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

[Zachary Turner via lldb-dev]

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



>
> At that point you're already 90% of the way towards what I'm proposing,
> and it's useful independently.
>
>
> I think that “90%” figure is a little off :-) But please don’t take my
> questions as opposition to the general idea. I find the idea very
> interesting, and we could maybe use something similar internally so I am
> interested. That’s why I’m asking questions.
>

Hmm, well I think the 90% figure is pretty accurate.  Because if we
envision a hypothetical command line tool which ingests DWARF from a binary
or set of binaries, and has some command line interface that allows you to
query it in the same way our SymbolFile plugins can be queried, and dumps
its output in some intermediate format (maybe JSON, maybe something else)
and is sufficiently descriptive to make a Clang AST or build LLDB's
internal symbol & type hierarchy out of it, then at that point the only
thing missing from my original proposal is a socket to send that over the
wire and something on the other end to make the Clang AST and LLDB type /
symbol hierarchy.
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