[lldb-dev] [Reproducers] SBReproducer RFC

[Pavel Labath via lldb-dev]

On 04/01/2019 22:19, Jonas Devlieghere via lldb-dev wrote:
> Hi Everyone,
> 
> In September I sent out an RFC [1] about adding reproducers to LLDB. 
> Over the
> past few months, I landed the reproducer framework, support for the GDB 
> remote
> protocol and a bunch of preparatory changes. There's still an open code 
> review
> [2] for dealing with files, but that one is currently blocked by a change to
> the VFS in LLVM [3].
> 
> The next big piece of work is supporting user commands (e.g. in the 
> driver) and
> SB API calls. Originally I expected these two things to be separate, but 
> Pavel
> made a good case [4] that they're actually very similar.
> 
> I created a prototype of how I envision this to work. As usual, we can
> differentiate between capture and replay.
> 
> ## SB API Capture
> 
> When capturing a reproducer, every SB function/method is instrumented 
> using a
> macro at function entry. The added code tracks the function identifier
> (currently we use its name with __PRETTY_FUNCTION__) and its arguments.
> 
> It also tracks when a function crosses the boundary between internal and
> external use. For example, when someone (be it the driver, the python 
> binding
> or the RPC server) call SBFoo, and in its implementation SBFoo calls 
> SBBar, we
> don't need to record SBBar. When invoking SBFoo during replay, it will 
> itself
> call SBBar.
> 
> When a boundary is crossed, the function name and arguments are 
> serialized to a
> file. This is trivial for basic types. For objects, we maintain a table that
> maps pointer values to indices and serialize the index.
> 
> To keep our table consistent, we also need to track return for functions 
> that
> return an object by value. We have a separate macro that wraps the returned
> object.
> 
> The index is sufficient because every object that is passed to a 
> function has
> crossed the boundary and hence was recorded. During replay (see below) 
> we map
> the index to an address again which ensures consistency.
> 
> ## SB API Replay
> 
> To replay the SB function calls we need a way to invoke the corresponding
> function from its serialized identifier. For every SB function, there's a
> counterpart that deserializes its arguments and invokes the function. These
> functions are added to the map and are called by the replay logic.
> 
> Replaying is just a matter looping over the function identifiers in the
> serialized file, dispatching the right deserialization function, until 
> no more
> data is available.
> 
> The deserialization function for constructors or functions that return 
> by value
> contains additional logic for dealing with the aforementioned indices. The
> resulting objects are added to a table (similar to the one described 
> earlier)
> that maps indices to pointers. Whenever an object is passed as an 
> argument, the
> index is used to get the actual object from the table.
> 
> ## Tool
> 
> Even when using macros, adding the necessary capturing and replay code is
> tedious and scales poorly. For the prototype, we did this by hand, but we
> propose a new clang-based tool to streamline the process.
> 
> For the capture code, the tool would validate that the macro matches the
> function signature, suggesting a fixit if the macros are incorrect or 
> missing.
> Compared to generating the macros altogether, it has the advantage that we
> don't have "configured" files that are harder to debug (without faking line
> numbers etc).
> 
> The deserialization code would be fully generated. As shown in the prototype
> there are a few different cases, depending on whether we have to account for
> objects or not.
> 
> ## Prototype Code
> 
> I created a differential [5] on Phabricator with the prototype. It 
> contains the
> necessary methods to re-run the gdb remote (reproducer) lit test.
> 
> ## Feedback
> 
> Before moving forward I'd like to get the community's input. What do you 
> think
> about this approach? Do you have concerns or can we be smarter 
> somewhere? Any
> feedback would be greatly appreciated!
> 
> Thanks,
> Jonas
> 
> [1] http://lists.llvm.org/pipermail/lldb-dev/2018-September/014184.html
> [2] https://reviews.llvm.org/D54617
> [3] https://reviews.llvm.org/D54277
> [4] https://reviews.llvm.org/D55582
> [5] https://reviews.llvm.org/D56322
> 
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[Adding Tamas for his experience with recording and replaying APIs.]


Thank you for sharing the prototype Jonas. It looks very interesting, 
but there are a couple of things that worry me about it.

The first one is the usage of __PRETTY_FUNCTION__. That sounds like a 
non-starter even for an initial implementation, as the string that 
expands to is going to differ between compilers (gcc and clang will 
probably agree on it, but I know for a fact it will be different on 
msvc). It that was just an internal property of the serialization 
format, then it might be fine, but it looks like you are hardcoding the 
values in code to connect the methods with their replayers, which is 
going to be a problem.

I've been thinking about how could this be done better, and the best 
(though not ideal) way I came up with is using the functions address as 
the key. That's guaranteed to be unique everywhere. Of course, you 
cannot serialize that to a file, but since you already have a central 
place where you list all intercepted functions (to register their 
replayers), that place can be also used to assign unique integer IDs to 
these functions. So then the idea would be that the SB_RECORD macro 
takes the address of the current function, that gets converted to an ID 
in the lookup table, and the ID gets serialized.

The part that bugs me about this is that taking the address of an 
overloaded function is extremely tedious (you have to write something 
like static_cast<function_prototype>(&SBFoo::Bar)). That would mean all 
of these things would have to be passed to the RECORD macro. OTOH, the 
upshot of this would be that the macro would now have sufficient 
information to perform pretty decent error checking on its invocation. 
Another nice about this could be that once you already have a prototype 
and an address of the function, it should be possible (with sufficient 
template-fu) to synthesize replay code for the function automatically, 
at least in the simple cases, which would avoid the repetitiveness of 
the current replay code. Together, this might obviate the need for any 
clang plugins or other funny build steps.


The second thing I noticed is the usage of pointers for identifying 
object. A pointer is good for that but only while the object it points 
to is alive. Once the object is gone, the pointer can (and most likely 
will) be reused. So, it sounds to me like you also need to track the 
lifetime of these objects. That may be as simple as intercepting 
constructor/destructor calls, but I haven't seen anything like that yet 
(though I haven't looked at all details of the patch).

Tying into that is the recording of return values. It looks like the 
current RECORD_RETURN macro will record the address of the temporary 
object in the frame of the current function. However, that address will 
become invalid as soon as the function returns as the result object will 
be copied into a location specified by the caller as a part of the 
return processing. Are you handling this in any way?

The final thing, which I noticed is the lack of any sign of threading 
support. I'm not too worried about that, as that sounds like something 
that could be fitted into the existing framework incrementally, but it 
is something worth keeping in mind, as you're going to run into that 
pretty soon.