Re: PATCH: Optimize protected call for i386

[Alan Modra <amodra at bigpond dot net dot au>]

On Wed, May 14, 2003 at 04:24:39PM -0700, Richard Henderson wrote:
> On Thu, May 15, 2003 at 08:32:53AM +0930, Alan Modra wrote:
> > I see.  I think this is wrong.  gcc needs to emit the normal PIC call,
> > ie. "call foo@PLT" so the the linker can set up a plt entry if
> > needed.
> 
> I disagree.
> 
> > A plt entry is needed for protected symbols so that function
> > pointer comparisons work, since the x86 ABI uses the .plt entry for
> > the value of a function symbol referenced by a dynamic object.
> > Consider liba.so exporting a protected symbol foo.  Will a function
> > pointer, value foo, passed to libb.so compare equal to &foo in
> > libb.so?  What about a function pointer, value foo, passed from
> > libb.so back to liba.so?
> 
> None of which has anything to do with call instructions.

Well, it may be that the linker will set up a plt entry anyway,
thus making the function pointer comparisons work properly.  The
problem is that if gcc emits "call foo" for protected syms, you'll get
an R_386_PC32 reloc on foo.  How is the linker supposed to distinguish
this reloc from an R_386_PC32 on ".long foo - ."?  In the second case,
you really want to use the plt entry address.

> If liba.so contains a call to foo, ld *knows* that foo *must*
> resolve to the version in liba.so and nowhere else.  Thus we
> can direct the call to foo without going through a PLT entry.

Exactly.  ld can do this even when gcc emits "call foo@PLT".

> If you take the address of foo, in *any* DSO, you will not be
> using either R_386_PC32 or R_386_PLT32.  You'll be using either
> R_386_32 or R_386_GOT32.

Does the ABI preclude use of R_386_PC32 from assembly?

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre