[avr-gcc-list] Add 'OS_main' and 'OS_task' attributes in avr-gcc 4.2.1

[Anatoly Sokolov]

Hi.

This patch add "OS_main" and 'OS_task' attributes in GCC 4.2.1. Function with 
"OS_main" and 'OS_task' attributes do not save any "call-saved" registers.

1. "OS_main" attribute  used when there IS guarantee that interrupts are 
disabled at that time when function is called. For example "main" function in 
avr-libc. If this function has local variables that a prologue will look so:
  in r28, 0x3d ; 61
  in r29, 0x3e ; 62
  sbiw r28, 0x08 ; 8
  out 0x3e, r29 ; 62
  out 0x3d, r28 ; 61

No save/clear/restore "I" flag.

2. "OS_task" attribute  used when there is NO guarantee that interrupts are 
disabled at that time when function is called. For example task functions 
multi-threading operating systems. In this case save/clear/restore "I" flag is 
need for changing SP register. Prologue (if this function has local variables) 
will look so:

  in r28, 0x3d ; 61
  in r29, 0x3e ; 62
  sbiw r28, 0x08 ; 8
  in r0, 0x3f ; 63
  cli
  out 0x3e, r29 ; 62
  out 0x3f, r0 ; 63
  out 0x3d, r28 ; 61


Testcase:
__attribute__ ((noinline))
int fn0(long a1, long a2, long a3, long a4, long a5)
{
    return 0;
}

__attribute__ ((OS_main))
int main(void)
{
   volatile long long a; // local var, need function frame

   a = 1;

   return fn0(1, 2, 3, 4, 5); // use call-saved regs
}

PS: At this moment, I have not found a simple way to add "OS_main" attribute 
in the avr-gcc 4.3. So presence of this attribute in GCC 4.3  in doubt.

Anatoly. 


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