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From: "fxue at os dot amperecomputing.com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/107066] New: Field initialized before ctor is
 mis-optimized away by DSE
Date: Wed, 28 Sep 2022 10:55:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107066

            Bug ID: 107066
           Summary: Field initialized before ctor is mis-optimized away by
                    DSE
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fxue at os dot amperecomputing.com
  Target Milestone: ---

By means of user-defined new operator, it is possible that a field is
initialized before constructor.

#include <stddef.h>

class A {
public:
  int f1;
  int f2;

  A() : f2(2) { }

  void *operator new(size_t size)
  {
      void *mem =3D ::operator new(size);
      A *obj =3D static_cast<A *>(mem);

      obj->f1 =3D 1;
      return obj;
  }

};

A* foo ()
{
  return new A();
}


The original gimple code of foo() is:

struct A * foo ()
{
  void * D.2444;
  void * _9;

  <bb 2> :
  _9 =3D operator new (8);=20
  MEM[(struct A *)_9].f1 =3D 1;
  MEM[(struct A *)_9] =3D{v} {CLOBBER};
  MEM[(struct A *)_9].f2 =3D 2;
  return _9;

}

In gimple, there exists a pseudo clobber statement marking beginning of
constructor code. Although the statement is of no side effect, it is regard=
ed
as normal store by DSE when determining store redundancy. Consequently, DSE
thought that "MEM[(struct A *)_9].f1 =3D 1" was killed by "MEM[(struct A *)=
_9]
=3D{v} {CLOBBER}", and removed it. After DSE pass,the foo becomes:

struct A * foo ()
{
  void * D.2444;
  void * _9;

  <bb 2> :
  _9 =3D operator new (8);
  MEM[(struct A *)_9] =3D{v} {CLOBBER};
  MEM[(struct A *)_9].f2 =3D 2;
  return _9;

}=