[llvm-dev] [RFC] Adding a new option to lld to make it easy to maintain compatibility with other linkers

[James Y Knight via llvm-dev]

I believe this will report an error on more command-lines than a
traditional linker would fail to link. E.g.:
  ld.lld foo.o A.a B.a A.a B.a
That "should" work fine if the libs are circularly-dependent, as long as
there's only a 1-level cycle. I've seen that sort of construct used a fair
amount, and while the start-group/end-group flags are pretty clearly a
superior option, but are not universally used.

It will also fail to detect some problematic situations in the face of
symbols defined in multiple objects within a group. For example, if you
have:
ld foo.o --start-group A.a B.a --end-group
and:
 foo.o needs a definition of "f"
 A.a:a.o provides "g".
 B.a:b.o provides "f" and requires "g"
 B.a:b2.o provides "g".

IIUC, lld will pull in b1.o to get "f", and then a.o to get "g". This flag
won't report a failure, as they're in a group. But GNU ld will pull in b1.o
to get "f" and then continue searching to the right before circling back
around, and thus find b2.o to provide "g". So, despite no warning message,
you'll end up with a different link. I mean -- obviously don't do crazy
things like that, but still, no warning.

However, both those issues seem perfectly acceptable for an opt-in warning
flag, and should just be documented so that people know its limitations.


On Wed, Apr 4, 2018 at 6:47 PM Rui Ueyama via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi guys,
>
> I'd like to propose a new lld command line option,
> --check-library-dependency. We already have a patch (
> https://reviews.llvm.org/D45195), and I'm satisfied with it, but I'd like
> to get feedback from those who are using lld before submitting.
>
> --check-library-dependency is an option to prevent an undefined reference
> from fetching an archive member written earlier in the command line. It can
> be used to keep your program compatible with GNU linkers after you switch
> to lld. I'll explain the feature and why you may find it useful below.
>
> lld's symbol resolution semantics is more relaxed than traditional Unix
> linkers. For example,
>
>   ld.lld foo.a bar.o
>
> succeeds even if bar.o contains an undefined symbol that have to be
> resolved by some object file in foo.a. Traditional Unix linkers don't allow
> this kind of backward reference, as they visit each file only once from
> left to right in the command line while resolving all undefined symbols at
> the moment of visiting.
>
> In the above case, since there's no undefined symbol when a linker visits
> foo.a, no files are pulled out from foo.a, and because the linker forgets
> about foo.a after visiting, it can't resolve undefined symbols in bar.o
> that could have been resolved otherwise.
>
> That lld accepts more relaxed form means that (besides it'd make more
> sense) you can accidentally write a command line or a build file that works
> only with lld, even if you have a plan to distribute it to wider users who
> may be using GNU linkers. With --check-library-dependency, you can detect a
> library order that doesn't work with other Unix linkers.
>
> The option is also useful to detect cyclic dependencies between static
> archives. Again, lld accepts
>
>   ld.lld foo.a bar.a
>
> even if foo.a and bar.a depend on each other. With
> --check-library-dependency, it is handled as an error.
>
> Here is how the option works. We assign a group ID to each file. A file
> with a smaller group ID can pull out object files from an archive file with
> an equal or greater group ID. Otherwise, it is a reverse dependency and an
> error.
>
> A file outside --{start,end}-group gets a fresh ID when instantiated. All
> files within the same --{start,end}-group get the same group ID. E.g.
>
>   ld.lld A B --start-group C D --end-group E
>
> A and B form group 0. C, D and their member object files form group 1. E
> forms group 2. I think that you can see how this group assignment rule
> simulates the traditional linker's semantics.
>
> I think this is a reasonable option to add. What do you guys think?
>
> Rui
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