[llvm-dev] Confused by how to register a pass with opt

[Krzysztof Parzyszek via llvm-dev]

You need to call "initializeMyExamplePass" ahead of time. Look at 
include/llvm/InitializePasses.h, and for the actual calls that 
initialize individual passes, at lib/Analysis/Analysis.cpp, 
lib/Transforms/Scalar/Scalar.cpp, etc.

These are all called from "main" in opt.cpp.

-Krzysztof


On 11/5/2018 3:07 PM, Jon Chesterfield via llvm-dev wrote:
> I have a pre instruction selection pass that is registered by calling 
> addPass from Target::addPreISel. This works fine - it runs at the 
> appropriate point in the pipeline and does what it is supposed to.
> 
> I would like to call the pass from opt in order to write some test cases 
> in IR. Unfortunately I'm missing something - despite a determined 
> attempt to do what other passes do, mine doesn't show up in opt. With 
> variables renamed, the pass amounts to:
> 
> #define DEBUG_TYPE "my-pass"
> #define EXAMPLE_NAME "A longer description of my pass"
> 
> namespace {
> class MyExample : public FunctionPass {
> public:
>    static char ID;
>    MyExample() : FunctionPass(ID) {
>      initializeMyExamplePass(*PassRegistry::getPassRegistry());
>    }
> 
>    bool runOnFunction(Function &) override;
>    StringRef getPassName() const override { return EXAMPLE_NAME; }
> 
>    void getAnalysisUsage(AnalysisUsage &AU) const override {
>      AU.addRequired<DominatorTreeWrapperPass>();
>      AU.addRequired<LoopInfoWrapperPass>();
>      AU.addRequired<ScalarEvolutionWrapperPass>();
>    }
> };
> } // namespace
> char MyExample::ID = 0;
> 
> INITIALIZE_PASS_BEGIN(MyExample, DEBUG_TYPE, EXAMPLE_NAME, false, false)
> INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
> INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
> INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
> INITIALIZE_PASS_END(MyExample, DEBUG_TYPE, EXAMPLE_NAME, false, false)
> 
> FunctionPass *llvm::createMyExamplePass() { return new MyExample(); }
> 
> I think the problem is that addPreISel isn't called by opt, so the 
> constructor is never run and the pass never added to the list. This is 
> somewhat confirmed by the following hack:
> 
>   static struct once_t {
>    once_t() { createMyExamplePass(); }
> } unused_variable;
> 
> which creates an instance of the pass then immediately throws it away, 
> in which case opt will let me run the pass as desired.
> 
> So while I have a workaround, said workaround looks absurd - what's the 
> correct way to use this API?
> 
> Thanks!
> 
> Jon
> 
> 
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