How to prevent Arduino compiler from merging changing of port pin outputs into a single operation

Question

I want to set one pin's output value, and then set a different pin's output value so that the two pins change value one after the other. However, the compiler optimises the two calls to change both pins in one go.

PORTD |= _BV(0); //set pd0 to high PORTD |= _BV(1); //set pd1 to high PORTD &= ~(_BV(1)); //set pd1 to low

The two pins go high simultaneously. I tried inserting a NOP between these but it doesn't change behaviour.

(I originally intended to include an oscilloscope screengrab to show what was happening - that's how I knew (or got the wrong impression) that the two pins were changing at the same time. But the photo wasn't shifting from my Nexus 7 for me to access it from my laptop.)

• Ignacio Vazquez-Abrams: I can't duplicate this behavior. Which version of GCC is in use?
• BrettFolkins: What application requires those pins to be turned on one after the other, but so quickly that you don't need any code to cause a delay between the two?
• mackenir: Cool - there's an Arduino stackexchange! Thanks to the mods for migrating the question.
• mackenir: It sounds like I've just got myself confused here, since the behaviour I've seen isnt repro-able.
• mackenir: (by others, that is).

Answer 1

I certainly can't duplicate what you are saying. The small code snippet:

void setup() {
 PORTD |= _BV(0);
 PORTD |= _BV(1);
 PORTD &= ~(_BV(1));
}
void loop() {
}

compiles into:

000000a6 <setup>:
 a6: 58 9a sbi 0x0b, 0 ; 11
 a8: 59 9a sbi 0x0b, 1 ; 11
 aa: 59 98 cbi 0x0b, 1 ; 11
 ac: 08 95 ret
000000ae <loop>:
 ae: 08 95 ret

That is clearly two separate bit sets followed by a bit clear.

I am using UECIDE that is set to use GCC 4.3.2

If I switch to GCC 4.8.1 instead, the code generated is identical.

Even turning on full -O3 optimization I still get the same results.

So how are you determining that it's happening at the same time?

• Ignacio Vazquez-Abrams: Which makes sense since the memory location has been declared as volatile.
• mackenir: I think I'm using Arduino IDE 1.06 but I will need to check when I get home.
• mackenir: I observed the two pins changing on an oscilloscope, but I will have to take a step back and do a test with the Arduino Micro removed from the project I'm working on. Sounds like I'm just mistaken.
• Edgar Bonet: Same code generated here with avr-gcc 5.4.0.