[llvm-dev] Linker option to dump dependency graph

Wed Feb 27 19:55:26 PST 2019

+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
> > _______________________________________________
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> > llvm-dev at lists.llvm.org
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