CODEWITHSUNDEEP
c++inline-functions
Milad Khajavi
Milad Khajavi

Reputation: 2859

Why using inline function in c++ doesn't grow binary size?

I have written this code:

inline int a_plus_b_power2(int a, int b) {
  return (a + b) * (a + b); 
}

int main() {
  for(int a = 0; a < 9999999999999; ++a)
    for(int b = 0; b < 999999999999; ++b)
      a_plus_b_power2(a, b);  

  return 0;
}

but why the binary of this program doesn't differ with this program:

inline int a_plus_b_power2(int a, int b) {
  return (a + b) * (a + b); 
}

int main() {
  for(int a = 0; a < 9; ++a)
    for(int b = 0; b < 9; ++b)
      a_plus_b_power2(a, b);  

  return 0;
}

Upvotes: 0

Views: 466

Answers (2)

Thomas Matthews
Thomas Matthews

Reputation: 57749

Inlined functions are only "pasted" once per invocation.

In both your examples, the inlined function is only invoked once, although it is called many times.

I believe you want something like this:

for (unsigned int a = 0; a < 9; ++a)
{
  for (unsigned int b = 0; b < 9; b+= 3) // Incremented by 3 because of 3 calls in loop.
  {
    a_plus_b_power_2(a, b + 0);
    a_plus_b_power_2(a, b + 1);
    a_plus_b_power_2(a, b + 2);
  }
}

The above example may cause the compiler to paste the code inside your inline function 3 times within the loop and increase the size of the binary.

Note: turn off optimizations because optimizations may cause the compiler to convert the inline function into a standalone function inside the loop.

Upvotes: 2

us2012
us2012

Reputation: 16263

You are confusing function inlining with loop unrolling:

Loop unrolling means transforming

for (int i = 0; i < 4; i++)
  a(i);

into

a(0); a(1); a(2); a(3);

while function inlining means transforming 

void a(int i) { cout << i; }

for (int i = 0; i < 4; i++)
  a(i);

into

for (int i = 0; i < 4; i++)
  cout << i;

Compilers do have options to enable loop unrolling (look at -funroll-loops and related options for gcc), but unless you poke them really hard, most of them will be very reluctant to unroll 999999999999 iterations... (the resulting binary would be multiple terabytes).

Upvotes: 6

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