FunctionImportUtils.h

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//===- FunctionImportUtils.h - Importing support utilities -----*- 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
//
//===----------------------------------------------------------------------===//
//
// This file defines the FunctionImportGlobalProcessing class which is used
// to perform the necessary global value handling for function importing.
//
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_TRANSFORMS_UTILS_FUNCTIONIMPORTUTILS_H
#define LLVM_TRANSFORMS_UTILS_FUNCTIONIMPORTUTILS_H
 
#include "llvm/ADT/SetVector.h"
#include "llvm/IR/ModuleSummaryIndex.h"
 
namespace llvm {
class Module;
 
/// Class to handle necessary GlobalValue changes required by ThinLTO
/// function importing, including linkage changes and any necessary renaming.
class FunctionImportGlobalProcessing {
  /// The Module which we are exporting or importing functions from.
  Module &M;
 
  /// Module summary index passed in for function importing/exporting handling.
  const ModuleSummaryIndex &ImportIndex;
 
  /// Globals to import from this module, all other functions will be
  /// imported as declarations instead of definitions.
  SetVector<GlobalValue *> *GlobalsToImport;
 
  /// Set to true if the given ModuleSummaryIndex contains any functions
  /// from this source module, in which case we must conservatively assume
  /// that any of its functions may be imported into another module
  /// as part of a different backend compilation process.
  bool HasExportedFunctions = false;
 
  /// Set to true (only applicatable to ELF -fpic) if dso_local should be
  /// dropped for a declaration.
  ///
  /// On ELF, the assembler is conservative and assumes a global default
  /// visibility symbol can be interposable. No direct access relocation is
  /// allowed, if the definition is not in the translation unit, even if the
  /// definition is available in the linkage unit. Thus we need to clear
  /// dso_local to disable direct access.
  ///
  /// This flag should not be set for -fno-pic or -fpie, which would
  /// unnecessarily disable direct access.
  bool ClearDSOLocalOnDeclarations;
 
  /// Set of llvm.*used values, in order to validate that we don't try
  /// to promote any non-renamable values.
  SmallPtrSet<GlobalValue *, 4> Used;
 
  /// Keep track of any COMDATs that require renaming (because COMDAT
  /// leader was promoted and renamed). Maps from original COMDAT to one
  /// with new name.
  DenseMap<const Comdat *, Comdat *> RenamedComdats;
 
  /// Check if we should promote the given local value to global scope.
  bool shouldPromoteLocalToGlobal(const GlobalValue *SGV, ValueInfo VI);
 
#ifndef NDEBUG
  /// Check if the given value is a local that can't be renamed (promoted).
  /// Only used in assertion checking, and disabled under NDEBUG since the Used
  /// set will not be populated.
  bool isNonRenamableLocal(const GlobalValue &GV) const;
#endif
 
  /// Helper methods to check if we are importing from or potentially
  /// exporting from the current source module.
  bool isPerformingImport() const { return GlobalsToImport != nullptr; }
  bool isModuleExporting() const { return HasExportedFunctions; }
 
  /// If we are importing from the source module, checks if we should
  /// import SGV as a definition, otherwise import as a declaration.
  bool doImportAsDefinition(const GlobalValue *SGV);
 
  /// Get the name for a local SGV that should be promoted and renamed to global
  /// scope in the linked destination module.
  std::string getPromotedName(const GlobalValue *SGV);
 
  /// Process globals so that they can be used in ThinLTO. This includes
  /// promoting local variables so that they can be reference externally by
  /// thin lto imported globals and converting strong external globals to
  /// available_externally.
  void processGlobalsForThinLTO();
  void processGlobalForThinLTO(GlobalValue &GV);
 
  /// Get the new linkage for SGV that should be used in the linked destination
  /// module. Specifically, for ThinLTO importing or exporting it may need
  /// to be adjusted. When \p DoPromote is true then we must adjust the
  /// linkage for a required promotion of a local to global scope.
  GlobalValue::LinkageTypes getLinkage(const GlobalValue *SGV, bool DoPromote);
 
public:
  FunctionImportGlobalProcessing(Module &M, const ModuleSummaryIndex &Index,
                                 SetVector<GlobalValue *> *GlobalsToImport,
                                 bool ClearDSOLocalOnDeclarations)
      : M(M), ImportIndex(Index), GlobalsToImport(GlobalsToImport),
        ClearDSOLocalOnDeclarations(ClearDSOLocalOnDeclarations) {
    // If we have a ModuleSummaryIndex but no function to import,
    // then this is the primary module being compiled in a ThinLTO
    // backend compilation, and we need to see if it has functions that
    // may be exported to another backend compilation.
    if (!GlobalsToImport)
      HasExportedFunctions = ImportIndex.hasExportedFunctions(M);
 
#ifndef NDEBUG
    SmallVector<GlobalValue *, 4> Vec;
    // First collect those in the llvm.used set.
    collectUsedGlobalVariables(M, Vec, /*CompilerUsed=*/false);
    // Next collect those in the llvm.compiler.used set.
    collectUsedGlobalVariables(M, Vec, /*CompilerUsed=*/true);
    Used = {Vec.begin(), Vec.end()};
#endif
  }
 
  bool run();
};
 
/// Perform in-place global value handling on the given Module for
/// exported local functions renamed and promoted for ThinLTO.
bool renameModuleForThinLTO(
    Module &M, const ModuleSummaryIndex &Index,
    bool ClearDSOLocalOnDeclarations,
    SetVector<GlobalValue *> *GlobalsToImport = nullptr);
 
} // End llvm namespace
 
#endif