Fully integrating the pass into the pass registry – Optimizing IR

To fully integrate the new pass into LLVM, the source of the plugin needs to be structured slightly differently. The main reason for this is that the constructor of the pass class is called from the pass registry, which requires the class interface to be put into a header file.
Like before, you must put the new pass into the Transforms component of LLVM. Begin the implementation by creating the llvm-project/llvm/include/llvm/Transforms/PPProfiler/PPProfiler.h header file. The content of that file is the class definition; put it into the llvm namespace. No other changes are required:

ifndef LLVM_TRANSFORMS_PPPROFILER_PPPROFILER_H
define LLVM_TRANSFORMS_PPPROFILER_PPPROFILER_H
include “llvm/IR/PassManager.h”
namespace llvm {
class PPProfilerIRPass
: public llvm::PassInfoMixin {
public:
llvm::PreservedAnalyses
run(llvm::Module &M, llvm::ModuleAnalysisManager &AM);
private:
void instrument(llvm::Function &F,
llvm::Function *EnterFn,
llvm::Function *ExitFn);
};
} // namespace llvm
endif

Next, copy the source file of the pass plugin, PPProfiler.cpp, into the new directory, llvm-project/llvm/lib/Transforms/PPProfiler. This file needs to be updated in the following way:

1. Since the class definition is now in a header file, you must remove the class definition from this file. At the top, add the include directive for the header file:

include “llvm/Transforms/PPProfiler/PPProfiler.h”

2. The llvmGetPassPluginInfo() function must be removed because the pass wasn’t built into a shared library of its own.
   As before, you also need to provide a CMakeLists.txt file for the build. You must declare the new pass as a new component:

add_llvm_component_library(LLVMPPProfiler
PPProfiler.cpp
LINK_COMPONENTS
Core
Support
)

After, like in the previous section, you need to include the new source directory by adding the following line to the CMakeLists.txt file in the parent directory:

add_subdirectory(PPProfiler)

Inside LLVM, the available passes are kept in the llvm/lib/Passes/ PassRegistry.def database file. You need to update this file. The new pass is a module pass, so we need to search inside the file for the section in which module passes are defined, for example, by searching for the MODULE_PASS macro. Inside this section, add the following line:

MODULE_PASS(“ppprofiler”, PPProfilerIRPass())

This database file is used in the llvm/lib/Passes/PassBuilder.cpp class. This file needs to include your new header file:

include “llvm/Transforms/PPProfiler/PPProfiler.h”

These are all required source changes based on the plugin version of the new pass.
Since you created a new LLVM component, it is also necessary to add a link dependency in the llvm/lib/Passes/CMakeLists.txt file. Under the LINK_COMPONENTS keyword, you need to add a line with the name of the new component:

PPProfiler

Et voilà – you are ready to build and install LLVM. The new pass, ppprofiler, is now available to all LLVM tools. It has been compiled into the libLLVMPPProfiler.a library and available in the build system as the PPProfiler component.
So far, we have talked about how to create a new pass. In the next section, we will examine how to use the ppprofiler pass.