llvm dev - Sep 2020 - LLD: Can we make --warn-backrefs the default?

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

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.

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.)

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/20200903/757f66a2/attachment.html>

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.

- 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/20200904/5b7dd30a/attachment.html>