lldb has two RegisterLocation classes that do slightly different things.

UnwindPlan::Row::RegisterLocation (new: AbstractRegisterLocation) has a
description of how to find a register's value or location, not specific
to a particular stopping point. It may say that at a given offset into a
function, the caller's register has been spilled to stack memory at CFA
minus an offset. Or it may say that the caller's register is at a DWARF
exprssion.

UnwindLLDB::RegisterLocation (new: ConcreteRegisterLocation) is a
specific address where the register is currently stored, or the register
it has been copied into, or its value at this point in the current
function execution.

When lldb stops in a function, it interprets the
AbstractRegisterLocation's instructions using the register context and
stack memory, to create the ConcreteRegisterLocation at this point in
time for this stack frame.

I'm not thrilled with AbstractRegisterLocation and
ConcreteRegisterLocation, but it's better than the same name and it will
be easier to update them if someone suggests a better pair.

The stack validation heuristic is counter-productive in this case, as
the unaligned address is most likely the thing that caused the signal in
the first place.

Most of our code was including Log.h even though that is not where the
"lldb" log channel is defined (Log.h defines the generic logging
infrastructure). This worked because Log.h included Logging.h, even
though it should.

After the recent refactor, it became impossible the two files include
each other in this direction (the opposite inclusion is needed), so this
patch removes the workaround that was put in place and cleans up all
files to include the right thing. It also renames the file to LLDBLog to
better reflect its purpose.

Identified with readability-redundant-control-flow.

Add calls into LanguageRuntime when finding the unwind method to
use out of the 0th (currently executing) stack frame.

Allow for the LanguageRuntimes to indicate if this stack frames
should be treated like a zeroth-frame -- symbolication should be
done based on the saved pc address, not decremented like normal ABI
function calls.

Add methods to RegisterContext and StackFrame to get a pc value
suitable for symbolication, to reduce the number of places in lldb
where we decrement the saved pc values before symbolication.

<rdar://problem/70398009>
Differential Revision: https://reviews.llvm.org/D97644

Summary:
This is the only real unwinder, and things have been this way for quite
a long time. At this point, the class has accumulated so many features
it is unlikely that anyone will want to reimplement the whole thing.

The class is also fairly closely coupled (through UnwindPlans and
FuncUnwinders) with a lot of other lldb components that it is hard to
imagine a different unwinder implementation being substantially
different without reimplementing all of those.

The existing unwinding functionality is nonetheless fairly complex and
there is space for adding more structure to it, but I believe a more
worthwhile effort would be to take the existing UnwindLLDB class and try
to break it down and introduce extension/customization points, instead
of writing a brand new Unwind implementation.

Reviewers: jasonmolenda, JDevlieghere, xiaobai

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D75848