MustExecute.h

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//===- MustExecute.h - Is an instruction known to execute--------*- 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
//
//===----------------------------------------------------------------------===//
/// \file
/// Contains a collection of routines for determining if a given instruction is
/// guaranteed to execute if a given point in control flow is reached.  The most
/// common example is an instruction within a loop being provably executed if we
/// branch to the header of it's containing loop.
///
//===----------------------------------------------------------------------===//

#ifndef LLVM_ANALYSIS_MUSTEXECUTE_H
#define LLVM_ANALYSIS_MUSTEXECUTE_H

#include "llvm/ADT/DenseMap.h"
#include "llvm/Analysis/EHPersonalities.h"
#include "llvm/Analysis/InstructionPrecedenceTracking.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instruction.h"

namespace llvm {

class Instruction;
class DominatorTree;
class Loop;

/// Captures loop safety information.
/// It keep information for loop blocks may throw exception or otherwise
/// exit abnormaly on any iteration of the loop which might actually execute
/// at runtime.  The primary way to consume this infromation is via
/// isGuaranteedToExecute below, but some callers bailout or fallback to
/// alternate reasoning if a loop contains any implicit control flow.
/// NOTE: LoopSafetyInfo contains cached information regarding loops and their
/// particular blocks. This information is only dropped on invocation of
/// computeLoopSafetyInfo. If the loop or any of its block is deleted, or if
/// any thrower instructions have been added or removed from them, or if the
/// control flow has changed, or in case of other meaningful modifications, the
/// LoopSafetyInfo needs to be recomputed. If a meaningful modifications to the
/// loop were made and the info wasn't recomputed properly, the behavior of all
/// methods except for computeLoopSafetyInfo is undefined.
class LoopSafetyInfo {
  // Used to update funclet bundle operands.
  DenseMap<BasicBlock *, ColorVector> BlockColors;

protected:
  /// Computes block colors.
  void computeBlockColors(const Loop *CurLoop);

public:
  /// Returns block colors map that is used to update funclet operand bundles.
  const DenseMap<BasicBlock *, ColorVector> &getBlockColors() const;

  /// Copy colors of block \p Old into the block \p New.
  void copyColors(BasicBlock *New, BasicBlock *Old);

  /// Returns true iff the block \p BB potentially may throw exception. It can
  /// be false-positive in cases when we want to avoid complex analysis.
  virtual bool blockMayThrow(const BasicBlock *BB) const = 0;

  /// Returns true iff any block of the loop for which this info is contains an
  /// instruction that may throw or otherwise exit abnormally.
  virtual bool anyBlockMayThrow() const = 0;

  /// Return true if we must reach the block \p BB under assumption that the
  /// loop \p CurLoop is entered.
  bool allLoopPathsLeadToBlock(const Loop *CurLoop, const BasicBlock *BB,
                               const DominatorTree *DT) const;

  /// Computes safety information for a loop checks loop body & header for
  /// the possibility of may throw exception, it takes LoopSafetyInfo and loop
  /// as argument. Updates safety information in LoopSafetyInfo argument.
  /// Note: This is defined to clear and reinitialize an already initialized
  /// LoopSafetyInfo.  Some callers rely on this fact.
  virtual void computeLoopSafetyInfo(const Loop *CurLoop) = 0;

  /// Returns true if the instruction in a loop is guaranteed to execute at
  /// least once (under the assumption that the loop is entered).
  virtual bool isGuaranteedToExecute(const Instruction &Inst,
                                     const DominatorTree *DT,
                                     const Loop *CurLoop) const = 0;

  LoopSafetyInfo() = default;

  virtual ~LoopSafetyInfo() = default;
};


/// Simple and conservative implementation of LoopSafetyInfo that can give
/// false-positive answers to its queries in order to avoid complicated
/// analysis.
class SimpleLoopSafetyInfo: public LoopSafetyInfo {
  bool MayThrow = false;       // The current loop contains an instruction which
                               // may throw.
  bool HeaderMayThrow = false; // Same as previous, but specific to loop header

public:
  virtual bool blockMayThrow(const BasicBlock *BB) const;

  virtual bool anyBlockMayThrow() const;

  virtual void computeLoopSafetyInfo(const Loop *CurLoop);

  virtual bool isGuaranteedToExecute(const Instruction &Inst,
                                     const DominatorTree *DT,
                                     const Loop *CurLoop) const;

  SimpleLoopSafetyInfo() : LoopSafetyInfo() {};

  virtual ~SimpleLoopSafetyInfo() {};
};

/// This implementation of LoopSafetyInfo use ImplicitControlFlowTracking to
/// give precise answers on "may throw" queries. This implementation uses cache
/// that should be invalidated by calling the methods insertInstructionTo and
/// removeInstruction whenever we modify a basic block's contents by adding or
/// removing instructions.
class ICFLoopSafetyInfo: public LoopSafetyInfo {
  bool MayThrow = false;       // The current loop contains an instruction which
                               // may throw.
  // Contains information about implicit control flow in this loop's blocks.
  mutable ImplicitControlFlowTracking ICF;
  // Contains information about instruction that may possibly write memory.
  mutable MemoryWriteTracking MW;

public:
  virtual bool blockMayThrow(const BasicBlock *BB) const;

  virtual bool anyBlockMayThrow() const;

  virtual void computeLoopSafetyInfo(const Loop *CurLoop);

  virtual bool isGuaranteedToExecute(const Instruction &Inst,
                                     const DominatorTree *DT,
                                     const Loop *CurLoop) const;

  /// Returns true if we could not execute a memory-modifying instruction before
  /// we enter \p BB under assumption that \p CurLoop is entered.
  bool doesNotWriteMemoryBefore(const BasicBlock *BB, const Loop *CurLoop)
      const;

  /// Returns true if we could not execute a memory-modifying instruction before
  /// we execute \p I under assumption that \p CurLoop is entered.
  bool doesNotWriteMemoryBefore(const Instruction &I, const Loop *CurLoop)
      const;

  /// Inform the safety info that we are planning to insert a new instruction
  /// \p Inst into the basic block \p BB. It will make all cache updates to keep
  /// it correct after this insertion.
  void insertInstructionTo(const Instruction *Inst, const BasicBlock *BB);

  /// Inform safety info that we are planning to remove the instruction \p Inst
  /// from its block. It will make all cache updates to keep it correct after
  /// this removal.
  void removeInstruction(const Instruction *Inst);

  ICFLoopSafetyInfo(DominatorTree *DT) : LoopSafetyInfo(), ICF(DT), MW(DT) {};

  virtual ~ICFLoopSafetyInfo() {};
};

}

#endif