https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96945

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |tree-optimization
   Last reconfirmed|                            |2023-07-13
             Status|UNCONFIRMED                 |NEW
            Summary|optimizations regression    |unused std::vector is not
                   |when defaulting copy        |always optimized away
                   |constructor                 |
     Ever confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

In the case of having a move constructor, there is a loop generated for the
constructor (and since this is an empty struct, the loop is empty).
In the case of not having a move constructo, we get:
  MEM <unsigned char[3]> [(char * {ref-all})_30] = MEM <unsigned char[3]>
[(char * {ref-all})&D.26026];



In the end DSE does not remove that store even though it is dead right before
the operator delete.

  _30 = operator new (3);
  MEM <unsigned char[3]> [(char * {ref-all})_30] = MEM <unsigned char[3]>
[(char * {ref-all})&D.26026];
  D.26026 ={v} {CLOBBER(eol)};
  operator delete (_30, 3); [tail call]

Also if we add an field and initialize it to 0, we get a memcpy:
  __builtin_memcpy (_40, &D.26033, 12);
Which is also not DSEd:
  _40 = operator new (12);
  __builtin_memcpy (_40, &D.26033, 12);
  D.26033 ={v} {CLOBBER(eol)};
  operator delete (_40, 12); [tail call]

Maybe the clobber is getting in the way here ....

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