f3discovery/09-clocks-and-timers: aux9::nop() optimized away

[guoshimin]

In https://docs.rust-embedded.org/discovery/f3discovery/09-clocks-and-timers/nop.html, it was suggested that doing a aux9::nop() would prevent the loop from being optimized away. When I tried it, I got the following assembly:

(gdb) disassemble /m
Dump of assembler code for function _ZN17clocks_and_timers5delay17h4e291ad62a2b0e58E:
7	fn delay(tim6: &tim6::RegisterBlock, ms: u16) {
   0x08000220 <+0>:	push	{r4, r5, r7, lr}
   0x08000222 <+2>:	add	r7, sp, #8
   0x08000224 <+4>:	movs	r4, #0

8	    const K: u16 = 3; // this value needs to be tweaked
9	    for _ in 0..(K * ms) {
10	        aux9::nop()
11	    }
12	}
   0x08000232 <+18>:	pop	{r4, r5, r7, pc}

End of assembler dump.

In the empty loop case, the delay function was not even called in the assembly. With a nop, the function did get called, but the loop itself was skipped.

Did the compiler get smarter or did I do something wrong? I'm using rust 1.57 on a mac.