[Bug c++/109421] New: Compilation takes a long time for struct that contains a large default-initialized array

[ejakobs at boerboeltrading dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109421

            Bug ID: 109421
           Summary: Compilation takes a long time for struct that contains
                    a large default-initialized array
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ejakobs at boerboeltrading dot com
  Target Milestone: ---

This is similar to #59659, but is still a problem in GCC 11.2.1 and GCC 10.2.1. 

The following code takes a very long time to compile with -O3 enabled:


#include <vector>

#ifndef SIZE
#define SIZE 100000
#endif
struct S
{
  int a;
};

struct T
{
  std::vector<S> arr[SIZE]{}; // remove the default-initializer {} to compile
fast
};

int main(int argc, char** argv)
{
  T t;
  for (int i=0; i<SIZE; ++i) {
    t.arr[i].push_back(S{argc * i}); // use argc so that gcc doesn't optimize
everything out
  }
  return t.arr[SIZE-1][0].a;
}