[llvm-dev] Linker option to dump dependency graph

Fri Jun 21 05:08:42 PDT 2019

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
>> >>>> > 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
>> >>
>> >> _______________________________________________
>> >> LLVM Developers mailing list
>> >> llvm-dev at lists.llvm.org
>> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>> >
>> >
>> > --
>> > --
>> > 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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