https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111870
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Or maybe the issue is you don't understand the cmpxchg instruction and how it
gives back the original value too.
The RTL form for the "lock;cmpxchg " is:
(insn:TI 14 10 17 5 (parallel [
(set (reg:DI 0 ax [108])
(unspec_volatile:DI [
(mem/v:DI (reg/v/f:DI 5 di [orig:105 map ] [105]) [-1
S8 A64])
(reg:DI 0 ax [108])
(reg:DI 4 si [107])
(const_int 32773 [0x8005])
] UNSPECV_CMPXCHG))
(set (mem/v:DI (reg/v/f:DI 5 di [orig:105 map ] [105]) [-1 S8
A64])
(unspec_volatile:DI [
(const_int 0 [0])
] UNSPECV_CMPXCHG))
(set (reg:CCZ 17 flags)
(unspec_volatile:CCZ [
(const_int 0 [0])
] UNSPECV_CMPXCHG))
]) "/app/example.c":15:22 9336 {atomic_compare_and_swapdi_1}
(expr_list:REG_DEAD (reg:DI 4 si [107])
(nil)))
