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From: "jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107163] [10/11/12/13 Regression] huge Compile time increase
 when using templated base classes, virtual method, and Wall since
 r10-2823-g6a07489267e55084
Date: Thu, 23 Mar 2023 19:59:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107163

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gn=
u.org

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
This isn't specific to virtual functions; the same slowdown can be seen with

struct BaseType  {
  int i;
};

template< int Seq >
class DerivedType : public DerivedType< Seq - 1 > { };

template<>
class DerivedType< -1 > : public BaseType { };

int main() {
  DerivedType< COUNT > d;
  d.i =3D 42;
}

The reference to the base member is represented with a long sequence of
COMPONENT_REFs, like
d.derived<5>.derived<4>.derived<3>.derived<2>.derived<1>.derived<0>.derived=
<-1>.base.i
=3D 42

and with r10-2823 we check sequence points across each of those .s, with hi=
gh
algorithmic complexity.  Checking across COMPONENT_REF seems unnecessary, s=
ince
it just selects a subobject of the LHS, there's no additional evaluation.=