Why not optimize _Destroy for non-standard allocators, too?

[KHO,THOMAS WIN]

In stl_construct.h in gcc 4.1, _Destroy puts extra effort to check that
the standard allocator is used before optimizing for types that have
trivial destructors.

This behavior is described by the code comment:
   * Destroy a range of objects using the supplied allocator.  For
   * nondefault allocators we do not optimize away invocation of
   * destroy() even if _Tp has a trivial destructor.

In a benchmark that uses a custom allocator that reflects a large
application, we found over a 1% improvement after allowing the same
optimization for non-default allocators. For us, this speedup comes from
faster calls to vector::clear().

What's the intention for not optimizing this function for non-default
allocators?

Regards,

Thomas Kho