Feature request: disable codegeneration for atomics

The option to disable atomics has also been mentioned in the original atomics issue https://sourceforge.net/p/sdcc/feature-requests/624/

The option --norestartseqatomics has been implemented in th eatomics branch, and can be tested by users.

Currently it only affects the mcs51 and ds390 ports, and mostly disables atomics support for these two ports.

I did not call it --noatomics, and did not make it disable all atomics support; it just disables the restartable sequence support routines for atomics; depending on the target architecture and how the user uses atomics, stuff might work or not; basically, one could just try to compile with --norestartseqatomics, and the atomics will either work, or there will be a diagnostic from the linker.

P.S.: E.g. without restartable sequence support routines mcs51 and ds390 loose virtually all atomics support except volatile sig_atomic_t, but AFAIK all other targets can still do atomic_flag and once we have support for more atomic types and improve atomics support, there will be even more differences (e.g. mcs51 hardware could handle an atomic_flag in __idata without the restartable sequence support routines, but not in __xdata).

Ok, i built sdcc from atomics branch and make smoke tests for mcs51.

sdcc --version
SDCC : mcs51/z80/z180/r2k/r2ka/r3ka/r4k/r5k/r6k/sm83/tlcs90/ez80/z80n/r800/ds390/pic16/pic14/TININative/ds400/hc08/s08/stm8/pdk13/pdk14/pdk15/mos6502/mos65c02/f8/f8l TD- 4.5.24 #16459 (Linux)
published under GNU General Public License (GPL)

Wow At first look its worked.

Details:
First test is try to build main.c with content
void main(void) {}

If no --norestartseqatomics compiler flag the size of firmware is
Name Start End Size Max

ROM/EPROM/FLASH 0x0000 0x00ae 175 65536
main.asm contains atomic related code like
; restartable atomic support routines
.ds 5
sdcc_atomic_exchange_rollback_start::
nop
nop

If add --norestartseqatomics compiler flag
Name Start End Size Max

ROM/EPROM/FLASH 0x0000 0x0068 105 65536

Second test on main.c

#include <stdatomic.h>

volatile atomic_flag lock = ATOMIC_FLAG_INIT;

void critical_section(void) {
    // Spin until we acquire the lock
    while (atomic_flag_test_and_set(&lock)) {
        // In a real application, you might do something here
        // to avoid tight spinning, or just wait for the lock.
    }

    // --- CRITICAL SECTION START ---
    // Example: Protected access to a shared resource
    // (e.g., updating a 16-bit variable, sending serial data)
    __critical {
        // Even within this lock, you might need __critical 
        // if interrupts can also try to grab this same lock.
    }
    // --- CRITICAL SECTION END ---

    // Release the lock
    atomic_flag_clear(&lock);    
}


void main(void) {
    while (1) {
        critical_section();
    }
}

Without --norestartseqatomics

Other memory:
Name Start End Size Max

PAGED EXT. RAM 0 256
EXTERNAL RAM 0 65536
ROM/EPROM/FLASH 0x0000 0x00d6 215 65536

Try to compile with --norestartseqatomics is failed (and there is expected behavior)
?ASlink-Warning-Undefined Global sdcc_atomic_exchange_gptr_impl referenced by module

The feature is now in trunk.