[llvm-dev] Linker option to dump dependency graph

[Rui Ueyama via llvm-dev]

Peter,

Thank you for sharing your experiences and thoughts!

On Wed, Feb 27, 2019 at 2:37 AM Peter Smith <peter.smith at linaro.org> wrote:

> 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.
>

Slightly offtopic, but if that's option is useful, maybe we should
implement that to lld?

>>>>  - 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.
>

I agree with the summary. I believe there are some analyses that are
popular and useful for a large number of users, and that kind of analyses
should probably be implemented directly to lld, so that the analyses are
usable out of the box. Other analyses would be implemented as offline
analyses.

As to the file format, I don't have a strong preference, but if the output
could easily become too large that graphviz can't practically handle, the
dot format may not make sense that much. Perhaps JSON is something that
people are most familiar with, so I'm inclined to it. (But file format is
not the most important topic -- what we'll output is more important.)

Peter
>
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