SAOC LLDB D integration: 15th Weekly Update

[Luís Ferreira]

Hi D community!

Sorry for being late. I'm here again, to describe what I've done 
during the
fifteenth week of Symmetry Autumn of Code.

## LLVM upstream changes: LLD D demangling

I didn't work on the demangler patches but I touched on some 
other existing
ones, such as implementation of `DW_TAG_immutable_type` on the 
LLVM core which
had some missing pieces and added tests. (See
[here](https://reviews.llvm.org/D113633))

I also added support for other demanglers other than Itanium on 
LLD linker.
This included the freshly added D demangler along with Rust and 
other future
demanglers added to LLVM core.

So now instead of:

```
app.d:16: error: undefined reference to '_D3app7noexistFZi'
```

You will have this:

```
app.d:16: error: undefined reference to 'app.noexist()'
```

This came along with my work on adding D demangler on the LLVM 
core. You can
read more about this change, 
[here](https://reviews.llvm.org/D116279).

## Type name dumping and value dumping

I added D type kind mapping to type name for the rest of the 
built-in types.

I also have found the missing part to make value dumping working. 
I needed to
implement two missing parts:

- A way to discover the bit size based on the D type wrapper type 
kind.
- A way to get the type information based on a type kind using
   `lldb::TypeFlags`

This way LLDB can understand if a certain type kind is built-in, 
has value, is
signed, is integer, is scalar, etc...

So finally, I can print a simple runtime boolean value:

```
(lldb) ta v
Global variables for app.d in app:
(bool) app.falseval = false
(bool) app.trueval = true
```

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

## Expanding value dumping to other built-in types

Having this implemented, I now need to compare and check if the 
DWARF bit size
and encoding match a certain D type kind. The implementation of 
other types are
not yet pushed, since I faced a problem while adding logic to 
platform-specific
size types, such as `real`.

### The `real` problem

Since `real` is, according to D specification, platform-specific, 
I need to
accomudate the right bit size according to a certain target and 
discover the
right floating point encoding. This quite a challange because 
DWARF doesn't
specify the floating point encoding. To try to understand why, I 
did a bit of
research about that, and found
[this](https://gcc.gnu.org/legacy-ml/gcc/2015-10/msg00015.html) 
mailing list
thread from 2015 about distiguish different floating point 
encoding in DWARF.

Right now, there is no way and it seems there is no intention to 
distiguish
target-specific floating point formats on DWARF, because 
according to them,
this should be specified on the target ABI. But what if the ABI 
doesn't specify
this behaviour? We should at least have a way to distiguish IEEE 
interchangable
format and non-interchangable formats, like 128-bit x86 SSE 
floating points.

Fortunately, we don't have to worry much about this, since we 
don't use 128-bit
in any of D implementation, although our spec say:

     real: largest floating point size available

     Implementation Defined: The real floating point type has at 
least the range
     and precision of the double type. On x86 CPUs it is often 
implemented as
     the 80 bit Extended Real type supported by the x86 FPU.

This is wrong, because, AFAIK, on x86-64 System V ABI, 128-bit 
floating point
is the largest available, since AMD64 CPUs are required to have 
at least SSE
extensions, which have support for 128-bit XMM registers to 
perform
floating-point operations.

So, LDC and DMD generates binaries with System V as target ABI 
but uses x87 FPU
instead of SSE for `real`, which means they are out of spec?

Anyway, according to Mathias and as I suggested, the simple way 
to do this is
to hardcode this according the target triple and the DWARF type 
name, but I
think this can be problematic for either when we support 128-bit 
floats or when
the ABI doesn't specify the floating point encoding format.

That said, I would like to have some thoughts on this, specially 
if someone
knows if there is any special case for certain targets and how 
DMD/LDC/GDC
interprets the D spec and target ABI spec.

## What is next?

I plan to finish support for built-in type value dumping and 
hopefully start
implementing DIDerivedType which includes DWARF tags for `const` 
type
modifiers, `alias`/`typedef`s,...