[PATCH] C++ space optimization: de-cloning con/de/structors

[Jason Merrill]

On Wed, 7 Aug 2002 18:48:37 -0700, Mike Stump <mrs@apple.com> wrote:

> On Wednesday, August 7, 2002, at 05:41 PM, Jason Merrill wrote:

>> Similarly, in the case of the decloning thunks, the compiler is not
>> allowed to add new copy constructor calls.  So this is a correctness
>> issue, not just an optimization.
>
> Doesn't the latitude in:
>
>    12.2  Temporary objects                              [class.temporary]
>...
>    Here, an implementation might use a temporary in  which  to  construct
>    X(2)  before  passing  it  to f() using X's copy-constructor; alterna-
>    tively, X(2) might be constructed in the space used to hold the  argu-
>    ment.
>
> cover this case?

No, it doesn't.  It doesn't give us the latitude to copy the object again
*after* passing it to f().

>> Why not?  These objects, created by copy constructors, are passed to the
>> thunk by invisible reference.
>
> They are?  Why do you think that?  I believe in the code stuart submitted,
> that in fact they are not on darwin.

Dude, I'm talking about basic ABI stuff here.  A type with a non-trivial
copy constructor has TREE_ADDRESSABLE set, and therefore is passed by
invisible reference.

Jason