FunctionImport.h

//===- llvm/Transforms/IPO/FunctionImport.h - ThinLTO importing -*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_TRANSFORMS_IPO_FUNCTIONIMPORT_H
#define LLVM_TRANSFORMS_IPO_FUNCTIONIMPORT_H

#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/ModuleSummaryIndex.h"
#include "llvm/IR/PassManager.h"
#include "llvm/Support/Error.h"
#include <functional>
#include <map>
#include <memory>
#include <string>
#include <system_error>
#include <unordered_set>
#include <utility>

namespace llvm {

class Module;

/// The function importer is automatically importing function from other modules
/// based on the provided summary informations.
class FunctionImporter {
public:
  /// Set of functions to import from a source module. Each entry is a set
  /// containing all the GUIDs of all functions to import for a source module.
  using FunctionsToImportTy = std::unordered_set<GlobalValue::GUID>;

  /// The different reasons selectCallee will chose not to import a
  /// candidate.
  enum ImportFailureReason {
    None,
    // We can encounter a global variable instead of a function in rare
    // situations with SamplePGO. See comments where this failure type is
    // set for more details.
    GlobalVar,
    // Found to be globally dead, so we don't bother importing.
    NotLive,
    // Instruction count over the current threshold.
    TooLarge,
    // Don't import something with interposable linkage as we can't inline it
    // anyway.
    InterposableLinkage,
    // Generally we won't end up failing due to this reason, as we expect
    // to find at least one summary for the GUID that is global or a local
    // in the referenced module for direct calls.
    LocalLinkageNotInModule,
    // This corresponds to the NotEligibleToImport being set on the summary,
    // which can happen in a few different cases (e.g. local that can't be
    // renamed or promoted because it is referenced on a llvm*.used variable).
    NotEligible,
    // This corresponds to NoInline being set on the function summary,
    // which will happen if it is known that the inliner will not be able
    // to inline the function (e.g. it is marked with a NoInline attribute).
    NoInline
  };

  /// Information optionally tracked for candidates the importer decided
  /// not to import. Used for optional stat printing.
  struct ImportFailureInfo {
    // The ValueInfo corresponding to the candidate. We save an index hash
    // table lookup for each GUID by stashing this here.
    ValueInfo VI;
    // The maximum call edge hotness for all failed imports of this candidate.
    CalleeInfo::HotnessType MaxHotness;
    // most recent reason for failing to import (doesn't necessarily correspond
    // to the attempt with the maximum hotness).
    ImportFailureReason Reason;
    // The number of times we tried to import candidate but failed.
    unsigned Attempts;
    ImportFailureInfo(ValueInfo VI, CalleeInfo::HotnessType MaxHotness,
                      ImportFailureReason Reason, unsigned Attempts)
        : VI(VI), MaxHotness(MaxHotness), Reason(Reason), Attempts(Attempts) {}
  };

  /// Map of callee GUID considered for import into a given module to a pair
  /// consisting of the largest threshold applied when deciding whether to
  /// import it and, if we decided to import, a pointer to the summary instance
  /// imported. If we decided not to import, the summary will be nullptr.
  using ImportThresholdsTy =
      DenseMap<GlobalValue::GUID,
               std::tuple<unsigned, const GlobalValueSummary *,
                          std::unique_ptr<ImportFailureInfo>>>;

  /// The map contains an entry for every module to import from, the key being
  /// the module identifier to pass to the ModuleLoader. The value is the set of
  /// functions to import.
  using ImportMapTy = StringMap<FunctionsToImportTy>;

  /// The set contains an entry for every global value the module exports.
  using ExportSetTy = DenseSet<ValueInfo>;

  /// A function of this type is used to load modules referenced by the index.
  using ModuleLoaderTy =
      std::function<Expected<std::unique_ptr<Module>>(StringRef Identifier)>;

  /// Create a Function Importer.
  FunctionImporter(const ModuleSummaryIndex &Index, ModuleLoaderTy ModuleLoader,
                   bool ClearDSOLocalOnDeclarations)
      : Index(Index), ModuleLoader(std::move(ModuleLoader)),
        ClearDSOLocalOnDeclarations(ClearDSOLocalOnDeclarations) {}

  /// Import functions in Module \p M based on the supplied import list.
  Expected<bool> importFunctions(Module &M, const ImportMapTy &ImportList);

private:
  /// The summaries index used to trigger importing.
  const ModuleSummaryIndex &Index;

  /// Factory function to load a Module for a given identifier
  ModuleLoaderTy ModuleLoader;

  /// See the comment of ClearDSOLocalOnDeclarations in
  /// Utils/FunctionImportUtils.h.
  bool ClearDSOLocalOnDeclarations;
};

/// The function importing pass
class FunctionImportPass : public PassInfoMixin<FunctionImportPass> {
public:
  PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
};

/// Compute all the imports and exports for every module in the Index.
///
/// \p ModuleToDefinedGVSummaries contains for each Module a map
/// (GUID -> Summary) for every global defined in the module.
///
/// \p ImportLists will be populated with an entry for every Module we are
/// importing into. This entry is itself a map that can be passed to
/// FunctionImporter::importFunctions() above (see description there).
///
/// \p ExportLists contains for each Module the set of globals (GUID) that will
/// be imported by another module, or referenced by such a function. I.e. this
/// is the set of globals that need to be promoted/renamed appropriately.
void ComputeCrossModuleImport(
    const ModuleSummaryIndex &Index,
    const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
    StringMap<FunctionImporter::ImportMapTy> &ImportLists,
    StringMap<FunctionImporter::ExportSetTy> &ExportLists);

/// Compute all the imports for the given module using the Index.
///
/// \p ImportList will be populated with a map that can be passed to
/// FunctionImporter::importFunctions() above (see description there).
void ComputeCrossModuleImportForModule(
    StringRef ModulePath, const ModuleSummaryIndex &Index,
    FunctionImporter::ImportMapTy &ImportList);

/// Mark all external summaries in \p Index for import into the given module.
/// Used for distributed builds using a distributed index.
///
/// \p ImportList will be populated with a map that can be passed to
/// FunctionImporter::importFunctions() above (see description there).
void ComputeCrossModuleImportForModuleFromIndex(
    StringRef ModulePath, const ModuleSummaryIndex &Index,
    FunctionImporter::ImportMapTy &ImportList);

/// PrevailingType enum used as a return type of callback passed
/// to computeDeadSymbolsAndUpdateIndirectCalls. Yes and No values used when
/// status explicitly set by symbols resolution, otherwise status is Unknown.
enum class PrevailingType { Yes, No, Unknown };

/// Update call edges for indirect calls to local functions added from
/// SamplePGO when needed. Normally this is done during
/// computeDeadSymbolsAndUpdateIndirectCalls, but can be called standalone
/// when that is not called (e.g. during testing).
void updateIndirectCalls(ModuleSummaryIndex &Index);

/// Compute all the symbols that are "dead": i.e these that can't be reached
/// in the graph from any of the given symbols listed in
/// \p GUIDPreservedSymbols. Non-prevailing symbols are symbols without a
/// prevailing copy anywhere in IR and are normally dead, \p isPrevailing
/// predicate returns status of symbol.
/// Also update call edges for indirect calls to local functions added from
/// SamplePGO when needed.
void computeDeadSymbolsAndUpdateIndirectCalls(
    ModuleSummaryIndex &Index,
    const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols,
    function_ref<PrevailingType(GlobalValue::GUID)> isPrevailing);

/// Compute dead symbols and run constant propagation in combined index
/// after that.
void computeDeadSymbolsWithConstProp(
    ModuleSummaryIndex &Index,
    const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols,
    function_ref<PrevailingType(GlobalValue::GUID)> isPrevailing,
    bool ImportEnabled);

/// Converts value \p GV to declaration, or replaces with a declaration if
/// it is an alias. Returns true if converted, false if replaced.
bool convertToDeclaration(GlobalValue &GV);

/// Compute the set of summaries needed for a ThinLTO backend compilation of
/// \p ModulePath.
//
/// This includes summaries from that module (in case any global summary based
/// optimizations were recorded) and from any definitions in other modules that
/// should be imported.
//
/// \p ModuleToSummariesForIndex will be populated with the needed summaries
/// from each required module path. Use a std::map instead of StringMap to get
/// stable order for bitcode emission.
void gatherImportedSummariesForModule(
    StringRef ModulePath,
    const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
    const FunctionImporter::ImportMapTy &ImportList,
    std::map<std::string, GVSummaryMapTy> &ModuleToSummariesForIndex);

/// Emit into \p OutputFilename the files module \p ModulePath will import from.
std::error_code EmitImportsFiles(
    StringRef ModulePath, StringRef OutputFilename,
    const std::map<std::string, GVSummaryMapTy> &ModuleToSummariesForIndex);

/// Based on the information recorded in the summaries during global
/// summary-based analysis:
/// 1. Resolve prevailing symbol linkages and constrain visibility (CanAutoHide
///    and consider visibility from other definitions for ELF) in \p TheModule
/// 2. (optional) Apply propagated function attributes to \p TheModule if
///    PropagateAttrs is true
void thinLTOFinalizeInModule(Module &TheModule,
                             const GVSummaryMapTy &DefinedGlobals,
                             bool PropagateAttrs);

/// Internalize \p TheModule based on the information recorded in the summaries
/// during global summary-based analysis.
void thinLTOInternalizeModule(Module &TheModule,
                              const GVSummaryMapTy &DefinedGlobals);

} // end namespace llvm

#endif // LLVM_TRANSFORMS_IPO_FUNCTIONIMPORT_H