While we cannot make std::pair or std::tuple trivial for now for ABI reasons, it should still be safe to use memcpy-type optimizations for them when it is safe for each member. We probably don't want to lie to the user in is_trivial or is_trivially_copyable (?), but we could at least introduce an internal version of those traits, specialized for a few types like pair/tuple, and use them so _GLIBCXX_MOVE_BACKWARD3 would use memmove on std::pair<int,int> for instance.

According to some benchmark, this might (they weren't exactly testing this) change the average performance of insert/erase in boost::flat_map<,,std::vector<>> by a factor of 2. Of course, it won't affect the default flat_map, which uses boost's vector and traits, so it isn't a real solution, just a small band-aid.

The exact traits to specialize depend on PR 68350.

#include <vector>
#include <utility>
#ifdef FAST
struct A {
  int first,second;
  A(int a,int b):first(a),second(b){}
  A()=default;
};
#else
typedef std::pair<int,int> A;
#endif
typedef std::vector<A> V;

int main(int argc,char**){
  V v;
  for(int i=0;i<100000;++i){
    v.insert(v.begin(),{i,i});
  }
  return v[argc].second;
}

At -O3, I get 3.41s for std::pair, 1.00s for the struct, and an intermediate 1.99s for the struct minus the default constructor.