[LLVMdev] [RFC] Internal command line options should not be statically initialized.

[Andrew Trick]

LLVM's internal command line library needs to evolve. We have an immediate need to build LLVM as a library free of static initializers, but before brute-force fixing this problem, I'd like outline the incremental steps that will lead to a desirable long term solution. We want infrastructure in place to provide an evolutionary path.

In the near term, clients who need llvm-the-library with no static initializers will build with LLVM_NO_STATICINIT. In this mode, existing users of cl::opt will default to being optimized away as constant initializers, and those options will not be available for command line parsing.

A new option class will need be defined, cl::toolopt. Initially, this will share the implementation and API with cl::opt. The only difference will be that cl::toolopt is immune to LLVM_NO_STATICINIT. Options that are required for tool support can simply be type-renamed to toolopt. Since these are not defined in a library, their static initializers are irrelevant.

Eventually, we would like to eliminate the special LLVM_NO_STATICINIT build mode. This can be done by making the -Asserts build just as strict. In the meantime, we would like to run as many unit tests as possible with LLVM_NO_STATICINIT builds. This will be solved by gradually moving cl::opt definitions buried within LLVM libraries to to a new pattern that avoids static initialization.

One easy pattern to follow is to register the option during pass initialization with all the convenient flags and parameters, but refer to a globally defined option storage that enforces the singleton and provides visibility. As long as pass initialization happens before parseCommandLine, usage should be consistent.

Strawman:

cl::optval<bool> MyOption; // Just the storage, no initialization.

MyPass() {
  // Only registers an option with the same optval once.
  Option cl::registerOpt(MyOption, cl::init(false), cl::Hidden,
                         cl::desc("Descriptive string..."), );
}

-Andy