[llvm-dev] Linker option to dump dependency graph

Rui Ueyama via llvm-dev llvm-dev at lists.llvm.org
Tue Feb 26 16:05:52 PST 2019

On Tue, Feb 26, 2019 at 3:31 PM Michael Spencer <bigcheesegs at gmail.com>

> 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.
>>  - 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.
>>  - 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.

  <list of symbols>
  <list of sections>
 1 2 3  // 1st section depends on 3rd section via 2nd symbol
 5 1 4  // likewise
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