Re: Using the PLT for vtables (or not)

[Brian Ryner <bryner at brianryner dot com>]

I am on i686-pc-linux-gnu.  I'm seeing some odd results in my testcase 
which have me a little confused, maybe someone can explain what's going 
on.  I have this class:

class A
{
  virtual void Foo();
};


and the definition of A::Foo() in a DSO.

If you unpack the attached testcase and build it, you'll get executables 
called test1 and test2.  In test1, the vtable slot for A::Foo points 
directly to A::Foo, not to a PLT stub.  In test2, it points to a PLT 
stub.  The only difference is that in test2.cpp, I have an (unused) 
function which calls A::Foo() directly, rather than through the vtable. 
 This is with gcc 3.3.2, binutils version is 2.13.90.0.18-9 (rpm).


Ian Lance Taylor wrote:
> > Well, consider that if I were to build up a struct or array of
> > function pointers from a class's constructor, I would be getting the
> > real function address, not a PLT stub.  This seems like the same thing
> > conceptually as a vtable, and yet it would not have the PLT overhead.
>
>
> Hmmm, I think I made an incorrect assumption here.  What target are
> you talking about?  The behaviour I see for a vtable on
> i686-pc-linux-gnu is the same as building an array of function
> pointers by hand.  What actually happens depends upon whether you use
> -fpic when compiling and whether you link the vtable into a shared
> library.

--
-Brian Ryner
bryner@brianryner.com

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