Arduino loops add clock cycles, why? - loops

Question

It seems that on Arduino, using loops results in more clock cycles. The default loop adds about 4 cycles per iteration, any other loop (while, for etc.) adds 2. Thus, not using any loop at all, and instead explicitly typing in the repeats, results in fewer cycles. Why is that?
Here are some examples of what I mean:
Example 1
void loop() {
for (int i=0;i<4;i++) {
PORTB |= 0b00010000;
PORTB &= 0b11101111;
}
}
screenshot of measurement
Example 2
void loop() {
PORTB |= 0b00010000;
PORTB &= 0b11101111;
PORTB |= 0b00010000;
PORTB &= 0b11101111;
PORTB |= 0b00010000;
PORTB &= 0b11101111;
PORTB |= 0b00010000;
PORTB &= 0b11101111;
}
screenshot of measurement
Example 3
void loop() {
while (1) {
PORTB |= 0b00010000;
PORTB &= 0b11101111;
PORTB |= 0b00010000;
PORTB &= 0b11101111;
PORTB |= 0b00010000;
PORTB &= 0b11101111;
PORTB |= 0b00010000;
PORTB &= 0b11101111;
}
}
screenshot of measurement

Answer 1

The simple story is:
In this case, example 2 and 3 are the same (and usually the compiler optimize the code and remove useless instructions). Those examples have only 2 instructions:
PORTB |= 0b00010000;
PORTB &= 0b11101111;
Conclusion for those example the code only change the port value
For example 1 you have 5 instructions (1 once per for and 4 for each iteration):
int i = 0 (once per for)
i < 4
PORTB |= 0b00010000;
PORTB &= 0b11101111;
i++
Conclusion for this example the code change the port value and handle the for index
In order to understand why you should learn what a compiler does and how a processor works. Here is the assembly code for your examples
Example 1
PORTB:
.zero 4
loop():
push rbp
mov rbp, rsp
mov DWORD PTR [rbp-4], 0
.L3:
cmp DWORD PTR [rbp-4], 3
jg .L4
mov eax, DWORD PTR PORTB[rip]
or eax, 16
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
and eax, 239
mov DWORD PTR PORTB[rip], eax
add DWORD PTR [rbp-4], 1
jmp .L3
.L4:
nop
pop rbp
ret
Example 2
PORTB:
.zero 4
loop():
push rbp
mov rbp, rsp
mov eax, DWORD PTR PORTB[rip]
or eax, 16
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
and eax, 239
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
or eax, 16
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
and eax, 239
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
or eax, 16
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
and eax, 239
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
or eax, 16
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
and eax, 239
mov DWORD PTR PORTB[rip], eax
nop
pop rbp
ret
Example 3
PORTB:
.zero 4
loop():
push rbp
mov rbp, rsp
.L2:
mov eax, DWORD PTR PORTB[rip]
or eax, 16
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
and eax, 239
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
or eax, 16
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
and eax, 239
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
or eax, 16
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
and eax, 239
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
or eax, 16
mov DWORD PTR PORTB[rip], eax
mov eax, DWORD PTR PORTB[rip]
and eax, 239
mov DWORD PTR PORTB[rip], eax
jmp .L2