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Re: PowerpC pointer_equality_needed optimisation

On Mon, Feb 25, 2008 at 07:57:27PM -0800, H.J. Lu wrote:
> On Mon, Feb 25, 2008 at 7:51 PM, Alan Modra <> wrote:
> > On Mon, Feb 25, 2008 at 03:39:40PM -0800, H.J. Lu wrote:
> >  > I am not sure if it should work. and have
> >
> > > different ABIs. This behavior isn't well defined.
> >
> >  Yes, the ELF gABI is sadly lacking in defining the behaviour of weak
> >  symbols.
> >
> >  I think that a good guiding principle is that shared libraries should
> >  behave as closely as possible to archives.  Therefore, it would be
> >  good if, as much as possible, weak references to definitions in a
> >  shared library behaved the same as weak references to definitions in a
> >  static library.
> >
> >  With shared libraries you always have the possibility of updating the
> >  library, then running your executable using the new library version.
> >  Conceptually this is like relinking the executable object files
> >  against a new static library.  Of course if you relink with a new
> >  static library, weak references may change.  They will resolve to zero
> >  (if the archive doesn't define the symbol in question), or to a symbol
> >  in the library (if the archive does define the symbol), regardless of
> >  how they resolved using an old static library.  We don't currently
> >  support this for shared libraries due to not emitting the necessary
> >  dynamic relocations.
> >
> I don't think we support changing ABI with the same soname. Removing
> a symbol from a DSO is a ABI change. Many applications linked with
> the old DSO will simply crash with normal reference. I don't think
> weak reference should be an exception here.

I agree that removing an exported symbol from a dynamic library is an
ABI change for the library in question.  I claim that removing an
exported symbol from a static library is exactly the same.  It changes
the library ABI too.  Relinking with a normal reference to the removed
symbol will result in a link-time failure.

Undefined weak references are designed to handle such ABI changes.

Alan Modra
Australia Development Lab, IBM

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