[llvm-dev] Linker option to dump dependency graph

Thu Feb 28 17:14:42 PST 2019

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
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