[llvm-dev] LLD handling of orphaned sections

[Roger Pau Monné via llvm-dev]

On Fri, Jun 21, 2019 at 09:04:34PM +0900, Rui Ueyama wrote:
> I think Geroge (cc'ed) knows better than me in that area.

Thanks for the reply.

> lld is underdocumented, and in particular there is virtually no
> documentation about its linker script support. Our basic strategy is to
> follow the GNU's documentation and the implementations unless it is too
> hard or doesn't match with the rest of lld.

Here LLD is diverging from GNU ld, and TBH I think this is going to
cause issues to other projects that also use linker scripts.

One of the reasons of using linker scripts is to have fine grained
control about section placement. Without a formal statement about LLD
behaviour regarding orphaned section handling in linker scripts the
only option to have fine grained control is to use
--orphan-handling=error and thus to be sure every input section is
handled, which is likely to be hard to justify as a change in order to
support LLD.

> So, as long as it works just
> like GNU, it's fine, but if not, it is hard to say whether it is intended
> or not, just as you explained. This is an area that we need to improve.

As I understand it when there's no linker script LLD has a lot more
freedom to layout sections differently from GNU ld, but I think doing
it for the linker script case is likely to cause trouble in the long
run for projects trying to switch to LLD or trying to support both GNU
ld and LLD (like Xen).

Roger.

> On Thu, Jun 20, 2019 at 11:18 PM Roger Pau Monné via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> 
> > Hello,
> >
> > The handling of orphaned sections in LLD 8 has changed from GNU LD
> > behavior (note that LLD 7 didn't show this behavior). I've reported
> > this as:
> >
> > https://bugs.llvm.org/show_bug.cgi?id=42327
> >
> > It's not clear to me however whether this is intentional or a
> > regression when compared to LLD 7.
> >
> > As stated in that bug report it would be helpful for me to get some
> > kind of documentation that describes LLD intended handling of orphaned
> > sections, so that I can justify fixes in order to support LLD, and
> > that I can be sure what I modify is going to be robust against future
> > changes in LLD. For example GNU LD has:
> >
> > https://sourceware.org/binutils/docs/ld/Orphan-Sections.html
> >
> > Having something like this for LLD (if it's going to diverge from GNU
> > LD behavior) would be very helpful, and would allow me to implement
> > future proof support for LLD in Xen.
> >
> > Thanks, Roger.
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