[Bug target/78357] New: nios2 uses non-standard atomic functions

["sebastian dot huber at embedded-brains dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78357

            Bug ID: 78357
           Summary: nios2 uses non-standard atomic functions
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sebastian.huber@embedded-brains.de
  Target Milestone: ---

Some Nios II variants lack support for atomic operations in hardware.  The GCC
Nios II support generates in this case non-standard __sync_* function calls,
e.g.

#include <stdatomic.h>

_Bool cas(atomic_uint *a, unsigned int *e, unsigned int d)
{
        return atomic_compare_exchange_strong_explicit(a, e, d,
memory_order_relaxed, memory_order_relaxed);
}

unsigned int add(atomic_uint *a, unsigned int v)
{
        return atomic_fetch_add_explicit(a, v, memory_order_relaxed);
}

        .file   "atomic.c"
        .global __sync_val_compare_and_swap_4
        .section        .text
        .align  2
        .global cas
        .type   cas, @function
cas:
        addi    sp, sp, -12
        stw     r17, 4(sp)
        stw     ra, 8(sp)
        stw     r16, 0(sp)
        ldw     r16, 0(r5)
        mov     r17, r5
        mov     r5, r16
        call    __sync_val_compare_and_swap_4
        cmpeq   r16, r2, r16
        bne     r16, zero, .L2
        stw     r2, 0(r17)
.L2:
        mov     r2, r16
        ldw     ra, 8(sp)
        ldw     r17, 4(sp)
        ldw     r16, 0(sp)
        addi    sp, sp, 12
        ret
        .size   cas, .-cas
        .global __sync_fetch_and_add_4
        .align  2
        .global add
        .type   add, @function
add:
        addi    sp, sp, -4
        stw     ra, 0(sp)
        call    __sync_fetch_and_add_4
        ldw     ra, 0(sp)
        addi    sp, sp, 4
        ret
        .size   add, .-add

Thus, this target is not covered by libatomic. There are at least two options
to fix this problem:

1. Change the Nios II support to emit standard __atomic_* function calls:

https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

2. Add the __sync_* functions to libatomic.