Terminate the simulation on exit() from firmware

[edgar-bonet]

The firmware can end the simulation by sleeping with interrupts disabled. While this way of hanging the CPU makes a lot of sense, it is not the most intuitive.

The standard way to terminate a C program is by either calling exit() or returning from main(). It turns out the AVR C runtime supports stopping the firmware in both these standard ways. Returning from main() jumps to exit(), which disables interrupts and hangs the CPU in an infinite loop. The implementation is roughly like this:

; after the C runtime initializations:
.section .init9
    call main  ; run the firmware
    jmp exit   ; if main() returns, go to exit()

exit:
_exit:
    cli        ; disable interrupts
    rjmp .-2   ; hang the CPU

This pull request makes run_avr gracefully quit if the firmware executes the CPU-hanging instruction rjmp .-2 with interrupts disabled.

The actual condition used in the code is:

if (avr->state == cpu_Running && new_pc == avr->pc
    && avr->flash[new_pc] == 0xff && avr->flash[new_pc+1] == 0xcf
    && !avr->sreg[S_I])

Some of those tests may be redundant but, as I am not sure to understand all the subtleties of the simulation, I opted for defensive programming.

The feature has been tested with the following program:

#include <stdlib.h>
#include <avr/io.h>
#include <avr/sleep.h>
#include "avr_mcu_section.h"

AVR_MCU(F_CPU, "atmega328p");
AVR_MCU_SIMAVR_CONSOLE(&GPIOR0);

int main(void)
{
    const char *s = "Exiting.\r\n";
    for (const char *t = s; *t; t++)
        GPIOR0 = *t;
#if defined(QUIT_BY_SLEEP)
    sleep_mode();
#elif defined(QUIT_BY_EXIT)
    exit(0);
#endif
}

[edgar-bonet]

I checked that, with this pull request, abort() is also a valid way of terminating the simulation. According to the avr-libc documentation, abort() is implemented as:

cli();
_exit(1);

[kittennbfive]

+1 for this PR, i wondered why cli();while(1); does not stop the simulation.