https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121973
--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The nop comes from the need of debugging. Specifically from cfgcleanup:
```
/* If not optimizing, preserve the locus of the single edge between
blocks A and B if necessary by emitting a nop. */
if (!optimize
&& !forward_edge_locus
&& !DECL_IGNORED_P (current_function_decl))
{
emit_nop_for_unique_locus_between (a, b);
a_end = BB_END (a);
}
```
The mv comes from the expansion of the return:
```
(insn 10 6 11 2 (clobber (reg/i:TI 10 a0)) "/app/example.c":32:1 -1
(nil))
(insn 11 10 14 2 (clobber (reg:TI 134 [ <retval> ])) "/app/example.c":32:1 -1
(nil))
(insn 14 11 8 2 (const_int 0 [0]) "/app/example.c":32:1 -1
(nil))
(insn 8 14 9 2 (set (reg:DI 10 a0)
(subreg:DI (reg:TI 134 [ <retval> ]) 0)) "/app/example.c":32:1 -1
(nil))
(insn 9 8 12 2 (set (reg:DI 11 a1 [+8 ])
(subreg:DI (reg:TI 134 [ <retval> ]) 8)) "/app/example.c":32:1 -1
(nil))
(insn 12 9 0 2 (use (reg/i:TI 10 a0)) "/app/example.c":32:1 -1
(nil))
```
The easy workaround is to add:
__builtin_unreachable();
right before the last asm.
