[lldb-dev] [Reproducers] SBReproducer RFC

[Pavel Labath via lldb-dev]

Thank you for the update Jonas. I haven't looked at the patch in detail, 
but (for better or worse) it seems more-or-less like what I would expect 
at a first glance.

One of the things that occurred to me while looking at this is that it 
would be great to be able to test this code in greater isolation instead 
of just the full "run lldb.exe" test. (I've already said this on the 
file provider patch, but I think that is doubly true here.) There 
shouldn't be anything (except the prolific use of the string SB) tying 
this code to the SB api, and so we should be able to use it to 
record/replay a dummy api for testing purposes.

That would make it easier to test some of the trickier corner cases. For 
instance you could use placement new to make sure you get two objects at 
the same address instead of hoping that this will eventually happen 
during the full lldb run. It would also enable you to make incremental 
patches while you work out the remaining issues instead of having to 
have a fully working system from the get-go.

I think the only big change that you would need to make for this to work 
is to move the core of this code out of the API folder (maybe all the 
way down to Utility?), as in the way the build is set up now, it won't 
get exported in a way that can be used by anyone else.

I think the patch tool you've made strikes a good compromise between 
ease of use and ease of developing it. I expect it will be particularly 
useful during the initial stages, when you need to annotate a large 
number of functions in batches.


On 17/01/2019 03:29, Jonas Devlieghere wrote:
> I've put up a (WIP) patch for the tool (https://reviews.llvm.org/D56822) 
> in case anybody is curious about that.
> 
> On Tue, Jan 15, 2019 at 1:41 PM Jonas Devlieghere <jonas at devlieghere.com 
> <mailto:jonas at devlieghere.com>> wrote:
> 
>     I've updated the patch with a new version of the prototype:
>     https://reviews.llvm.org/D56322
> 
>     It uses Pavel's suggestion to use the function address as a runtime
>     ID. All the deserialization code is generated using templates, with
>     automatic mapping on indices during serialization and deserialization.
> 
>     I (again) manually added the macros for the same set of functions I
>     had in the original prototype. Unsurprisingly this is very
>     error-prone. It's easy to forget to add the right macros for the
>     registry, the function, and the return type. Some of these things
>     can be detected at compile time, other only blow up at run-time. I
>     strongly believe that a tool to add the macros is the way forward.
>     It would be more of a developer tool rather than something that
>     hooks up in the build process.
> 
>     Note that it's still a prototype, there are outstanding issues like
>     void pointers, callbacks and other types of argument that require
>     some kind of additional information to serialize. I also didn't get
>     around yet to the lifetime issue yet that was discussed on IRC last
>     week.
> 
>     Please let me know what you think.
> 
>     Thanks,
>     Jonas
> 
>     On Wed, Jan 9, 2019 at 8:58 AM Jonas Devlieghere
>     <jonas at devlieghere.com <mailto:jonas at devlieghere.com>> wrote:
> 
> 
> 
>         On Wed, Jan 9, 2019 at 8:42 AM Pavel Labath <pavel at labath.sk
>         <mailto:pavel at labath.sk>> wrote:
> 
>             On 09/01/2019 17:15, Jonas Devlieghere wrote:
>              >
>              >
>              > On Wed, Jan 9, 2019 at 5:05 AM Pavel Labath
>             <pavel at labath.sk <mailto:pavel at labath.sk>
>              > <mailto:pavel at labath.sk <mailto:pavel at labath.sk>>> wrote:
>              >
>              >     On 08/01/2019 21:57, Jonas Devlieghere wrote:
>              >      > Before I got around to coding this up I realized
>             you can't take the
>              >      > address of constructors in C++, so the function
>             address won't
>              >     work as an
>              >      > identifier.
>              >      >
>              >
>              >     You gave up way too easily. :P
>              >
>              >
>              > I counted on you having something in mind, it sounded too
>             obvious for
>              > you to have missed.  ;-)
>              >
>              >     I realized that constructors are going to be tricky,
>             but I didn't want
>              >     to dive into those details until I knew if you liked
>             the general idea.
>              >     The most important thing to realize here is that for
>             the identifier
>              >     thingy to work, you don't actually need to use the
>             address of that
>              >     method/constructor as the identifier. It is
>             sufficient to have
>              >     something
>              >     that can be deterministically computed from the
>             function. Then you can
>              >     use the address of *that* as the identifier.
>              >
>              >
>              > I was thinking about that yesterday. I still feel like it
>             would be
>              > better to have this mapping all done at compile time. I
>             was considering
>              > some kind of constexpr hashing but that sounded overkill.
>              >
> 
>             Well.. most of this is done through template
>             meta-programming, which
>             _is_ compile-time. And the fact that I have a use for the new
>             construct/invoke functions I create this way means that even
>             the space
>             used by those isn't completely wasted (although I'm sure
>             this could be
>             made smaller with hard-coded IDs). The biggest impact of
>             this I can
>             think of is the increased number of dynamic relocations that
>             need to be
>             performed by the loader, as it introduces a bunch of
>             function pointers
>             floating around. But even that shouldn't too bad as we have
>             plenty of
>             other sources of dynamic relocs (currently about 4% of the
>             size of
>             liblldb and 10% of lldb-server).
> 
> 
>         Yeah of course, it wasn't my intention to critique your
>         approach. I was talking specifically about the mapping (the
>         std::map) in the prototype, something I asked about earlier in
>         the thread. FWIW I think this would be an excellent trade-off is
>         we don't need a tool to generate code for us. I'm hopeful that
>         we can have the gross of the deserialization code generated this
>         way, most of the "special" cases are still pretty similar and
>         dealing with basic types. Anyway, that should become clear later
>         today as I integrate this into the lldb prototype.
>