[llvm-dev] [lldb-dev] RFC: libtrace

Zachary Turner via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 26 17:14:03 PDT 2018

Yes that’s what I’ve been thinking about as well.

One thing I’ve been giving a lot of thought to is whether to serialize the
handling of trace events.  I want to balance the “this is a library and you
should be able to get it to work for you no matter what your use case is”
aspect with the “you really just don’t want to go there, we know what’s
best for you” aspect.  Then there’s the  fact that not all platforms behave
the same, but we’d like a consistent set of expectations that makes it easy
to use for everyone.

So I’m leaning towards having the library serialize all tace events,
because it’s a nice common denominator that every platform can implement.

To be clear though, I don’t mean that if 2 processes are being traced
simultaneously and A stops followed by B stopping, then the tool will
necessarily block before handling  B’s stop.  I just mean that A and B’s
stop handlers will be invoked on a single thread (not the threads which are
tracing  A or B).

So A stops, posts its stop event on the blessed thread and waits.  Then B
stops and does the same thing.  A’s handler runs, for whatever reason
decides it will continue later, saves off the event somewhere, then
processes B’s.  Later something happens, it decides to continue A, signals
A’s thread which wakes up.

I think this kind of design eliminates a large class of race conditions
without sacrificing any performance.

LLDB doesn’t currently work like this, but it would be nice not to end up
with another split similar to the dwarf split, so I’m curious if you can
think of any fundamental assumptions of LLDB’s architecture that this would
violate.  This way we’d at least know that it’s possible to use the api in
lldb (assuming it does everything lldb needs obviously)


On Tue, Jun 26, 2018 at 1:09 PM Jim Ingham <jingham at apple.com> wrote:

> You'd probably need to pull the Unwinder in if you want backtraces, but
> that part shouldn't be that hard to disentangle.  I don't think you'd need
> much else?
> Basing your work on NativeProcess rather than lldb proper would also cut
> the number of observer processes in half and avoid the context switches
> between the server and the debugger.  That seems more appropriate for a
> lightweight tool.
> Jim
> > On Jun 26, 2018, at 12:59 PM, Jim Ingham via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >
> > So you aren't planning to print values at all, just stop points (i.e.
> you are only interested in the line table and function symbols part of
> >
> > Given what you've described so far, I'm wondering if what you really
> want is the NativeProcess classes with some symbol-file reading pulled in?
> Is there anything that you couldn't do from there?
> >
> > Jim
> >
> >
> >> On Jun 26, 2018, at 12:48 PM, Zachary Turner <zturner at google.com>
> wrote:
> >>
> >> no expression parser or knowledge of any specific programming language.
> >>
> >> Basically I just mean that the parsing of the native DWARF format
> itself is in scope, but anything beyond that is out of scope.  For
> symbolication we have things like llvm-symbolizer that already just work
> and are built on top of LLVM's dwarf parsing code.  Similarly, LLDB's type
> system could be built on top of it as well.  Given that I think everyone
> mostly agrees that unifying on one DWARF parser is a good idea in
> principle, this would mean no functional change from LLDB's point of view,
> it would just continue to do exactly what it does regarding parsing C++
> expressions and converting these into types that clang understands.
> >>
> >> It will probably be useful someday to have an expression parser and
> language specific type system, but when that comes I don't think we'd want
> anything radically different than what LLDB already has.
> >>
> >> On Tue, Jun 26, 2018 at 12:26 PM Jim Ingham <jingham at apple.com> wrote:
> >> Just to be clear, by "no clang integration" do you mean "no expression
> parser" or do you mean something more radical?  For instance, adding a
> TypeSystem and its DWARF parser for C family languages that uses a
> different underlying representation than Clang AST's to store the results
> would be a lot of work that wouldn't be terribly interesting to lldb.  I
> don't think that's what you meant, but wanted to be sure.
> >>
> >> Jim
> >>
> >>> On Jun 26, 2018, at 11:58 AM, Zachary Turner via lldb-dev <
> lldb-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.  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.
> >>>
> >>> 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.
> >>>
> >>> Anyone have any thoughts / strong opinions on this proposal, or where
> the code should live?  Also, does anyone have any suggestions on things
> they’d like to see come out of this?  Whether it’s a specific new tool, new
> functionality to an existing tool, an architectural or design change to
> some existing tool or library, or something else entirely, all feedback and
> ideas are welcome.
> >>>
> >>> Thanks,
> >>> Zach
> >>>
> >>> _______________________________________________
> >>> lldb-dev mailing list
> >>> lldb-dev at lists.llvm.org
> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
> >>
> >
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