Creating an optimization pipeline – Optimizing IR-2

by Nancy Rohan Creating a new TableGen tool, Drawbacks of TableGen, Exams of IT, Exploring the lli tool, ITCertification Exams
  1. Now, we must replace the existing emit() function with a new version. Additionally, we must declare the required PassBuilder instance at the top of the function:

bool emit(StringRef Argv0, llvm::Module *M,
llvm::TargetMachine *TM,
StringRef InputFilename) {
PassBuilder PB(TM);

  1. To implement the support for pass plugins given on the command line, we must loop through the list of plugin libraries given by the user and try to load the plugin. We’ll emit an error message if this fails; otherwise, we’ll register the passes: for (auto &PluginFN : PassPlugins) {
    auto PassPlugin = PassPlugin::Load(PluginFN);
    if (!PassPlugin) {
    WithColor::error(errs(), Argv0)
    << “Failed to load passes from ‘” << PluginFN << “‘. Request ignored.\n”; continue; } PassPlugin->registerPassBuilderCallbacks(PB);
    }
  2. The information from the static plugin registry is used in a similar way to register those plugins with our PassBuilder instance:

define HANDLE_EXTENSION(Ext) \
getExtPluginInfo().RegisterPassBuilderCallbacks( \
PB);
include “llvm/Support/Extension.def”

  1. Now, we need to declare variables for the different analysis managers. The only parameter is the debug flag: LoopAnalysisManager LAM(DebugPM);
    FunctionAnalysisManager FAM(DebugPM);
    CGSCCAnalysisManager CGAM(DebugPM);
    ModuleAnalysisManager MAM(DebugPM);
  2. Next, we must populate the analysis managers with calls to the respective register method on the PassBuilder instance. Through this call, the analysis manager is populated with the default analysis passes and also runs registration callbacks. We must also make sure that the function analysis manager uses the default alias-analysis pipeline and that all analysis managers know about each other: FAM.registerPass(
    [&] { return PB.buildDefaultAAPipeline(); });
    PB.registerModuleAnalyses(MAM);
    PB.registerCGSCCAnalyses(CGAM);
    PB.registerFunctionAnalyses(FAM);
    PB.registerLoopAnalyses(LAM);
    PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
  3. The MPM module pass manager holds the pass pipeline that we constructed. The instance is initialized with the debug flag: ModulePassManager MPM(DebugPM);
  4. Now, we need to implement two different ways to populate the module pass manager with the pass pipeline. If the user provided a pass pipeline on the command line – that is, they have used the –passes option – then we use this as the pass pipeline: if (!PassPipeline.empty()) {
    if (auto Err = PB.parsePassPipeline(
    MPM, PassPipeline)) {
    WithColor::error(errs(), Argv0)
    << toString(std::move(Err)) << “\n”;
    return false;
    }
    }
  5. Otherwise, we use the chosen optimization level to determine the pass pipeline to construct. The name of the default pass pipeline is default, and it takes the optimization level as a parameter: else {
    StringRef DefaultPass;
    switch (OptLevel) {
    case 0: DefaultPass = “default”; break;
    case 1: DefaultPass = “default”; break;
    case 2: DefaultPass = “default”; break;
    case 3: DefaultPass = “default”; break;
    case -1: DefaultPass = “default”; break;
    case -2: DefaultPass = “default”; break;
    }
    if (auto Err = PB.parsePassPipeline(
    MPM, DefaultPass)) {
    WithColor::error(errs(), Argv0)
    << toString(std::move(Err)) << “\n”;
    return false;
    }
    }
  6. With that, the pass pipeline to run transformations on the IR code has been set up. After this step, we need an open file to write the result to. The system assembler and LLVM IR output are text-based, so we should set the OF_Text flag for them: std::error_code EC;
    sys::fs::OpenFlags OpenFlags = sys::fs::OF_None;
    CodeGenFileType FileType = codegen::getFileType();
    if (FileType == CGFT_AssemblyFile)
    OpenFlags |= sys::fs::OF_Text;
    auto Out = std::make_unique(
    outputFilename(InputFilename), EC, OpenFlags);
    if (EC) {
    WithColor::error(errs(), Argv0)
    << EC.message() << ‘\n’;
    return false;
    }