'for loop' seems to change the execution of pow()

Question

The first time, pow() works. But inside of a 'for loop', or again afterwards, it doesn't behave !? (I'm using a Chinese Pro Mini with Arduino 1.8.16)

EDIT To clarify what I am asking, my question is: "Why does pow() only give the correct answer the first time?"

uint8_t places;
uint8_t temp8;
uint32_t n;

void setup() {
Serial.begin(115200);
}

void loop() {
  places  = 5U;

  n = pow( 10, places);
  Serial.println(n);

  Serial.println("\nHERE'S THE SAME CODE INSIDE OF A 'FOR LOOP'\n");

  for (temp8 = 0; temp8 < places; temp8++)  {
    n = pow( 10, places);
    Serial.println(n);
  }

  Serial.println("\nNOW THE EXACT SAME CODE AS BEFORE GIVES A DIFFERENT RESULT\n");

  n = pow( 10, places);
  Serial.println(n);
 
  while(1){}
}

Here's what the serial monitor shows:

100000

HERE'S THE SAME CODE INSIDE OF A 'FOR LOOP'

99999

99999

99999

99999

99999

NOW THE EXACT SAME CODE AS BEFORE GIVES A DIFFERENT RESULT

99999

Answer 1

This seems like a case of a confused compiler. More specifically, a confused optimizer.

On the first occurrence of pow(10, places), the compiler noticed that places must be equal to 5, and so replaced the expression with its value. If you disassemble the compiled code, you will notice it calls

Serial.println(100000);

On the following occurrences, the compiler did not notice that places can not be anything other than 5, so it did not optimize-out the call to pow(), and got the result 99999.9765625. This is 3 ULPs less than the exact result, and is consistent with the accuracy you can expect from pow(). Converting to an integer implicitly rounds towards zero, hence the printed result: 99999.