[SOLVED] Lambda vs. manually inlined code changes GCC's optimizer behavior

Issue

The following code:

#include <vector>

extern std::vector<int> rng;

int main()
{
  auto is_even=[](int x){return x%2==0;};
  int res=0;
  for(int x:rng){
    if(is_even(x))res+=x;
  }

  return res;
}

is optimized by GCC 11.1 (link to Godbolt) in a very different way than:

#include <vector>

extern std::vector<int> rng;

int main()
{
  int res=0;
  for(int x:rng){
    if(x%2==0)res+=x;
  }

return res;
}

(Link to Godbolt.) Besides, the second version (where the lambda has been replaced by direct, manual injection of its body in the place of call), is much faster than the first one.

Is this a GCC bug?

Solution

It's a quirk of the code generation. There is no reason why the lambda version shouldn't be vectorized. In fact, clang vectorizes it as-is. If you specify return type as int, GCC vectorizes it too:

auto is_even = [](int x) -> int { return x % 2 == 0; };

If you use std::accumulate, it's also vectorized. You can report this to GCC so they can fix it.

Answered By - Ayxan Haqverdili