Re: container<POD> refactoring

[Joe Buck <Joe dot Buck at synopsys dot COM>]

On Mon, Sep 20, 2004 at 01:28:00PM -0500, Gabriel Dos Reis wrote:
> Joe Buck <Joe.Buck@synopsys.COM> writes:
> 
> | Has anyone done any thinking about adding specializations for avoiding
> | bloat with containers of POD types?  For std::vector, it seems that all
> | of the out-of-line functions other than the == and < operators depend
> | only on the size of the POD, and nothing else.
> 
> I would say this is something best done at the linker level.
> User-codes may detect -- at program source level -- whether we
> specialize or not.

The linker could resolve cases where the exact same code is generated.
However, relying on that has two limitations:

- While the exact same code will work, the compiler may not generate the
  exact same code.  Consider vector<int> vs vector<float>.  The latter
  might generate floating point copies for insertion functions.

- Some opportunities for refactoring are missed.  For most of these
  functions, the same code could handle PODs of different sizes with
  nearly no penalty (the __insert_aux function for a POD of four bytes
  and a POD of eight bytes is basically the same; at some point there
  is a multiplication by the size).

- Speed penalty (though I don't know if this is significant).  If we
  can avoid expanding, compiling, and then discarding function bodies
  for many STL functions in common cases, there may be a speedup; this
  has to weigh against the extra cost of parsing extra specializations,
  of course.