[llvm-dev] How to make LLVM go faster? (original) (raw)

Andrew Kelley via llvm-dev llvm-dev at lists.llvm.org
Tue Sep 11 17:48:15 PDT 2018

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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