C: Calculated machine epsilon differs from limits.h

I tried to use the following program to estimate the machine epsilon with a simple C program

#include <stdio.h>
#include <limits.h>
#include <float.h>

int main(){
  float f = 1.0f;
  float prev_f = 1.0f;

  while( 1 + f != 1 ){ 
    prev_f = f;
    f /= 2;  
  }
  prev_f *= 2;  

  printf("Calculated epsilon for float is %.10g", prev_f);
  printf("The actual value is %.10f", FLT_EPSILON);
  return 0;
}

where my output is

Calculated epsilon for float is 2.802596929e-45

The actual value is 0.0000001192

Can anyone explain this deviation to me? Is it architecture specific? Compiler dependent? Am i doing something wrong?

EDIT: The problem seemed to be due to me using the -Ofast optimization in gcc. Using -O instead solves the problem.

First, remove prev_f *= 2;. Since the loop remembers the value of f for which 1 + f != 1 failed by storing it in prev_f, the value of prev_f when the loop ends is the last value that caused 1+f not to equal 1, which is the result you want.¹

Second, C implementations are allowed to evaluate floating-point expressions with more precision than their nominal types. It appears your C implementation is effectively evaluating 1+f != 1 with infinite precision (which is possible by recognizing at compile time that 1+f != 1 evaluated to infinite precision is true iff f != 0, so optimization can change it to f != 0). Thus, the loop terminates only when f becomes zero, at which point the previous f was the smallest possible representable positive value, which, for the format commonly used for float is 2⁻¹⁴⁹. (And your current code doubles this and prints 2⁻¹⁴⁸.) Note that the (effective) infinite precision occurs only in evaluating 1 + f != 1, not in the assignment f /= 2;. This is because the C standard requires implementations to “discard” excess precision when casts and assignments are performed. This gives us the solution to this problem: Change 1 + f != 1 to (float) (1 + f) != 1. This will force the evaluation to be done as if in the float format.²

Footnotes

¹ Sort of. The machine episilon is sometimes incorrectly stated to be the smallest value x such that evaluating 1+x produces a value greater than x. However, it is defined to be the different between 1 and the next greater representable value, say 1+𝜀. If we let x be slightly greater than ½𝜀 (such as ½𝜀(1+𝜀)), then eavluating 1+x would produce 1+𝜀 due to rounding, even though x is less than the machine epsilon. However, this code will find the correct value provided the issues described above are fixed and the floating-point radix is two, since it never tests one of these false candidates for the machine epsilon and therefore never finds one.

² Generally, double-rounding problems can occur: When the C implementation uses excess precision to evaluate an expression, it may round the ideal mathematical result to that excess precision. When it is forced to “discard” the excess precision by a cast or assignment, it rounds to the nominal precision. It is possible for these two roundings to cause a different result than if there were only one rounding to the nominal precision. However, that will not be a problem in this particular case.