[llvm-dev] How to make LLVM go faster?

[Andrew Kelley via llvm-dev]

Here is some timing information from running the Zig standard library tests:

$ ./zig test ../std/index.zig --enable-timing-info
                Name       Start         End    Duration     Percent
          Initialize      0.0000      0.0010      0.0010      0.0001
   Semantic Analysis      0.0010      0.9968      0.9958      0.1192
     Code Generation      0.9968      1.4000      0.4032      0.0483
    LLVM Emit Output      1.4000      8.1759      6.7760      0.8112
  Build Dependencies      8.1759      8.3341      0.1581      0.0189
           LLVM Link      8.3341      8.3530      0.0189      0.0023
               Total      0.0000      8.3530      8.3530      1.0000

81% of the time was spent waiting for LLVM to turn a Module into an object
file. This is with optimizations off, FastISel, no module verification, etc.

How can I speed this up? Any tips or things to look into?

Here's the function that 81% of the time is spent inside:

bool ZigLLVMTargetMachineEmitToFile(LLVMTargetMachineRef targ_machine_ref,
LLVMModuleRef module_ref,
        const char *filename, ZigLLVM_EmitOutputType output_type, char
**error_message, bool is_debug, bool is_small)
{
    std::error_code EC;
    raw_fd_ostream dest(filename, EC, sys::fs::F_None);
    if (EC) {
        *error_message = strdup((const char
*)StringRef(EC.message()).bytes_begin());
        return true;
    }
    TargetMachine* target_machine =
reinterpret_cast<TargetMachine*>(targ_machine_ref);
    target_machine->setO0WantsFastISel(true);

    Module* module = unwrap(module_ref);

    PassManagerBuilder *PMBuilder = new(std::nothrow) PassManagerBuilder();
    if (PMBuilder == nullptr) {
        *error_message = strdup("memory allocation failure");
        return true;
    }
    PMBuilder->OptLevel = target_machine->getOptLevel();
    PMBuilder->SizeLevel = is_small ? 2 : 0;

    PMBuilder->DisableTailCalls = is_debug;
    PMBuilder->DisableUnitAtATime = is_debug;
    PMBuilder->DisableUnrollLoops = is_debug;
    PMBuilder->SLPVectorize = !is_debug;
    PMBuilder->LoopVectorize = !is_debug;
    PMBuilder->RerollLoops = !is_debug;
    // Leaving NewGVN as default (off) because when on it caused issue #673
    //PMBuilder->NewGVN = !is_debug;
    PMBuilder->DisableGVNLoadPRE = is_debug;
    PMBuilder->VerifyInput = assertions_on;
    PMBuilder->VerifyOutput = assertions_on;
    PMBuilder->MergeFunctions = !is_debug;
    PMBuilder->PrepareForLTO = false;
    PMBuilder->PrepareForThinLTO = false;
    PMBuilder->PerformThinLTO = false;

    TargetLibraryInfoImpl tlii(Triple(module->getTargetTriple()));
    PMBuilder->LibraryInfo = &tlii;

    if (is_debug) {
        PMBuilder->Inliner = createAlwaysInlinerLegacyPass(false);
    } else {
        target_machine->adjustPassManager(*PMBuilder);

        PMBuilder->addExtension(PassManagerBuilder::EP_EarlyAsPossible,
addDiscriminatorsPass);
        PMBuilder->Inliner =
createFunctionInliningPass(PMBuilder->OptLevel, PMBuilder->SizeLevel,
false);
    }

    addCoroutinePassesToExtensionPoints(*PMBuilder);

    // Set up the per-function pass manager.
    legacy::FunctionPassManager FPM = legacy::FunctionPassManager(module);
    auto tliwp = new(std::nothrow) TargetLibraryInfoWrapperPass(tlii);
    FPM.add(tliwp);

FPM.add(createTargetTransformInfoWrapperPass(target_machine->getTargetIRAnalysis()));
    if (assertions_on) {
        FPM.add(createVerifierPass());
    }
    PMBuilder->populateFunctionPassManager(FPM);

    // Set up the per-module pass manager.
    legacy::PassManager MPM;

MPM.add(createTargetTransformInfoWrapperPass(target_machine->getTargetIRAnalysis()));
    PMBuilder->populateModulePassManager(MPM);

    // Set output pass.
    TargetMachine::CodeGenFileType ft;
    if (output_type != ZigLLVM_EmitLLVMIr) {
        switch (output_type) {
            case ZigLLVM_EmitAssembly:
                ft = TargetMachine::CGFT_AssemblyFile;
                break;
            case ZigLLVM_EmitBinary:
                ft = TargetMachine::CGFT_ObjectFile;
                break;
            default:
                abort();
        }

        if (target_machine->addPassesToEmitFile(MPM, dest, ft)) {
            *error_message = strdup("TargetMachine can't emit a file of
this type");
            return true;
        }
    }

    // run per function optimization passes
    FPM.doInitialization();
    for (Function &F : *module)
      if (!F.isDeclaration())
        FPM.run(F);
    FPM.doFinalization();

    MPM.run(*module);

    if (output_type == ZigLLVM_EmitLLVMIr) {
        if (LLVMPrintModuleToFile(module_ref, filename, error_message)) {
            return true;
        }
    }

    return false;
}
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