SAOC LLDB D integration: 14th Weekly Update

[Luís Ferreira]

Hi D community!

I'm here again, to describe what I've done during the fourteenth 
week of
Symmetry Autumn of Code. Updates on my COVID situation: I 
successfully
recovered from it, just feel some tiredness and lack of energy 
sometimes, but
other than that, dry cough is now occasional and I already 
recovered from the
other symptoms such as lack of smell and taste, and fever. 
Nothing to worry,
from now on :)

## Milestone 4 rough task list

Since the route of the project changed a bit and I had no 
concrete plans for
the 4th milestone on my previous milestone list, I wrote some 
rough list of
tasks to tick off during development. This list can be quite 
extensive, since
there is a lot to integrate with LLDB. The general idea would be 
to adapt Clang
logic and decouple it as much as possible to make that logic more 
language
agnostic and therefore fit D or other future languages:

- **Implement DWARF basic type parsing and integration with the 
TypeSystem:**
   To implement this, it is required to support the majority of 
the basic types
   not including type modifiers (only DWARF base type, 
`DW_TAG_base_type`). It
   is required to have a map between DType wrapper and the 
following LLDB
   enumerations:

   * `lldb::Format`
   * `lldb::Encoding`
   * `lldb::BasicType`

   It is also required to have some knowledge of the bit size of 
each basic
   type. Other built-in types like vectors may be skipped, if 
complicated to
   integrate at this stage.

   This enumeration mapping is responsible of formatting the 
output value with
   the right encoding, according to a certain type. From what I'm 
aware of and
   according to my knowledge on the LLVM infrastructure, this is 
the only thing
   required to get value dumping done, but some additional step 
may be required,
   in practice. I still need to understand a bit more what `Dump`, 
`DumpValue`,
   `DumpSummary` and other dump related methods are for.

   After this, the next step is to support other types such as:

   - **Support type modifiers:** To fully support types it is 
required to read
     other considered type modifiers such as pointers, _typedefs_ 
(aliases) and
     types associated with a type qualifier such as const, 
immutable, ...

   - **Support aggregate types:** Aggregate types such as structs, 
unions or
     classes need to be supported. Only simple support is desired, 
since stuff
     like inheritance can be a bit more complicated to handle. At 
least struct
     fields or base class members may be simple enough.

   - **Support arrays:** Not to be confused with D slices, which 
are technically
     interpreted as structs. D slices can be later recognized as 
native dynamic
     arrays, but synthetic formatters can handle it for now (work 
done on the
     Milestone 2). The idea here, at this stage, would be to 
support static
     stack allocated arrays.

   - **Support other types:** Other types like enumerations can be 
supported.

- **Implement DWARF function parsing and integration with the 
TypeSystem:** To
   implement this, it is required to support
   `DW_TAG_subprogram`/`DW_TAG_subroutine_type` tags which is 
dependent of a
   special type for functions called subroutine types. Support D 
linkage is
   blocked due to inconsistent calling convention on both DMD/LDC
   implementations (see my attempt to fix it
   [here](https://github.com/dlang/dmd/pull/13287)), so C linkage 
should be a
   simple way to support it or rely on GDC implementation.

   After having support for this special type, we should implement 
the
   `ParseFunctionFromDWARF` DWARFASTParser override, which is 
responsible of
   parsing all the function child tags, such as formal parameters 
including
   normal parameters and variadic parameters, and inner functions 
(functions
   with the context of the parent outer function).

   I haven't explored much about function features on LLDB, so 
there might be
   some hidden dependency I don't know about.


## LLVM upstream updates

During the development of DType mapping to lldb::BasicType, I 
found out that
UTF-8 char basic type was missing, so I wrote two patches to 
implement it on
the upstream:

- https://reviews.llvm.org/D116136
- https://reviews.llvm.org/D116138

With that in mind, I also got the missing part to make unicode8 
formatting
working on Clang-based TypeSystem, completing the D language 
plugin as well.
I've done a partial patch to fix that in the past
([D112564](https://reviews.llvm.org/D112564)), so I amended it 
and added the
requested tests to be ready to merge.

## Helpers in DType wrapper for LLDB enumerations

I implemented some helpers required in the DType wrapper to make 
the type kind
with:

   * `lldb::Format`
   * `lldb::Encoding`
   * `lldb::BasicType`

With that done, I started tinkering with LLDB dumping system, 
although, some
unknown dependency is still missing (`Dump` functions are not 
being called with
those helpers implemented). My guess is that some missing type 
information is
required in order to LLDB recognize a type as, e.g. scalar type.

## Type name dumping

Value dumping is not yet working, but I already got a way to dump 
the type
name. For now it is only dumping type name for `bool`. The output 
looks
something like this:

```
(lldb) ta v
Global variables for app.d in app:
(bool) app.bar = <No TLS data currently exists for this thread.>

app.ptr = <could not resolve type>
(bool) app.foobar =
```

You can consult the source code for those changes
[here](https://github.com/devtty63/llvm-project/tree/lldb-d/implement-typesystem-d).

## What is next?

I plan to extend type name dumping to the rest of the built-in 
type kinds by
mapping constant names to the kind enumeration. I also plan to 
hopefully have
some output about `app.foobar` value. For that, I'm going to 
investigate the
missing dependency and implement the missing logic. If that is 
successful, I
will extend it to the rest of the built-in types.

You can also read this on my blog,
[here](https://lsferreira.net/posts/d-saoc-2021-14/).