Investigate program breakage when using Serial functions

[beakthoven]

The Blink example program fails when serial functions are used, even with LTO disabled globally. The LED seems to be stuck in ON position. However, removing the serial code allows the LED to blink normally.

test code:

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);      // Initialize LED pin
  Serial.begin(9600);                    
  Serial.println();                 
  Serial.println("Hello world. I was built with clang!"); 
}

void loop() {
  digitalWrite(LED_BUILTIN, HIGH); 
  delay(1000);                  
  digitalWrite(LED_BUILTIN, LOW);   
  delay(1000);                  
}

how ever by removing the serial code the program works normally:

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);      // Initialize LED pin
}

void loop() {
  digitalWrite(LED_BUILTIN, HIGH); 
  delay(1000);                  
  digitalWrite(LED_BUILTIN, LOW);   
  delay(1000);                  
}

[beakthoven]

LLVM issue opened: llvm/llvm-project#128536