OK, right, that's the clarity of thought I was missing.
But in this case the compiler still misses the optimization with '(double)0.02f'. https://godbolt.org/z/az7819nKM
I think this is because the optimization isn't safe. I wrote a program to find a counter example to your claim that "the optimization should in theory still apply". It found one. Here's the code:
#include <stdio.h>
#include <stdlib.h>
float mul_as_float(float t) {
t += 0.02f * (float)17;
return t;
}
float mul_as_double(float t) {
t += (double)0.02f * (float)17;
return t;
}
int main() {
while (1) {
unsigned r = rand();
float t = *((float*)&r);
float result1 = mul_as_float(t);
float result2 = mul_as_double(t);
if (result1 != result2) {
printf("Counter example when t is %f (0x%x)\n", t, *((unsigned*)&t));
printf("result1 is %f (0x%x)\n", result1, *((unsigned*)&result1));
printf("result2 is %f (0x%x)\n", result2, *((unsigned*)&result2));
return 0;
}
}
}
Counter example when t is 0.000000 (0x3477d43f)
result1 is 0.340000 (0x3eae1483)
result2 is 0.340000 (0x3eae1482)