[Bug target/112980] 64-bit powerpc ELFv2 does not allow nops to be generated before function global entry point

["linkw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112980

--- Comment #17 from Kewen Lin <linkw at gcc dot gnu.org> ---
(In reply to Michael Matz from comment #16)
> (In reply to Kewen Lin from comment #15)
> > I agree, thanks for the comments! btw, I'm not fighting for the current
> > implementation, just want to know more details why users are unable to make
> > use of the current implementation, is it just due to its inefficiency (like
> > the above sequence) or un-usability (unused at all). As your comments, I
> > think it's due to the former (inefficiency)?!
> 
> Okay.  So, yeah, I _think_ that other way (with NOPs between GEP and LEP,
> plus a jump around them) could be made to work with userspace live patching.
> It would just be inefficient.  But do note that that jump around was _not_
> part of the original way of -fpatchable-function-entry, so a change to
> codegen
> would have to have happened anyway to make that other way usable.  And it
> has the
> (perhaps theoretical, who knows :) ) problem of not using the normal 8-byte
> difference between GEP and LEP.
> 

Thanks again for confirming this understanding!

> I think your current proposal from comment #10 is the better from all
> perspectives.

Yeah, I agree. When reworking this support previously, comment #10 like
implementation was considered as a better one but it's not finally made due to
the concern that can break the assumption NOPs should be consecutive, based on
all the inputs here I think it's time to "fix" it by just underscoring this
special not-consecutive NOPs in documentation.