Difference between size() preceding and within a loop

beatle 0
Viewed 85

In terms of number of operations, compiling time and good practices, is there any difference by doing

for(int i = 0; i < x.size(); i++)

instead of

int size = x.size();
for(int i = 0; i < size; i++)

If there were no compiling optimization I'd say the latter would be preferable, because it is not calling the function stack every time, it assigns one time a variable and then accesses it instead of calling a function.

Does compiling optimization handle this underneath? Let's say I wanted to optimize the code to the most computationally economic as possible; would it be any different?

stack compilation optimization loops c++
2 Answers

Does compiling optimization handle this underneath?

Depends. If the loop content is simple enough, then it may handle it. Otherwise, probably not.

Note that if the loop body changes the size, then the loops are not interchangeable.

eerorika0

Does compiling optimization handle this underneath? Let's say I wanted to optimize the code to the most computationally economic as possible; would it be any different?

Usually, yes. Compiler optimizations handle the best method.

In your case, clang 13.1.6 produces identical code. I created a size1.cpp with your one line for() and a size2.cpp for the two line example.

% cc -O3 -std=c++20 -o size1 size1.cpp -lc++ ; ./size1 ; cc -O3 -std=c++20 -S size1.cpp ; ls -l size1.s ; cc -O3 -std=c++20 -o size2 size2.cpp -lc++ ; ./size2 ; cc -O3 -std=c++20 -S size2.cpp ; ls -l size2.s ; diff size1.s size2.s
size1 180
-rw-r--r--  1 risner  staff  13298 Sep 15 07:55 size1.s
size2 180
-rw-r--r--  1 risner  staff  13298 Sep 15 07:55 size2.s
435c435
<   .asciz  "size1 "
---
>   .asciz  "size2 "

Here are the files:

// size1.cpp
#include <iostream>
#include <array>
int main() {
    int a[8] = { 5, 10, 15, 20, 25, 30, 35, 40 };;
    std::array<int, 8> x = std::to_array(a);
    int z = 0;
    for(int i = 0; i < x.size(); i++)
        z += x[i];
    std::cout<< "size1 " << z << "\n";
}
// size2.cpp
#include <iostream>
#include <array>
int main() {
    int a[8] = { 5, 10, 15, 20, 25, 30, 35, 40 };;
    std::array<int, 8> x = std::to_array(a);
    int z = 0, size = x.size();
    for(int i = 0; i < size; i++)
        z += x[i];;
    std::cout<< "size2 " <<z << "\n";
}
James Risner0
Related