[TVM Discuss] [Questions] How is __tvm_module_startup invoked?

[JC Li via TVM Discuss]

I think I got it. The LLVM host module compilation generates it.

src/target/llvm/llvm_module.cc:
```
TVM_REGISTER_GLOBAL("target.build.llvm").set_body_typed([](IRModule mod, 
std::string target) {
  auto n = make_object<LLVMModuleNode>();
  n->Init(mod, target);
  return runtime::Module(n);
});
...
  void Init(const IRModule& mod, std::string target) {
    InitializeLLVM();
    tm_ = GetLLVMTargetMachine(target);
    ...
    module_ = cg->Finish();
    ...
}
```

The cg->Finish() calls src/target/llvm/codegen_llvm.cc:
```
std::unique_ptr<llvm::Module> CodeGenLLVM::Finish() {
  this->AddStartupFunction();
  for (size_t i = 0; i < link_modules_.size(); ++i) {
    CHECK(!llvm::Linker::linkModules(*module_, std::move(link_modules_[i])))
        << "Failed to link modules";
  }
  link_modules_.clear();
  // optimize
  this->Optimize();
  return std::move(module_);
}
```

The AddStartupFunction() is overridden in src/target/llvm/codegen_cpu.cc. Here 
you see the __tvm_module_startup. 
```
void CodeGenCPU::AddStartupFunction() {
  if (export_system_symbols_.size() != 0) {
    llvm::FunctionType* ftype = llvm::FunctionType::get(t_void_, {}, false);
    function_ = llvm::Function::Create(ftype, llvm::Function::InternalLinkage,
                                       "__tvm_module_startup", module_.get());
    llvm::BasicBlock* startup_entry = llvm::BasicBlock::Create(*ctx_, "entry", 
function_);
    builder_->SetInsertPoint(startup_entry);
    for (const auto& kv : export_system_symbols_) {
      llvm::Value* name = GetConstString(kv.first);
      builder_->CreateCall(f_tvm_register_system_symbol_,
                           {name, builder_->CreateBitCast(kv.second, 
t_void_p_)});
    }
    llvm::appendToGlobalCtors(*module_, function_, 65535);
    builder_->CreateRet(nullptr);
  }
}
```





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