[lldb-dev] RFC: Moving debug info parsing out of process

Zachary Turner via lldb-dev lldb-dev at lists.llvm.org
Fri Mar 1 13:43:35 PST 2019

On Wed, Feb 27, 2019 at 4:35 PM Frédéric Riss <friss at apple.com> wrote:

> On Feb 27, 2019, at 3:14 PM, Zachary Turner <zturner at google.com> wrote:
> On Wed, Feb 27, 2019 at 2:52 PM Frédéric Riss <friss at apple.com> wrote:
>> On Feb 27, 2019, at 10:12 AM, Zachary Turner <zturner at google.com> wrote:
>> For what it's worth, in an earlier message I mentioned that I would
>> probably build the server by using mostly code from LLVM, and making sure
>> that it supported the union of things currently supported by LLDB and
>> LLVM's DWARF parsers.  Doing that would naturally require merging the two
>> (which has been talked about for a long time) as a pre-requisite, and I
>> would expect that for testing purposes we might want something like
>> llvm-dwarfdump but that dumps a higher level description of the information
>> (if we change our DWARF emission code in LLVM for example, to output the
>> exact same type in slightly different ways in the underlying DWARF, we
>> wouldn't want our test to break, for example).  So for example imagine you
>> could run something like `lldb-dwarfdump -lookup-type=foo a.out` and it
>> would dump some description of the type that is resilient to insignificant
>> changes in the underlying DWARF.
>> At which level do you consider the “DWARF parser” to stop and the
>> debugger policy to start? In my view, the DWARF parser stop at the DwarfDIE
>> boundary. Replacing it wouldn’t get us closer to a higher-level abstraction.
> At the level where you have an alternative representation that you no
> longer have to access to the debug info.  In LLDB today, this
> "representation" is a combination of LLDB's own internal symbol hierarchy
> (e.g. lldb_private::Type, lldb_private::Function, etc) and the Clang AST.
> Once you have constructed those 2 things, the DWARF parser is out of the
> picture.
> A lot of the complexity in processing raw DWARF comes from handling
> different versions of the DWARF spec (e.g. supporting DWARF 4 & DWARF 5),
> collecting and interpreting the subset of attributes which happens be
> present, following references to other parts of the DWARF, and then at the
> end of all this (or perhaps during all of this), dealing with "partial
> information" (e.g. something that would have saved me a lot of trouble was
> missing, now I have to do extra work to find it).
> I'm treading DWARF expressions as an exception though, because it would be
> somewhat tedious and not provide much value to convert those into some text
> format and then evaluate the text representation of the expression since
> it's already in a format suitable for processing.  So for this case, you
> could just encode the byte sequence into a hex string and send that.
> I hinted at this already, but part of the problem (at least in my mind) is
> that our "DWARF parser" is intermingled with the code that *interprets the
> parsed DWARF*.  We parse a little bit, build something, parse a little bit
> more, add on to the thing we're building, etc.  This design is fragile and
> makes error handling difficult, so part of what I'm proposing is a
> separation here, where "parse as much as possible, and return an
> intermediate representation that is as finished as we are able to make it".
> This part is independent of whether DWARF parsing is out of process
> however.  That's still useful even if DWARF parsing is in process, and
> we've talked about something like that for a long time, whereby we have
> some kind of API that says "give me the thing, handle all errors
> internally, and either return me a thing which I can trust or an error".
> I'm viewing "thing which I can trust" as some representation which is
> separate from the original DWARF, and which we could test -- for example --
> by writing a tool which dumps this representation
> Ok, here we are talking about something different (which you might have
> been expressing since the beginning and I misinterpreted). If you want to
> decouple dealing with DIEs from creating ASTs as a preliminary, then I
> think this would be super valuable and it addresses my concerns about
> duplicating the AST creation logic.
> I’m sure Greg would have comments about the challenges of lazily parsing
> the DWARF in such a design.
Well, I was originally talking about both lumped into one thing.  Because
this is a necessary precursor to having it be out of process :)

Since we definitely agree on this portion, the question then becomes:
Suppose we have this firm API boundary across which we either return errors
or things that can be trusted.  What are the things which can be trusted?
Are they DIEs?  I'm not sure they should be, because we'd have to
synthesize DIEs on the fly in the case where we got something that was bad
but we tried to "fix" it (in order to sanitize the debug info into
something the caller can make basic assumptions about).  And additionally,
it doesn't really make the client's job much easier as far as parsing goes.

So, I think it should build up a little bit higher representation of the
debug info, perhaps by piecing together information from multiple DIEs and
sources, and return that.  Definitely laziness will have to be maintained,
but I don't think that's inherently more difficult with a design where we
return something higher level than DIEs.

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