Re: [PATCH] PowerPC64 ELFv2 ABI 3/6: PLT local entry point

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

Steven Munroe wrote:
> On Tue, 2013-11-12 at 22:22 +0100, Ulrich Weigand wrote:
> > Hello,
> > 
> > this is a follow-on to the previous patch to support the ELFv2 ABI in the
> > dynamic loader, split off into its own patch since it is just an optional
> > optimization.
> > 
> > In the ELFv2 ABI, most functions define both a global and a local entry
> > point; the local entry requires r2 to be already set up by the caller
> > to point to the callee's TOC; while the global entry does not require
> > the caller to know about the callee's TOC, but it needs to set up r12
> > to the callee's entry point address.
> > 
> > Now, when setting up a PLT slot, the dynamic linker will usually need
> > to enter the target function's global entry point.  However, if the
> > linker can prove that the target function is in the same DSO as the
> > PLT slot itself, and the whole DSO only uses a single TOC (which the
> > linker will let ld.so know via a DT_PPC64_OPT entry), then it is
> > possible to actually enter the local entry point address into the
> > PLT slot, for a slight improvement in performance.
> > 
> Would it simplify things if the dynamic linker always transferred to
> control to the global entry point. As this is only first call to each
> PLT entry, and only for self calls, it is not clear that calling the
> local entry point actually saves cycles considering the extra logic in
> pc64_local_entry_offset.

Actually, this is about which value is installed into the PLT, so the
optimization causes the local entry point to be used on *every*
successive call via the PLT, not just the first.  (I certainly
agree that if it were for the first call only, it wouldn't be a win.)

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU/Linux compilers and toolchain
  Ulrich.Weigand@de.ibm.com