[LLVMdev] Target Specific Parsing API

[Renato Golin]

Folks,

Following the discussion with Nico and others, I've created PR20683 to
discuss about the implementation of a generic and externalised target
specific parsing API for LLVM, Clang and others.

I have a vague plan involving a generic class (say TargetParser) in
lib/Target that is accessible as an API to any tool that needs target
specific parsing. The idea is then to let targets implement their own
versions of it on a generic part of the code (still lib/Target) so
that we don't break tools if we don't build every back-end (or any
back-end) with LLVM. Maybe this could be fixed on CMake, maybe not.

This class should also allow for customization from the part of the
tools, to either add functionality to existing functions (like
pre-parsing, post-parsing, de-mangling etc) or adding completely new
functions.

Also, since a common side effect of parsing architectural parameters
is to set flags in specific classes, we should expect that every tool
will *have* to override the "updateFlags" method inside the tool
(including LLVM's integrated assembler) to use their own
target-specific structures.

I'm hoping that this makes sense. Please let me know if there's any
major flaw, or existing infrastructure that we could be re-using. But
from the looks of target parsing in both LLVM and Clang, there
isn't... :/

cheers,
--renato