https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65697
--- Comment #17 from mwahab at gcc dot gnu.org ---
According to the GCC documentation, __atomic_compare_exchange(ptr, exp, des,
..) is: if (*ptr == *exp) *ptr = *exp; else *exp = *ptr;
On Aarch64 the else (*ptr != *exp) branch is a store rather than a
store-release.
This doesn't appear in your example but with
----
int cas(int* barf, int* expected, int* desired)
{
return __atomic_compare_exchange_n(barf, expected, desired, 0,
__ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
}
----
cas:
ldr w3, [x1]
.L3:
ldaxr w4, [x0]
cmp w4, w3
bne .L4
stlxr w5, w2, [x0] ; store-release
cbnz w5, .L3
.L4:
cset w0, eq
cbz w0, .L6
ret
.p2align 3
.L6:
str w4, [x1] ; store, no barrier.
ret
----
This looks odd to me but I'd need look into it more.
