Is math within macro computed at compile time?

Question

For example, does MIN_N_THINGIES below compile to 2? Or will I recompute the division every time I use the macro in code (e.g. recomputing the end condition of a for loop each iteration).

#define MAX_N_THINGIES  (10)
#define MIN_N_THINGIES  ((MAX_N_THINGIES) / 5)

uint8_t i;
for (i = 0; i < MIN_N_THINGIES; i++) {
  printf("hi");
}

This question stems from the fact that I'm still learning about the build process. Thanks!

Answer 1

Preprocessor will replace MIN_N_THINGIES with ((10)/5), then it is up to the compiler to optimize ( or not ) the expression.

Answer 2

If you pass -E to gcc it will show what the preprocessor stage outputted.

gcc -E test.c | tail -n11

Outputs:

# 3 "test.c" 2

int main() {
uint8_t i;
for (i = 0; i < ((10) / 5); i++) {
  printf("hi");
}
return 0;
}

Then if you pass -s flag to gcc you will see that the division was optimized out. If you also pass the -o flag you can set the output files and diff them to see that they generated the same code.

gcc -S test.c -o test-with-div.s
edit test.c to make MIN_N_THINGIES equal a const 2
gcc -S test.c -o test-constant.s
diff test-with-div.s test-constant.s
// for educational purposes you should look at the .s files generated.

Then as mentioned in another comment you can change the optimization flag by using -O...

gcc -S test.c -O2 -o test-unroll-loop.s

Will unroll the for loop even such that there isn't even a loop.