llvm-mirror/include/llvm/Analysis/BranchProbabilityInfo.h

//===--- BranchProbabilityInfo.h - Branch Probability Analysis --*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass is used to evaluate branch probabilties.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
#define LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H

#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Support/BranchProbability.h"

namespace llvm {
class LoopInfo;
class raw_ostream;

/// \brief Analysis pass providing branch probability information.
///
/// This is a function analysis pass which provides information on the relative
/// probabilities of each "edge" in the function's CFG where such an edge is
/// defined by a pair of basic blocks. The probability for a given block and
/// a successor block are always relative to the probabilities of the other
/// successor blocks. Another way of looking at it is that the probabilities
/// for a given block B and each of its successors should sum to exactly
/// one (100%).
class BranchProbabilityInfo : public FunctionPass {
public:
  static char ID;

  BranchProbabilityInfo() : FunctionPass(ID) {
    initializeBranchProbabilityInfoPass(*PassRegistry::getPassRegistry());
  }

  void getAnalysisUsage(AnalysisUsage &AU) const;
  bool runOnFunction(Function &F);
  void print(raw_ostream &OS, const Module *M = 0) const;

  /// \brief Get an edge's probability, relative to other out-edges of the Src.
  ///
  /// This routine provides access to the fractional probability between zero
  /// (0%) and one (100%) of this edge executing, relative to other edges
  /// leaving the 'Src' block. The returned probability is never zero, and can
  /// only be one if the source block has only one successor.
  BranchProbability getEdgeProbability(const BasicBlock *Src,
                                       const BasicBlock *Dst) const;

  /// \brief Test if an edge is hot relative to other out-edges of the Src.
  ///
  /// Check whether this edge out of the source block is 'hot'. We define hot
  /// as having a relative probability >= 80%.
  bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;

  /// \brief Retrieve the hot successor of a block if one exists.
  ///
  /// Given a basic block, look through its successors and if one exists for
  /// which \see isEdgeHot would return true, return that successor block.
  BasicBlock *getHotSucc(BasicBlock *BB) const;

  /// \brief Print an edge's probability.
  ///
  /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
  /// then prints that probability to the provided stream. That stream is then
  /// returned.
  raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
                                    const BasicBlock *Dst) const;

  /// \brief Get the raw edge weight calculated for the block pair.
  ///
  /// This returns the raw edge weight. It is guaranteed to fall between 1 and
  /// UINT32_MAX. Note that the raw edge weight is not meaningful in isolation.
  /// This interface should be very carefully, and primarily by routines that
  /// are updating the analysis by later calling setEdgeWeight.
  uint32_t getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const;

  /// \brief Set the raw edge weight for the block pair.
  ///
  /// This allows a pass to explicitly set the edge weight for a block. It can
  /// be used when updating the CFG to update and preserve the branch
  /// probability information. Read the implementation of how these edge
  /// weights are calculated carefully before using!
  void setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst,
                     uint32_t Weight);

private:
  typedef std::pair<const BasicBlock *, const BasicBlock *> Edge;

  // Default weight value. Used when we don't have information about the edge.
  // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
  // the successors have a weight yet. But it doesn't make sense when providing
  // weight to an edge that may have siblings with non-zero weights. This can
  // be handled various ways, but it's probably fine for an edge with unknown
  // weight to just "inherit" the non-zero weight of an adjacent successor.
  static const uint32_t DEFAULT_WEIGHT = 16;

  DenseMap<Edge, uint32_t> Weights;

  /// \brief Handle to the LoopInfo analysis.
  LoopInfo *LI;

  /// \brief Track the last function we run over for printing.
  Function *LastF;

  /// \brief Track the set of blocks directly succeeded by a returning block.
  SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable;

  /// \brief Get sum of the block successors' weights.
  uint32_t getSumForBlock(const BasicBlock *BB) const;

  bool calcUnreachableHeuristics(BasicBlock *BB);
  bool calcMetadataWeights(BasicBlock *BB);
  bool calcPointerHeuristics(BasicBlock *BB);
  bool calcLoopBranchHeuristics(BasicBlock *BB);
  bool calcZeroHeuristics(BasicBlock *BB);
  bool calcFloatingPointHeuristics(BasicBlock *BB);
  bool calcInvokeHeuristics(BasicBlock *BB);
};

}

#endif