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

Mon Sep 7 01:32:43 PDT 2020

On Fri, 4 Sep 2020 at 23:02, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Thu, Sep 3, 2020 at 5:15 PM Nico Weber <thakis at chromium.org> wrote:
>
>> I wanted to chime in and say that I think we should keep the current
>> default too, for three reasons:
>>
>> 1. The current default is more user friendly. Users shouldn't have to
>> worry about if they pass -lpthread before or after their .o files (...or
>> other libraries. I know I know for pthread it's `-pthread` and the driver
>> will figure it out).
>> https://stackoverflow.com/questions/45135/why-does-the-order-in-which-libraries-are-linked-sometimes-cause-errors-in-gcc
>> is a huge FAQ and a cause of confusion for beginners. LLD's current
>> defaults are much friendlier.
>>
>> 2. The current default is more self-consistent: It's consistent with
>> ld64, link.exe, and hence lld-link and (new) lld.ld64.
>>
>
> I think this misses the aspect of each of those linkers being consistent
> with the counterparts they're intended to emulate, right?
>
>
>> 3. As Peter says correctly, using this for layering isn't a great fit
>> since it only checks a linearized topological sort of the build graph
>> nodes. A good check needs to work on a higher abstraction level.
>>
>> You seem to focus on the use case where compiler and linker commands are
>> generated by some meta build system. I think humans and bash scripts also
>> do a lot of building, and for that case ease-of-use matters much more.
>> Experts who use meta build systems will have a much easier time opting in
>> to this than new folks will have to opting out of it. (And even then, given
>> 2, I think opting in to this is only attractive to non-win non-mac shops,
>> and even then, given 3, I'm not sure it's all that useful there either.)
>>
>
> Not sure there - projects like LLVM having lld provide bfd-ld-esque
> behavior gives a better chance the project won't regress the ability to
> link with bfd or gold (be a bit unfortunate if LLVM could only be linked
> with lld, or putting the cost on developers using bfd/gold to cleanup
> regressions other folks introduce). At least open source projects tend not
> to (so far as I know - probably fairly limited guess, admittedly) be "can
> only compile with this compile and only link with this linker" - probably
> have somewhat broad compatibility goals. Admittedly, for LLVM, maybe we'd
> want to opt-in to this behavior if using lld on any platform, not just
> ld.lld.
>

> I'm not massively invested in the default behavior one way or the other
> here - and can appreciate that users with no interest in linker portability
> with bfd.ld/gold might find the semantics to be an unnecessary annoyance.
> I'd have thought most projects - anymore than a scratch program to be
> thrown away, is probably intended to be compilable/linkable with platform
> defaults/not restricted to one linker.
>

I've not chimed in yet with any point of view on the overall discussion
yet, because our team hasn't had a chance to look at what the impact on us
would be just yet. However, I did want to point out that many (most?) of
those who use our downstream version of LLD probably don't care about any
platform other than ours and possibly one or two others, where GNU
ld.bfd/gold compatibility isn't a concern.

>
> - Dave
>
>
>>
>> Nico
>>
>> On Thu, Sep 3, 2020 at 7:08 PM Fāng-ruì Sòng <maskray at google.com> wrote:
>>
>>> On 2020-09-03, Peter Collingbourne wrote:
>>> >On Thu, Sep 3, 2020 at 2:00 PM Fāng-ruì Sòng <maskray at google.com>
>>> wrote:
>>> >
>>> >> On 2020-09-03, Peter Collingbourne wrote:
>>> >> >On Tue, Sep 1, 2020 at 5:35 PM Fāng-ruì Sòng via llvm-dev <
>>> >> >llvm-dev at lists.llvm.org> wrote:
>>> >> >
>>> >> >> On 2020-09-01, Petr Hosek wrote:
>>> >> >> >I see the GNU ld behavior as a limitation, not as a feature, as
>>> Peter
>>> >> >> Smith
>>> >> >> >also pointed out in https://reviews.llvm.org/D86762. While it
>>> can be
>>> >> >> argued
>>> >> >> >that there are certain cases where it can help detect layering
>>> >> >> >violations as you mentioned in your change, I'm not sure how
>>> valuable
>>> >> that
>>> >> >> >is in practice. Every case I've encountered so far either in
>>> Chrome or
>>> >> in
>>> >> >> >Fuchsia was a valid use case, most commonly interceptors. The
>>> solution
>>> >> >> >has always been the same, wrap all libraries in
>>> >> --start-group/--stop-group
>>> >> >> >and it's what most projects do by default to avoid dealing with
>>> these
>>> >> >> >issues, see for example [Chromium](
>>> >> >> >
>>> >> >>
>>> >>
>>> https://source.chromium.org/chromium/chromium/src/+/master:build/toolchain/gcc_toolchain.gni;l=409
>>> >> >> ).
>>> >> >> >In our case, compatibility with linkers on other platforms is more
>>> >> >> >important than compatibility with GNU ld, so I'd prefer to keep
>>> the
>>> >> >> current
>>> >> >> >behavior. Projects that care about compatibility with GNU ld can
>>> use
>>> >> >> >--warn-backrefs.
>>> >> >>
>>> >> >> I totally understand that some users may not want to deal with GNU
>>> ld
>>> >> >> compatibility:) I'll then question about Chromium's addition of -z
>>> defs:
>>> >> >> https://crrev.com/843583006 :)
>>> >> >>
>>> >> >> -z defs is like a layering checking tool for shared objects while
>>> >> >> --warn-backrefs is for archives. For performance, ABI concerns and
>>> ease
>>> >> >> of deployment, many projects tend to build their own components as
>>> >> >> archives instead of shared objects. In this sense --warn-backrefs
>>> will
>>> >> >> probably be more useful than -z defs.
>>> >> >>
>>> >> >> (
>>> >> >> TIL lorder and tsort were created to define an order of archives in
>>> >> >> early versions of Unix.
>>> >> >>
>>> >>
>>> https://www.gnu.org/software/coreutils/manual/html_node/tsort-background.html
>>> >> >> It seems that the article missed the point that proper library
>>> layering
>>> >> is
>>> >> >> still useful
>>> >> >> )
>>> >> >>
>>> >> >
>>> >> >I'm not a fan of this idea of reframing GNU ld behavior as a
>>> "layering
>>> >> >checking tool". It is an incomplete layering checking tool because
>>> it does
>>> >> >not detect the scenario where, for example, you have the intended
>>> >> >dependency graph:
>>> >> >
>>> >> >A -> B
>>> >> >A -> C
>>> >> >B -> D
>>> >> >C -> D
>>> >> >
>>> >> >(resulting in -la -lb -lc -ld) and you have an unexpected dependency
>>> B ->
>>> >> >C.
>>> >>
>>> >> Yes, the GNU ld layering checking behavior is incomplete (yet
>>> important
>>> >> and sufficient if we aim for compatibility).
>>> >>
>>> >
>>> >As Petr mentioned, only certain users care about this aspect of GNU ld
>>> >compatibility, and those users can just turn this feature on.
>>>
>>> OK, I will consider you and Petr are two users not caring about this
>>> aspect of GNU ld.
>>>
>>> If the other side turns out to be overwhelming: the users who don't want
>>> this
>>> feature can turn it off (--no-warn-backrefs).
>>>
>>> >The build system pick two orders:
>>> >> -la -lb -lc -ld
>>> >> -la -lc -lb -ld
>>> >>
>>> >
>>> >Unless you have multiple programs linking against A, you've just
>>> introduced
>>> >non-determinism in your build tool, which is generally considered to be
>>> a
>>> >Bad Thing.
>>> >
>>> >
>>> >> >There is already a way to detect layering problems, that detects
>>> >> >practical layering problems and not just theoretical ones, which is
>>> to
>>> >> link
>>> >> >programs that use subsets of the libraries. For example, linking a
>>> program
>>> >> >that depends only on B would result in detecting the invalid B -> C
>>> >> >dependency.
>>> >>
>>> >> This is actually cumbersome and is explicitly described in
>>> >> https://reviews.llvm.org/D86762
>>> >
>>> >
>>> >Right, and as I mention below even that doesn't catch all cases. My
>>> point
>>> >is that there is already a way to detect "practical" layering problems,
>>> >defined as "layering problems that cause an undefined symbol error in
>>> >programs that are linked at the same time as the library is built".
>>> >
>>> >- Users who don't care about GNU ld can link their libraries normally.
>>> >- Users who care about GNU ld can pass --warn-backrefs.
>>> >
>>> >Users who care more about "theoretical" layering problems, such as users
>>> >who ship prebuilt archive files to customers, will not be satisfied by
>>> >--warn-backrefs, as this will not catch every possible layering problem
>>> >before shipping the library, as they will not have a copy of every
>>> >customer's program. Instead, they will be better served by a separate
>>> tool.
>>> >
>>> >If B -> C is not specified,
>>> >>
>>> >> * If people write B_test ("linking a program that depends only on B"),
>>> >>    they will notice the dependency issue immediately via the
>>> "undefined
>>> >> symbol" diagnostic.
>>> >> * If such a B_test does not exist. The user may work on a large
>>> >>    application which depends on B (and transitively on D) but not on
>>> A.
>>> >>    OK, they will get an undefined symbol from C. However, it is not
>>> >>    appropriate for them to add a dependency on C because they don't
>>> use C
>>> >>    directly. See the "If adding the dependency does not form a cycle:"
>>> >>    paragraph in D86762.
>>> >>
>>> >>    If their application actually depends on A (thus they will get the
>>> >>    dependency on C), their link may or may not succeed with GNU ld,
>>> >>    depending on whether the build system picks
>>> >>    -la -lb -lc -ld or -la -lc -lb -ld, and the diagnostic can be very
>>> >>    confusing "undefined reference to" while C is actually linked.
>>> >>
>>> >> >It's also worth noting that even that would only detect the
>>> >> >layering problem if the program depends on the part of B that
>>> depends on
>>> >> C.
>>> >>
>>> >>
>>> >> >A better way to go about achieving layering checking would IMHO be to
>>> >> >implement a separate tool (not part of the linker) that is capable of
>>> >> >a complete layering check. Such a tool would only depend on symbol
>>> table
>>> >> >features common to all object formats, so it could probably be
>>> implemented
>>> >> >generically.
>>> >> >
>>> >> >Peter
>>> >>
>>> >> A standalone tool will not achieve sufficient ergonomics.
>>> >> It will read all input files and duplicate the symbol resolution work
>>> done
>>> >> by the linker,
>>> >> which can be slow. (ld.lld --warn-backrefs only imposes negligible
>>> >> overhead when a lazy object is fetched.)
>>> >>
>>> >
>>> >As I mentioned, it would do additional work that the linker is not
>>> >currently doing, which is necessary to implement a complete layering
>>> >checking tool, such as reading all archive members out of each archive
>>> >(whereas linkers only need to read depended-on archive members), and
>>> which
>>> >linkers should not be doing by default exactly for performance reasons.
>>> >Furthermore, the additional tool can be moved out of the critical
>>> >edit-build-run path, unlike a linker based tool, which should improve
>>> >performance as well.
>>>
>>> If the build system uses --start-lib more than archives, making the
>>> linker do
>>> the additional check may have little additional overhead. Lazy object
>>> files
>>> don't have the archive symbol table, which was much useful in the old
>>> days but
>>> limited in today's viewpoint. LLD already has quite a bit logic
>>> iterating the
>>> symbol table, doing one or two more things may not affect overall
>>> performance.
>>>
>>> >An additional tool would give the flexibility of allowing the interface
>>> to
>>> >specify the actual dependencies instead of just giving us only what we
>>> can
>>> >achieve as a result of historical design decisions. It would free build
>>> >tools like cmake or gn from needing to topologically sort libraries in
>>> >order to implement layering checking; instead, they can simply dump
>>> their
>>> >dependency graph.
>>> >
>>> >IMHO, selling this feature as a layering checker is worse than having no
>>> >layering checker at all, because it will mislead users into thinking
>>> "oh,
>>> >I'm using lld's layering checker, that must mean that my program is
>>> >properly layered".
>>> >
>>> >Peter
>>>
>>> The option name is still "--warn-backrefs", not "--check-layering". The
>>> selling
>>> point is more about the compatibility checker. While it does incomplete
>>> layering
>>> checking, it is an almost full GNU ld compatibility checking tool. I have
>>> personally checked dozens of executables with --warn-backrefs problems.
>>> Most
>>> don't link with GNU ld and the few remaining cases are whether symbol
>>> resolution
>>> will pick different definitions with LLD and GNU ld.
>>>
>>> >>
>>> >> >
>>> >> >> >On Mon, Aug 31, 2020 at 1:44 PM Fāng-ruì Sòng <maskray at google.com
>>> >
>>> >> wrote:
>>> >> >> >
>>> >> >> >> On Mon, Aug 31, 2020 at 1:29 PM David Blaikie <
>>> dblaikie at gmail.com>
>>> >> >> wrote:
>>> >> >> >> >
>>> >> >> >> >
>>> >> >> >> >
>>> >> >> >> > 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)?
>>> >> >> >>
>>> >> >> >> Yes, the ELF port (lld/ELF). No other port has implemented
>>> >> >> >> --warn-backrefs. The intion of --warn-backrefs is to capture
>>> behavior
>>> >> >> >> differences with GNU ld's ELF ports. If traditional ELF linkers
>>> don't
>>> >> >> >> behave like that, I can replace "traditional ELF linkers" in
>>> >> >> >> https://reviews.llvm.org/D86762 with "GNU linkers".
>>> >> >> >>
>>> >> >> >> >>
>>> >> >> >> >>
>>> >> >> >> >>
>>> >> >> >> >> > 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
>>> >> >> >>
>>> >> >>
>>> >> >>
>>> >> >> _______________________________________________
>>> >> >> LLVM Developers mailing list
>>> >> >> llvm-dev at lists.llvm.org
>>> >> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>> >> >>
>>> >> >
>>> >> >
>>> >> >--
>>> >> >--
>>> >> >Peter
>>> >>
>>> >
>>> >
>>> >--
>>> >--
>>> >Peter
>>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200907/f1d1c71b/attachment.html>