[Bug tree-optimization/107066] New: Field initialized before ctor is mis-optimized away by DSE

[Bug tree-optimization/107066] New: Field initialized before ctor is mis-optimized away by DSE

[fxue at os dot amperecomputing.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107066

            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 = ::operator new(size);
      A *obj = static_cast<A *>(mem);

      obj->f1 = 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 = operator new (8); 
  MEM[(struct A *)_9].f1 = 1;
  MEM[(struct A *)_9] ={v} {CLOBBER};
  MEM[(struct A *)_9].f2 = 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 regarded
as normal store by DSE when determining store redundancy. Consequently, DSE
thought that "MEM[(struct A *)_9].f1 = 1" was killed by "MEM[(struct A *)_9]
={v} {CLOBBER}", and removed it. After DSE pass,the foo becomes:

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

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

}

[Bug tree-optimization/107066] Field initialized before ctor is mis-optimized away by DSE

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107066

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
You need -fno-lifetime-dse as documented

https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Optimize-Options.html#index-flifetime-dse

C++ is defined this way otherwise.

[Bug tree-optimization/107066] Field initialized before ctor is mis-optimized away by DSE

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107066

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
See https://gcc.gnu.org/gcc-6/porting_to.html#flifetime-dse also. Which
documented when the change happened back in GCC 6.

[Bug tree-optimization/107066] Field initialized before ctor is mis-optimized away by DSE

[fxue at os dot amperecomputing.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107066

--- Comment #3 from Feng Xue <fxue at os dot amperecomputing.com> ---
Got it. Thanks for that.