https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86023
Bug ID: 86023
Summary: Fake triviality test for internal purposes
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: enhancement
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: glisse at gcc dot gnu.org
Target Milestone: ---
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.
