6X slowdown in 1.0.6 loop()

[dmd]

I'm seeing a 6X slowdown in arduino 1.0.6 loop() as compared to 1.0.5.

To reproduce, run the following sketch under 1.0.5, and note the printed values, which represent the delay in microseconds between the end of one iteration of the loop and the beginning of the next. Then try it again under 1.0.6.

Using 1.0.5 I get 12 uS
Using 1.0.6 I get 72 uS

This is on an ATmega2560.

unsigned long before, howlong, iters;

void setup() {
    Serial1.begin(57600);
}

void loop() {
    howlong = micros() - before;
    iters++;

    if (iters == 10000) {
        iters = 0;
        Serial1.println(howlong);
    }

    before = micros();
}

[cmaglie]

Hi @dmd !

Your test is not really measuring the time to perform 10000 loops (it measure just the latest one) BTW a slight rewrite reveals that there is indeed a problem:

unsigned long before, howlong, iters;

void setup() {
    Serial.begin(57600);
    iters = 0;
    before = micros();
}

void loop() {
    iters++;

    if (iters == 10000) {
        howlong = micros() - before;
        Serial.println(howlong);
        delay(500); // Wait for serial buffer to be emptied

        // Start a new count
        iters = 0;
        before = micros();
    }
}

Tested on a Mega1280, 1.0.5 gives 88000 and 1.0.6 480000.

With a git bisection I've found the following commit as the cause of the bug: f40e471
This change was introduced with #2057.

@matthijskooijman do you think we can revert it? There is a better solution to avoid the cast?

[matthijskooijman]

In that commit, it's not the type of the return expression that's cast to (unsigned) int, but it's the first operand to the modulo. I guess that doing modulo on a signed expression is significantly slower than doing it on an unsigned expression for some reason. The reason that this shows up is probably because Serialx.available() is called every loop to trigger serialEvent().

I guess we can just convert this commit, since it it doesn't actually do what it says - the cast isn't about the return type. Perhaps it would be good to add some parenthesis to emphasize this fact?

[cmaglie]

Fixed for 1.0.7.
Thanks @dmd!