[LLVMdev] Proposal: split tools/opt/opt.cpp to OptTool and a smaller main()

[Eli Bendersky]

Hello,

I started some refactoring work on tools/opt/opt.cpp in r201116, but
conceptually this is part of a larger effort. I'd like to consult with
llvmdev@ about the best way to move this forward.

Background: opt is a very useful swiss-army-knife tool, and its
capabilities may be useful to custom tools. However, as it stands now opt
is now very modular - almost all its functionality is contained within its
main opt.cpp file.

I think this is also the cause of some code duplication that currently
exists between different LLVM tools. For example, some code from opt.cpp is
duplicated in llc.cpp; the same is true for the logic of adding
optimization passes between opt.cpp and Clang (a comment in opt.cpp even
admits it "duplicates llvm-gcc behaviour", showing its age :-) A lot of
cl::opt definitions are duplicated between different tools and have to be
kept in sync for consistency.

What I'd like to see is most of opt's functionality moving outside of
opt.cpp to make it reusable in other tools. For example, this could be
encapsulated in a class named OptTool, or just a namespace with a bunch of
utility functions, as well as cl::opt definitions.

Such a transition can be done in steps: the first step would be to create
this OptTool within tools/opt. This would immediately enable building
custom opt-based tools by linking in the code from tools/opt, leaving
tools/opt/opt.cpp out. A more ambitious step would be to move the
functionality to a library (lib/Tools?) - enabling code reuse between
different LLVM tools as well as easier use within custom tools.

Any opinions / suggestions welcome. I'll be happy to send out piecemeal
patches that implement this refactoring.

Eli
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