[llvm-dev] LLD: Can we make --warn-backrefs the default?
Fangrui Song via llvm-dev
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)
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David Blaikie via llvm-dev
Fāng-ruì Sòng via llvm-dev
>
> 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.
> On Thu, Aug 27, 2020 at 10:15 PM Fangrui Song via llvm-dev <llvm...@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...@lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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David Blaikie via llvm-dev
On Fri, Aug 28, 2020 at 11:16 AM David Blaikie <dbla...@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.
Fāng-ruì Sòng via llvm-dev
>
>
>
> On Mon, Aug 31, 2020 at 1:24 PM Fāng-ruì Sòng <mas...@google.com> wrote:
>>
>> On Fri, Aug 28, 2020 at 11:16 AM David Blaikie <dbla...@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".
Petr Hosek via llvm-dev
Fāng-ruì Sòng via llvm-dev
>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
)
Peter Collingbourne via llvm-dev
Fāng-ruì Sòng via llvm-dev
Yes, the GNU ld layering checking behavior is incomplete (yet important
and sufficient if we aim for compatibility).
The build system pick two orders:
-la -lb -lc -ld
-la -lc -lb -ld
>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
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.)
Fāng-ruì Sòng via llvm-dev
Supplement: say the application is E, we have the following dependency
set and the link (lD -lB -lD) fail due to an undefined symbol
defined by C (because of the unspecified dependency from B to C):
E -> B
B -> D
C -> D
After adding E -> C, there is chance that --warn-backrefs will still
warn, because of a backward reference from B to C (-lD -lC -lB -lD). The
order topological order (-lD -lB -lC -lD) is good and accepted by GNU ld.
In the warning case, it will give the user actionable feedback on adding
the dependency at the appropriate level (B -> C).
My idea is that this is good enough and works well in practice
(compatible with GNU linkers).
Peter Collingbourne via llvm-dev
The build system pick two orders:
-la -lb -lc -ld
-la -lc -lb -ld
>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
Fāng-ruì Sòng via llvm-dev
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).
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.
Nico Weber via llvm-dev
David Blaikie via llvm-dev
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.
- Dave
James Henderson via llvm-dev
On Thu, Sep 3, 2020 at 5:15 PM Nico Weber <tha...@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.
James Henderson via llvm-dev
James Y Knight via llvm-dev
James Henderson via llvm-dev
Fāng-ruì Sòng via llvm-dev
<llvm...@lists.llvm.org> wrote:
>
> As mentioned previously, our code base is only intended to work with our downstream LLD port. It won't work with other linkers because they don't contain the private patches required to link our code. I haven't got a way of testing with another linker (BFD or gold) because that simply won't work, unrelated to the backrefs issue. I suspect, but have no way to easily verify, that the warning is telling me about both varieties in different places.
>
> On Mon, 21 Sep 2020 at 14:38, James Y Knight <jykn...@google.com> wrote:
>>
>> Does your codebase build correctly with gold or bfd linkers?
>>
>> By which I mean to distinguish:
>> - is the warning triggering on something which is empirically not a problem, or
>> - is it telling you about a problem which you happen to not care about, because you only support linking with lld.
I have identified/fixed more than 100 problems in our code base now.
Many were likely ODR violations. After https://reviews.llvm.org/D77522
I'll say in 99% detected cases GNU ld/gold will fail to link
("undefined reference to"), the remaining 1% cases are A.a B A2.so
(variant of ODR violation; see the last paragraph of
http://lld.llvm.org/ELF/warn_backrefs.html ) where LLD has a different
symbol resolution.
I have not seen any false positive.
I did ask binutils about the "memoryless" archive processing
https://sourceware.org/pipermail/binutils/2020-September/113194.html
but I think they will unlikely adopt a change.
>>
>> On Mon, Sep 21, 2020, 7:21 AM James Henderson via llvm-dev <llvm...@lists.llvm.org> wrote:
>>>
>>> It looks like the conversation has died, but I just wanted to post my own investigation based on our internal code base. The code base itself is quite a sprawling mass, involving multiple different build systems, some bits CMake based, some hand-curated and so on, and I don't fully comprehend it all. I do know that trying to change it is hard at best, and more likely impossible to do so safely. I ran the build using an LLD with the --warn-backrefs option always enabled and saw several dozen distinct instances of the warning being emitted. I can't say how many instances of this warning are genuine library layering violations that can be easily fixed, but I can say that getting any changes to fix things (even if they are genuine and bad violations) will be non-trivial, and unnecessary turbulence in my opinion.
Yeah I consider the conversation has died. Many(most?) users
subscribing to the list use one linker and the portability with GNU
ld/gold isn't a concern, unlikely the distribution world ;-)
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