko A. Ursulin]

Hello everyone!

I will try to explain some new functionality I am planning to implement in 
avr-gcc.

Imagine the following situation: We have an kernel, two tasks, and one 
interrupt. Kernel has its own stack space, as well as each task. The 
situation in RAM looks like this:

<Top of RAM>
[Kernel stack]
[Task 1 stack]
[...]
[Task N stack]
<free RAM>

If we have an interrupt which needs some stack space itself, we must expand 
each of the task stack space (and kernel space), to accomodate possible 
interrupt. But interrupt can happen only once at a time, so we have wasted 
IRQ_STACK_SIZE*NUMBER_OF_TASKS bytes of RAM.

What would I like to have is a enhanced "signal" function attribute, let's 
call it "global_signal". That kind of function should change SP as a first 
thing in it's prologue. Then we'd have situation like this:

<Top of RAM>
[Global interrupt SP storage]
[Global interrupt stack]
[Kernel stack]
[Task 1 stack]
[...]
[Task N stack]
<free RAM>

As "global_signal" must be able to restore original SP, it will use 1 bytes of 
"normal" stack space (to save __tmp_reg__ it needs to initialitze properly). 
But we still save possibly much space. Because we only must expand each tasks 
stack size by 1 (and kernel stack also).

Without this functionality, we have to expand each stack with as much as is 
needed for biggest global interrupt. 

The prologue of this function should look like this (possible bad operand 
usage):

push __tmp_reg__
in SP_L, __tmp_reg__
sts GLOBAL_IRQ_SP_L_STORAGE, __tmp_reg__
in SP_H, __tmp_reg__
sts GLOBAL_IRQ_SP_H_STORAGE, __tmp_reg__
out SP_L, GLOBAL_IRQ_STACK_L
out SP_H, GLOBAL_IRQ_STACK_H

and now proceed with almost normal "signal" prologue, but without saving 
__tmp_reg__ as it is already saved.

Epilogue:

Do standard "signal" epilogue only without restoring __tmp_reg__

lds GLOBAL_IRQ_SP_H_STORAGE, __tmp_reg__
out SP_H, __tmp_reg__
lds GLOBAL_IRQ_SP_L_STORAGE, __tmp_reg__
out SP_L, __tmp_reg__
pop __tmp_reg__


I hope so far it is clear to everyone what I am trying to do.

Now the problems: 

I am not fully competent to make such a complicated(?) patch. I don't have a 
clue how to make prologue and epilogue which don't use fixed RAM addresses 
for GLOBAL_IRQ_ variables, but can use symbols which can later be resolved by 
someone else. My idea is that addresses are like this:

GLOBAL_IRQ_SP_L_STORAGE = RAMEND 
GLOBAL_IRQ_SP_H_STORAGE = RAMEND - 1
GLOBAL_IRQ_STACK_L = RAMEND - 2
GLOBAL_IRQ_STACK_H = RAMEND - 3

Is it possible for gcc to know what is RAMEND based on --mmcu parameter on 
compilation time, and do this correctly?

Any comments are welcome and appreciated! (and patches too!) ;)


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