[llvm-dev] Linker option to dump dependency graph

Thu Jun 20 18:51:51 PDT 2019

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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