[llvm-dev] How to run InternalizePass

[Russell Wallace via llvm-dev]

I'm working on a whole program optimizer that uses LLVM as a library, and
one of the things I want to do is eliminate dead global functions and
variables even when they are not local to a module. (This understandably
doesn't happen by default because the optimizer has to assume it could be
compiling a library rather than a program.)

I've actually written a function to do this, but then I came across
InternalizePass which seems like it could allow my code to be discarded
because it could flag globals as internal (in the presence of a main
function, according to the documentation) which would allow the existing
dead global elimination optimization pass to do the job.

It seems that InternalizePass is, again understandably, not enabled by
default even when you select optimization level 3. How do you turn it on? I
tried setting PrepareForLTO but that doesn't seem to do anything. Here's
what I have so far:

    legacy::FunctionPassManager FPM(&M);
    legacy::PassManager MPM;

    PassManagerBuilder Builder;
    Builder.OptLevel = 3;
    Builder.PrepareForLTO = true;
    Builder.VerifyInput = true;
    Builder.VerifyOutput = true;
    Builder.populateFunctionPassManager(FPM);
    Builder.populateModulePassManager(MPM);

    FPM.doInitialization();
    for (Function &F : M)
      FPM.run(F);
    FPM.doFinalization();
    MPM.run(M);
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