CodeExtractor.h

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//===-- Transform/Utils/CodeExtractor.h - Code extraction util --*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// A utility to support extracting code from one function into its own
// stand-alone function.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_TRANSFORMS_UTILS_CODEEXTRACTOR_H
#define LLVM_TRANSFORMS_UTILS_CODEEXTRACTOR_H

#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/SetVector.h"

namespace llvm {
  class BasicBlock;
  class DominatorTree;
  class Function;
  class Loop;
  class Module;
  class RegionNode;
  class Type;
  class Value;

  /// \brief Utility class for extracting code into a new function.
  ///
  /// This utility provides a simple interface for extracting some sequence of
  /// code into its own function, replacing it with a call to that function. It
  /// also provides various methods to query about the nature and result of
  /// such a transformation.
  ///
  /// The rough algorithm used is:
  /// 1) Find both the inputs and outputs for the extracted region.
  /// 2) Pass the inputs as arguments, remapping them within the extracted
  ///    function to arguments.
  /// 3) Add allocas for any scalar outputs, adding all of the outputs' allocas
  ///    as arguments, and inserting stores to the arguments for any scalars.
  class CodeExtractor {
    typedef SetVector<Value *> ValueSet;

    // Various bits of state computed on construction.
    DominatorTree *const DT;
    const bool AggregateArgs;

    // Bits of intermediate state computed at various phases of extraction.
    SetVector<BasicBlock *> Blocks;
    unsigned NumExitBlocks;
    Type *RetTy;

  public:
    /// \brief Create a code extractor for a single basic block.
    ///
    /// In this formation, we don't require a dominator tree. The given basic
    /// block is set up for extraction.
    CodeExtractor(BasicBlock *BB, bool AggregateArgs = false);

    /// \brief Create a code extractor for a sequence of blocks.
    ///
    /// Given a sequence of basic blocks where the first block in the sequence
    /// dominates the rest, prepare a code extractor object for pulling this
    /// sequence out into its new function. When a DominatorTree is also given,
    /// extra checking and transformations are enabled.
    CodeExtractor(ArrayRef<BasicBlock *> BBs, DominatorTree *DT = nullptr,
                  bool AggregateArgs = false);

    /// \brief Create a code extractor for a loop body.
    ///
    /// Behaves just like the generic code sequence constructor, but uses the
    /// block sequence of the loop.
    CodeExtractor(DominatorTree &DT, Loop &L, bool AggregateArgs = false);

    /// \brief Create a code extractor for a region node.
    ///
    /// Behaves just like the generic code sequence constructor, but uses the
    /// block sequence of the region node passed in.
    CodeExtractor(DominatorTree &DT, const RegionNode &RN,
                  bool AggregateArgs = false);

    /// \brief Perform the extraction, returning the new function.
    ///
    /// Returns zero when called on a CodeExtractor instance where isEligible
    /// returns false.
    Function *extractCodeRegion();

    /// \brief Test whether this code extractor is eligible.
    ///
    /// Based on the blocks used when constructing the code extractor,
    /// determine whether it is eligible for extraction.
    bool isEligible() const { return !Blocks.empty(); }

    /// \brief Compute the set of input values and output values for the code.
    ///
    /// These can be used either when performing the extraction or to evaluate
    /// the expected size of a call to the extracted function. Note that this
    /// work cannot be cached between the two as once we decide to extract
    /// a code sequence, that sequence is modified, including changing these
    /// sets, before extraction occurs. These modifications won't have any
    /// significant impact on the cost however.
    void findInputsOutputs(ValueSet &Inputs, ValueSet &Outputs) const;

  private:
    void severSplitPHINodes(BasicBlock *&Header);
    void splitReturnBlocks();

    Function *constructFunction(const ValueSet &inputs,
                                const ValueSet &outputs,
                                BasicBlock *header,
                                BasicBlock *newRootNode, BasicBlock *newHeader,
                                Function *oldFunction, Module *M);

    void moveCodeToFunction(Function *newFunction);

    void emitCallAndSwitchStatement(Function *newFunction,
                                    BasicBlock *newHeader,
                                    ValueSet &inputs,
                                    ValueSet &outputs);
  };
}

#endif