[Bug c++/70723] Missed optimization opportunity for lambda converted to fun-ptr

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70723

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-04-19
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The initializer of erasedTypeVTable is not statically available but involves
runtime computation:

;; Function void __static_initialization_and_destruction_0(int, int) (null)
;; enabled by -tree-original


if (__initialize_p == 1)
  {
    if (__priority == 65535)
      {
        <<cleanup_point <<< Unknown tree: expr_stmt
  erasedTypeVTable.dtor = {anonymous}::<lambda(void*)>::operator void
(*)(void*) (&TARGET_EXPR <D.5974, {}>) >>>>>;
        <<< Unknown tree: expr_stmt
  <<< Unknown tree: void_cst >>> >>>;
      }
  }


;; Function (static initializers for t.C) (null)
;; enabled by -tree-original


<<cleanup_point <<< Unknown tree: expr_stmt
  __static_initialization_and_destruction_0 (1, 65535) >>>>>;


thats probably because the lambda is not evaluated in a constexpr context.
If I make erasedTypeVTable a constexpr I get

t.C: In instantiation of ‘constexpr const {anonymous}::ErasedTypeVTable
{anonymous}::erasedTypeVTable<{anonymous}::myType>’:
t.C:36:7:   required from here
t.C:20:41: error: call to non-constexpr function
‘{anonymous}::<lambda(void*)>::operator void (*)(void*)() const’
       static constexpr ErasedTypeVTable erasedTypeVTable = {
                                         ^~~~~~~~~~~~~~~~

not sure how I would make the lambda a constexpr.


There is no pass in GCC that would try to turn the runtime initialization
into static init again (optimizing the runtime initializers and parsing
them back to static initializers).