Investigate program breakage with LTO enabled for few files

[beakthoven]

LTO is currently disabled for both wiring.c due to program breakage when enabled. Investigate the root cause, resolve the issue, and remove the workarounds in place currently.

test code used: Blink example from Arduino IDE:

void setup() {
  // initialize digital pin LED_BUILTIN as an output.
  pinMode(LED_BUILTIN, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(LED_BUILTIN, HIGH);  // turn the LED on (HIGH is the voltage level)
  delay(1000);                      // wait for a second
  digitalWrite(LED_BUILTIN, LOW);   // turn the LED off by making the voltage LOW
  delay(1000);                      // wait for a second
}

breakage: The LED seems to blink once and gets stuck in ON position indefinitely.

[benshi001]

could you please give the incorrect assembly generated by clang/llvm? And create an issue at https://github.com/llvm/llvm-project/issues , with title "[AVR] ..." .

[beakthoven]

As requested, issue opened as llvm/llvm-project#127651 with relevant files attached.

[beakthoven]

Diagnosed problem: llvm/llvm-project#133579
This tracking issue will be closed once bug is resolved.