[llvm-dev] LLD: Can we make --warn-backrefs the default?

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Mon Aug 31 13:29:47 PDT 2020

On Mon, Aug 31, 2020 at 1:24 PM Fāng-ruì Sòng <maskray at google.com> wrote:

> On Fri, Aug 28, 2020 at 11:16 AM David Blaikie <dblaikie at gmail.com> wrote:
> >
> > Would you like to conduct the conversation here, or on the review
> thread? (I lean towards having them here, but don't mind if folks feel like
> it keeps the noise down & want to more post a notice saying "hey, here's
> this thing, if you're interested, go discuss it over there" - more an
> optional opt-in rather than requiring people to opt-out via muting the
> thread, etc)
> Yes, we can conduct the "should we enable --warn-backrefs by default"
> conversation here. Since the semantics --warn-backrefs of are a bit
> complex, we need a documentation. https://reviews.llvm.org/D86762 is
> put up to get wording suggestions. Explicitly adding the people to the
> CC list...
> FWIW for many code bases, --warn-backrefs should produce no warnings
> (error if --fatal-warnings). For some code bases, GNU ld may error
> "undefined reference".  --warn-backrefs can catch such problems.

One of the questions raised on the thread there was about different linker
semantics. I assume the "--warn-backrefs by default" we're discussing is
only related to the ld.lld frontend? Not the Windows linker lld behavior
(or ld64 (old or new) lld behavior)?

> > On Thu, Aug 27, 2020 at 10:15 PM Fangrui Song via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> Hi all, LLD's --warn-backrefs is a tool to identify potential
> >> incompatible archive selection semantics with traditional Unix linkers.
> >> I have improved it (via D77522,D77630 and D77512) to a state where a
> >> --warn-backrefs diagnostic almost assuredly means that the link will
> >> fail with GNU ld, or the symbol will get different resolution in GNU ld
> and LLD.
> >>
> >> My conclusion is that --warn-backrefs is a very useful layering check
> tool.
> >> I just wrote a documentation about the advantage (of GNU ld's archive
> >> selection semantics..... But we can do better with --warn-backrefs!
> >> GNU ld just reports "undefined reference" with no actionable feedback
> >> about the offending archive)
> >>
> >> https://reviews.llvm.org/D86762
> >>
> >> I am wondering whether in the next release we can make --warn-backrefs
> >> the default.  I have added many known users to the review.
> >> (There is no need for --no-warn-backrefs because
> --warn-backrefs-exclude='*' does the same job)
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> --
> 宋方睿
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