SPECIFYING A PASS PIPELINE – Optimizing IR

by Nancy Rohan Creating a new TableGen tool, Exams of IT, Extending the pass pipeline, ITCertification Exams, Simulating function calls

With the –-passes option, you can not only name a single pass but you can also describe a whole pipeline. For example, the default pipeline for optimization level 2 is named default<O2>. You can run the ppprofile pass before the default pipeline with the–-passes=”ppprofile,default<O2>” argument. Please note that the pass names in such a pipeline description must be of the same type.

Now, let’s turn to using the new pass with clang.

Plugging the new pass into clang

In the previous section, you learned how you can run a single pass using opt. This is useful if you need to debug a pass but for a real compiler, the steps should not be that involved.

To achieve the best result, a compiler needs to run the optimization passes in a certain order. The LLVM pass manager has a default order for pass execution. This is also called the default pass pipeline. Using opt, you can specify a different pass pipeline with the –passes option. This is flexible but also complicated for the user. It also turns out that most of the time, you just want to add a new pass at very specific points, such as before optimization passes are run or at the end of the loop optimization processes. These points are called extension points. The PassBuilder class allows you to register a pass at an extension point. For example, you can call the registerPipelineStartEPCallback() method to add a pass to the beginning of the optimization pipeline. This is exactly the place we need for the ppprofiler pass. During optimization, functions may be inlined, and the pass will miss those inline functions. Instead, running the pass before the optimization passes guarantees that all functions are instrumented.

To use this approach, you need to extend the RegisterCB() function in the pass plugin. Add the following code to the function:
  PB.registerPipelineStartEPCallback(
      [](ModulePassManager &PM, OptimizationLevel Level) {
        PM.addPass(PPProfilerIRPass());
      });

Whenever the pass manager populates the default pass pipeline, it calls all the callbacks for the extension points. We simply add the new pass here.

To load the plugin into clang, you can use the -fpass-plugin option. Creating the instrumented executable of the hello.c file now becomes almost trivial:
$ clang -fpass-plugin=./PPProfiler.so hello.c runtime.c

Please run the executable and verify that the run creates the ppprofiler.csv file.