[lldb-dev] lldb_private::RegisterContext vs lldb_private::RegisterInfoInterface

[Greg Clayton via lldb-dev]

Seems like this class was added for testing. RegisterInfoInterface is a class that creates a common API for getting lldb_private::RegisterInfo structures.  

A RegisterContext<OS>_<Arch> class uses one of these to be able to create a buffer large enough to store all registers defined in the RegisterInfoInterface and will actually read/write there registers to/from the debugged process. RegisterContext also caches registers values so they don't get read multiple times when the process hasn't resumed. A RegisterContext subclass is needed for each architecture so we can dynamically tell LLDB what the registers look like for a given architecture. It also provides abstractions by letting each register define its registers numbers for Compilers, DWARF, and generic register numbers like PC, SP, FP, return address, and flags registers. This allows the generic part of LLDB to say "I need you to give me the PC register for this thread" and we don't need to know that the register is "eip" on x86, "rip" on x86_64, "r15" on ARM. RegisterContext classes can also determine how registers are read/written: one at a time, or "get all general purpose regs" and "get all FPU regs". So if someone asks a RegisterContext to read the PC, it might go read all GPR regs and then mark them all as valid in the register context buffer cache, so if someone subsequently asks for SP, it will be already cached. 

So RegisterInfoInterface defines a common way that many RegisterContext classes can inherit from in order to give out the lldb_private::RegisterInfo (which is required by all subclasses of RegisterContext) info for a register context, and RegisterContext is the one that actually will interface with the debugged process in order to read/write and cache those registers as efficiently as possible for the current program being debugged.

> On Sep 12, 2017, at 10:59 PM, Ramana via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> 
> Hi,
> 
> When deriving RegisterContext<OS>_<Arch>, why some platforms (Arch+OS)
> are deriving it from lldb_private::RegisterContext while others are
> deriving from lldb_private::RegisterInfoInterface or in other words
> how to decide on the base class to derive from between those two and
> what are the implications?
> 
> Thanks,
> Ramana
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