[MCJIT][DebugInfo] [PATCH] Remove object-ownership from RuntimeDyld.

[Lang Hames]

Hi All,

The attached patch removes object file ownership from RuntimeDyld. Key
changes are:

(1) Instead of a taking a unique_ptr<ObjectFile>, the
RuntimeDyld::loadObject method will now take a const ObjectFile reference.
Instead of returning an owning wrapper (ObjectImage) for the loaded object,
RuntimeDyld::loadObject now returns a "LoadedObjectInfo" instance, which
can be queried for information about where the object's sections were
loaded.

(2) The classes that are currently used to manage object file ownership
within RuntimeDyld (ObjectBuffer and ObjectImage) are removed from the API
and turned into implementation details in MCJIT and RuntimeDyldELF
respectively.

(3) Minor changes are made to the DIContext and DWARFContext classes to
support debugging objects loaded by RuntimeDyld.

(4) All use-sites of loadObject are updated.

Background:

RuntimeDyld currently takes ownership of object files to be loaded/linked.
I'm not sure there was ever a strong reason for this, but these days the
only reason to have ownership in RuntimeDyld is to support the flawed
debugging registration system used by RuntimeDyldELF. This system relies on
rewriting objects in place to update address fields of the loaded
symbols/sections. The problem is that ObjectFile instances are backed by
MemoryBuffer instances, which (according to the MemoryBuffer contract)
represent constant memory. RuntimeDyldELF casts away the constness of the
underlying memory to make modifications. Usually the memory backing the
MemoryBuffer is writable, in which case this trick works. Sometimes the
memory is not writable (e.g. when mmaping files from disk), and you crash
the JIT. See http://llvm.org/PR20722 .

ELF debugging is important to a number of clients, and we haven't yet
fleshed out a safe, generic system of debugger registration for the JIT, so
I don't want to remove it. In lieu of that, this patch opts to clean up the
interface to RuntimeDyld, and contain the existing craziness within
RuntimeDyldELF. ELF objects will be cloned to writable memory before being
modified (still a violation of the MemoryBuffer contract, but safe in
practice, and no worse than what we had). MachO support is unaffected. Once
we have a better debugging registration system for the JIT (which is in the
works) we can go back and clean the rest of the nastiness out of
RuntimeDyldELF.

General feedback and style comments welcome. I could also particularly use
feedback from debugger experts (Dave, Eric, Adrian) on the changes to
DIContext/DWARFContext. I think they're sane, but I'm not intimately
familiar with the DebugInfo library.

Hopefully another step towards a cleaner LLVM JIT.

Cheers,
Lang.
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