[llvm-dev] RFC: libtrace

Zachary Turner via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 26 13:38:08 PDT 2018

On Tue, Jun 26, 2018 at 1:28 PM Adrian Prantl <aprantl at apple.com> wrote:

> > On Jun 26, 2018, at 11:58 AM, Zachary Turner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Hi all,
> >
> > We have been thinking internally about a lightweight llvm-based
> ptracer.  To address one question up front: the primary way in which this
> differs from LLDB is that it targets a more narrow use case -- there is no
> scripting support, no clang integration, no dynamic extensibility, no
> support for running jitted code in the target, and no user interface.  We
> have several use cases internally that call for varying levels of
> functionality from such a utility, and being able to use as little as
> possible of the library as is necessary for the given task is important for
> the scale in which we wish to use it.
> >
> > We are still in early discussions and planning, but I think this would
> be a good addition to the LLVM upstream.  Since we’re approaching this as a
> set of small isolated components, my thinking is to work on this completely
> upstream, directly under the llvm project (as opposed to making a separate
> subproject), but I’m open to discussion if anyone feels differently.
> >
> > LLDB has solved a lot of the difficult problems needed for such a tool.
> So in the spirit of code reuse, we think it’s worth trying componentize
> LLDB by sinking pieces into LLVM and rebasing LLDB as well as these smaller
> tools on top of these components, so that smaller tools can reduce code
> duplication and contribute to the overall health of the code base.
> Do you have a rough idea of what components specifically the new tool
> would need to function?

* process & thread control
* platform agnostic ptrace wrapper (not all platforms even have ptrace, and
those that do the usage and capabilities vary quite a bit)
* install various kinds of traps
* monitor cpu performance counters
* symbol file parsing
* symbol resolution (name <-> addr and line <-> addr)
* unwinding and backtrace generation

> >  At the same time we think that in doing so we can break things up into
> more granular pieces, ultimately exposing a larger testing surface and
> enabling us to create exhaustive tests, giving LLDB more fine grained
> testing of important subsystems.
> Are you thinking of the new utility as something that would naturally live
> in llvm/tools or as something that would live in the LLDB repository?
I would rather put it under LLDB and then link LLDB against certain pieces
in cases where that makes sense.

> >
> > A good example of this would be LLDB’s DWARF parsing code, which is more
> featureful than LLVM’s but has kind of evolved in parallel.  Sinking this
> into LLVM would be one early target of such an effort, although over time
> there would likely be more.
> As you are undoubtedly aware we've been carefully rearchitecting LLVM's
> DWARF parser over the last few years to eventually become featureful enough
> so that LLDB could use it, so any help on that front would be most welcome.
> As long as we are careful to not regress in performance/lazyness, features
> and fault-tolerance, deduplicating the implementations can only be good for
> LLVM and LLDB.
> Yea, this is the general idea.   Has anyone actively been working on this
specific effort recently?  To my knowledge someone started and then never
finished, but the efforts also never made it upstream, so my understanding
is that it's a goal, but one that nobody has made significant headway on.
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