[llvm-dev] Linker option to dump dependency graph

[Rui Ueyama via llvm-dev]

Sorry, I didn't notice that you are asking not to me but to Fangrui. Please
disregard my previous email.

On Fri, Jun 21, 2019 at 9:08 PM Rui Ueyama <ruiu at google.com> wrote:

> No I didn't.
>
> On Fri, Jun 21, 2019 at 10:52 AM Andrew Grieve via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Just wanted to check in on this - did your patches make it past the
>> prototype phase?
>>
>> On Tue, Mar 5, 2019 at 2:41 AM Fāng-ruì Sòng via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> > One thing a dependency graph might not capture is the order in which
>>> events occur, this can be very useful when debugging problems caused by
>>> library selection order.
>>>
>>> The event stream sounds like a more fine-grained --trace (-t).
>>>
>>> > (<from input section>, <symbol>, <to input section>)
>>>
>>> In --no-gc-sections mode and in some analysis, the file name part of
>>> the input section should be good enough.
>>>
>>> > section size and other section/symbol attributes
>>>
>>> If such customization is favored and the complexity isn't a big issue,
>>> it can probably be implemented as format specifiers (I'm thinking of
>>> printf, ps -o, date, ...). The design of
>>> https://github.com/Juniper/libxo can be used for reference.
>>>
>>> We shall flesh out the possible vertex/edge types and additional
>>> information that users may expect.
>>>
>>> On Fri, Mar 1, 2019 at 1:18 PM Peter Collingbourne via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>> >
>>> > You might have realized this already but it's probably not a good idea
>>> to use InputSection::Relocations for this because that ends up missing
>>> anything that becomes a dynamic relocation. I reckon that the code should
>>> be doing exactly what MarkLive.cpp is doing.
>>> >
>>> > Peter
>>> >
>>> > On Thu, Feb 28, 2019 at 5:15 PM Rui Ueyama via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>> >>
>>> >> I hacked up a patch to make lld output a dependency graph in the
>>> graphviz "dot" format.
>>> >>
>>> >> https://gist.github.com/rui314/4eab9f328a5568b682d11c84d328cdaa --
>>> this is a patch, which is just visiting all input sections and relocations.
>>> Note that this is far from completion but just a proof-of-concept.
>>> >>
>>> >> https://gist.github.com/rui314/5e85c559835ecddad46dcf02fe3ffafc is a
>>> result of static-linking a "hello world" program.
>>> >>
>>> >> https://rui314.github.io/hello.svg  -- I rendered the above dot file
>>> with graphviz `sfdp` engine. The rendered graph is too large and very hard
>>> to read. Apparently, I need a better visualization tool.
>>> >>
>>> >> On Wed, Feb 27, 2019 at 7:56 PM Zachary Turner <zturner at google.com>
>>> wrote:
>>> >>>
>>> >>> +1 for graphviz dot format, so that it can be consumed by any one of
>>> many existing graph visualization tools.
>>> >>>
>>> >>> On Wed, Feb 27, 2019 at 7:29 PM Shi, Steven via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>> >>>>
>>> >>>> >To summarise, I think we may
>>> >>>> > be able to do quite well with some very simple extra analysis in
>>> LLD,
>>> >>>> > a machine readable dependency graph would also be very useful for
>>> the
>>> >>>> > more complex cases.
>>> >>>>
>>> >>>> Strongly agree. The linker based dependency graph would be very
>>> useful for Uefi firmware. Below are my usage examples:
>>> >>>> 1. I need to detect the redundant code in my firmware, and I once
>>> wrote a analysis tool to compare the IR level symbols and call graph info
>>> before any optimization and after full optimization (e.g. LTO). But the IR
>>> level info does not support assembly code info well. So, there are many
>>> dependency information missing and false positive in my analysis tool. It
>>> will be more sound if the linker can help output complete and accurate
>>> dependency graph for final executable.
>>> >>>> 2. I need a tool to analyze and  track the firmware module accurate
>>> dependency for build cache soundness. Build performance is now a pain point
>>> in our CI system because every patch need to verify on many build targets
>>> in our side. We hope to enable the build cache (both module level and file
>>> level) to accelerate the build time. For module level build cache enabling,
>>> a very important problem is how to know the module's accurate dependency
>>> efficiently. I'm looking forward to the linker based dependency graph
>>> feature.
>>> >>>>
>>> >>>>
>>> >>>> Thanks
>>> >>>> Steven
>>> >>>> > -----Original Message-----
>>> >>>> > From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On
>>> Behalf Of Peter
>>> >>>> > Smith via llvm-dev
>>> >>>> > Sent: Wednesday, February 27, 2019 6:37 PM
>>> >>>> > To: Michael Spencer <bigcheesegs at gmail.com>
>>> >>>> > Cc: llvm-dev <llvm-dev at lists.llvm.org>
>>> >>>> > Subject: Re: [llvm-dev] Linker option to dump dependency graph
>>> >>>> >
>>> >>>> > Hello,
>>> >>>> >
>>> >>>> > I think outputting a dependency graph is a good idea and would
>>> enable
>>> >>>> > some offline analysis. I think that there is some advantage to
>>> >>>> > building some of the simpler ones in, particularly those that
>>> would
>>> >>>> > need heavy annotations to the dependency graph, in particular
>>> unless
>>> >>>> > we write a sample analysis tool that ships with the release, many
>>> >>>> > users are going to miss out on useful features as they aren't
>>> going to
>>> >>>> > have the time to build one. I've put some comments inline:
>>> >>>> >
>>> >>>> > On Wed, 27 Feb 2019 at 00:31, Michael Spencer via llvm-dev
>>> >>>> > <llvm-dev at lists.llvm.org> wrote:
>>> >>>> > >
>>> >>>> > > On Tue, Feb 26, 2019 at 4:06 PM Rui Ueyama <ruiu at google.com>
>>> wrote:
>>> >>>> > >>
>>> >>>> > >> On Tue, Feb 26, 2019 at 3:31 PM Michael Spencer
>>> >>>> > <bigcheesegs at gmail.com> wrote:
>>> >>>> > >>>
>>> >>>> > >>> On Tue, Feb 26, 2019 at 2:23 PM Rui Ueyama via llvm-dev <llvm-
>>> >>>> > dev at lists.llvm.org> wrote:
>>> >>>> > >>>>
>>> >>>> > >>>> Hi,
>>> >>>> > >>>>
>>> >>>> > >>>> I've heard people say that they want to analyze dependencies
>>> between
>>> >>>> > object files at the linker level so that they can run a
>>> whole-program analysis
>>> >>>> > which cannot be done at the compiler that works for one
>>> compilation unit at
>>> >>>> > a time. I'd like to start a discussion as to what we can do with
>>> it and how to
>>> >>>> > make it possible. I'm also sharing my idea about how to make it
>>> possible.
>>> >>>> > >>>>
>>> >>>> > >>>> Dependency analyses
>>> >>>> > >>>> First, let me start with a few examples of analyses I'm
>>> heard of or
>>> >>>> > thinking about. Dependencies between object files can be
>>> represented as a
>>> >>>> > graph where vertices are input sections and edges are symbols and
>>> >>>> > relocations. Analyses would work on the dependency graph.
>>> Examples of
>>> >>>> > analyses include but not limited to the following:
>>> >>>> > >>>>
>>> >>>> > >>>>  - Figure out why some library or an object file gets linked.
>>> >>>> > >>>>
>>> >>>> >
>>> >>>> > Arm's proprietary linker has a very helpful feature in verbose
>>> mode
>>> >>>> > where it will report on object loading: global/weak definitions
>>> and
>>> >>>> > global/weak references. For libraries you'd get a message like
>>> >>>> > selecting member.o from library.a to define symbol S. This
>>> resulted in
>>> >>>> > quite an effective trace of the linker output that could answer
>>> most
>>> >>>> > "why did this library and object file get loaded question?" One
>>> thing
>>> >>>> > a dependency graph might not capture is the order in which events
>>> >>>> > occur, this can be very useful when debugging problems caused by
>>> >>>> > library selection order.
>>> >>>> >
>>> >>>> > >>>>  - Finding a candidate to eliminate dependency by finding a
>>> "weak" link
>>> >>>> > to a library. We can for example say the dependency to a library
>>> is weak if
>>> >>>> > the library in the graph can be unreachable if we remove N edges
>>> from the
>>> >>>> > graph (which is likely to correspond to removing N function calls
>>> from the
>>> >>>> > code), where N is a small number.
>>> >>>> > >>>>
>>> >>>> > >>>>  - Understanding which of new dependencies increase the
>>> executable
>>> >>>> > size the most, compare to a previous build.
>>> >>>> > >>>>
>>> >>>> >
>>> >>>> > Arm's linker, being focused on embedded systems has a useful
>>> feature
>>> >>>> > that summarises the amount of content taken from each object
>>> broken
>>> >>>> > down into code, ro-data, rw-date etc. This can be helpful in the
>>> face
>>> >>>> > of comdat group elimination and optimisations such as garbage
>>> >>>> > collection and ICF that can be difficult to predict from a
>>> dependency
>>> >>>> > graph. It is true that this information could be added as
>>> attributes
>>> >>>> > but again it may just be easier to write a simple analysis pass
>>> over
>>> >>>> > the output in the linker.
>>> >>>> >
>>> >>>> > >>>>  - Finding bad or circular dependencies between
>>> sub-components.
>>> >>>> > >>>>
>>> >>>> > >>>> There would be many more analyses you want to run at the
>>> linker input
>>> >>>> > level. Currently, lld doesn't actively support such analyses.
>>> There are a few
>>> >>>> > options to make the linker emit dependency information (e.g.
>>> --cref or -Map),
>>> >>>> > but the output of the options is not comprehensive; you cannot
>>> reconstruct a
>>> >>>> > dependency graph from the output of the options.
>>> >>>> >
>>> >>>> >
>>> >>>> >
>>> >>>> > >>>>
>>> >>>> > >>>> Dumping dependency graph
>>> >>>> > >>>> So, I'm thinking if it would be desirable to add a new
>>> feature to the
>>> >>>> > linker to dump an entire dependency graph in such a way that a
>>> graph can be
>>> >>>> > reconstructed by reading it back. Once we have such feature, we
>>> can link a
>>> >>>> > program with the feature enabled and run any kind of dependency
>>> analysis
>>> >>>> > on the output. You can save dumps to compare to previous builds.
>>> You can
>>> >>>> > run any number of analyses on a dump, instead of invoking the
>>> linker for
>>> >>>> > each analysis.
>>> >>>> > >>>>
>>> >>>> > >>>> I don't have a concrete idea about the file output format,
>>> but I believe
>>> >>>> > it is essentially enough to emit triplets of (<from input
>>> section>, <symbol>,
>>> >>>> > <to input section>), which represents an edge, to reconstruct a
>>> graph.
>>> >>>> > >>>>
>>> >>>> > >>>> Thoughts?
>>> >>>> > >>>
>>> >>>> > >>>
>>> >>>> > >>> Back when I worked on the linker I pretty much always had a
>>> way to
>>> >>>> > dump a graphviz dot file to look at things.  Pretty much every
>>> graph
>>> >>>> > library/tool can read dot files, and they are easy to hack up a
>>> parser for.  You
>>> >>>> > can also add attributes to nodes and edges to store arbitrary
>>> data.
>>> >>>> > >>
>>> >>>> > >>
>>> >>>> > >> That's an interesting idea.
>>> >>>> > >>
>>> >>>> > >>> As for what to put it in, it really depends on how detailed
>>> it needs to be.
>>> >>>> > Should symbols and sections be collapsed together?  Should it
>>> include
>>> >>>> > relocation types? Symbol types/binding/size/etc?
>>> >>>> > >>
>>> >>>> > >>
>>> >>>> > >>  Maybe everything? We can for example emit all symbols and
>>> input
>>> >>>> > sections first, and then emit a graph as the second half of the
>>> output. E.g.
>>> >>>> > >>
>>> >>>> > >> Symbols:
>>> >>>> > >>   <list of symbols>
>>> >>>> > >> Sections:
>>> >>>> > >>   <list of sections>
>>> >>>> > >> Graph:
>>> >>>> > >>  1 2 3  // 1st section depends on 3rd section via 2nd symbol
>>> >>>> > >>  5 1 4  // likewise
>>> >>>> > >
>>> >>>> > >
>>> >>>> > > I suppose it's a question of if we want users to need to also
>>> read the inputs
>>> >>>> > if they want things like section size and other section/symbol
>>> attributes.  It
>>> >>>> > would be pretty trivial to include that data as long as we have a
>>> >>>> > format/syntax for it.
>>> >>>> > >
>>> >>>> > > dot supports listing nodes first with attributes and then
>>> referring to them by
>>> >>>> > name later when listing edges.
>>> >>>> > >
>>> >>>> > > - Michael Spencer
>>> >>>> > >
>>> >>>> >
>>> >>>> > I've experimented with dot files for this type of thing in the
>>> past.
>>> >>>> > The difficulty is that they get too large to be realistically
>>> viewed
>>> >>>> > very quickly. At that point you need to write scripts to process
>>> the
>>> >>>> > output and in that case you may as well use JSON or XML, which I
>>> guess
>>> >>>> > could easily be processed into dot files. To summarise, I think
>>> we may
>>> >>>> > be able to do quite well with some very simple extra analysis in
>>> LLD,
>>> >>>> > a machine readable dependency graph would also be very useful for
>>> the
>>> >>>> > more complex cases.
>>> >>>> >
>>> >>>> > Peter
>>> >>>> >
>>> >>>> >  _______________________________________________
>>> >>>> > > LLVM Developers mailing list
>>> >>>> > > llvm-dev at lists.llvm.org
>>> >>>> > > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>> >>>> > _______________________________________________
>>> >>>> > LLVM Developers mailing list
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>>> >>>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>> >>>> _______________________________________________
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>>> >>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>> >>
>>> >> _______________________________________________
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>>> >
>>> >
>>> >
>>> > --
>>> > --
>>> > Peter
>>> > _______________________________________________
>>> > 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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