[Bug c++/77443] New: Empty initializer on huge object array member slow down the compilation dramatically with -O1

["gcc at gyoo dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77443

            Bug ID: 77443
           Summary: Empty initializer on huge object array member slow
                    down the compilation dramatically with -O1
           Product: gcc
           Version: 5.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at gyoo dot com
  Target Milestone: ---

Adding an empty initializer on a huge array is multiplying the compilation time
by about x100 when using -O1:


    // time g++ -std=c++14 -O1 -Wall initialization.cc
    class MyObject
    {
    public:
        MyObject() {}
    };


    class MyClassFast
    {
        MyObject array[30000];     // ok: fast to build with -O1
    };


    class MyClassSlow
    {
        MyObject array[30000] {};  // problem: super slow to build with -O1
(x100 slower than the fast version)
    };


    int main()
    {
    //    MyClassFast fast;
        MyClassSlow slow;
    }

Was working fine with gcc4.9.3. Having this slow behaviour for gcc5.2.0,
gcc5.3.0 and gcc5.4.0

Looks like gcc is trying to unroll the constructor for each item of the array.
For instance if the constructor of MyObject is marked private instead of
public, the compilation will generate 30k (number of elements in the array)
compilation error messages.