[Bug c++/91859] Optnization -O2 removes valid and necessary code

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91859

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Status|RESOLVED                    |NEW
   Last reconfirmed|                            |2019-09-23
         Resolution|INVALID                     |---
     Ever confirmed|0                           |1

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The original report is invalid but the DSE optimization is **not** valid when
using C++20 destroying delete:

#include <stdio.h>
#include <stdlib.h>
#include <new>

struct Expression {
  int i = 0;
  void *operator new(size_t);
  void operator delete(Expression *, std::destroying_delete_t);
};

void * Expression::operator new(size_t sz)
{
  return malloc(sz);
}

void Expression::operator delete(Expression *p, std::destroying_delete_t)
{
  Expression * e = p;
  printf("%p : %d\n", e, e->i);
  p->~Expression();
  free(p);
}

int main()
{
  auto p = new Expression();
  p->i = 1;
  delete p;
}

With this feature the compiler doesn't automatically invoke the destructor, so
the store to p->i is not dead, and must not be eliminated.

Not a regression though, because destroying delete wasn't supported before GCC
9.