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 //===- SpillPlacement.h - Optimal Spill Code Placement ---------*- 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 analysis computes the optimal spill code placement between basic blocks.
 //
 // The runOnMachineFunction() method only precomputes some profiling information
 // about the CFG. The real work is done by prepare(), addConstraints(), and
 // finish() which are called by the register allocator.
 //
 // Given a variable that is live across multiple basic blocks, and given
 // constraints on the basic blocks where the variable is live, determine which
 // edge bundles should have the variable in a register and which edge bundles
 // should have the variable in a stack slot.
 //
 // The returned bit vector can be used to place optimal spill code at basic
 // block entries and exits. Spill code placement inside a basic block is not
 // considered.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIB_CODEGEN_SPILLPLACEMENT_H
 #define LLVM_LIB_CODEGEN_SPILLPLACEMENT_H
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/SparseSet.h"
 #include "llvm/CodeGen/MachineFunctionAnalysis.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/Support/BlockFrequency.h"
 
 namespace llvm {
 
 class BitVector;
 class EdgeBundles;
 class MachineBlockFrequencyInfo;
 class MachineFunction;
 class SpillPlacementWrapperLegacy;
 class SpillPlacementAnalysis;
 
 class SpillPlacement {
   friend class SpillPlacementWrapperLegacy;
   friend class SpillPlacementAnalysis;
 
   struct Node;
 
   const MachineFunction *MF = nullptr;
   const EdgeBundles *bundles = nullptr;
   const MachineBlockFrequencyInfo *MBFI = nullptr;
 
   std::unique_ptr<Node[]> nodes;
 
   // Nodes that are active in the current computation. Owned by the prepare()
   // caller.
   BitVector *ActiveNodes = nullptr;
 
   // Nodes with active links. Populated by scanActiveBundles.
   SmallVector<unsigned, 8> Linked;
 
   // Nodes that went positive during the last call to scanActiveBundles or
   // iterate.
   SmallVector<unsigned, 8> RecentPositive;
 
   // Block frequencies are computed once. Indexed by block number.
   SmallVector<BlockFrequency, 8> BlockFrequencies;
 
   /// Decision threshold. A node gets the output value 0 if the weighted sum of
   /// its inputs falls in the open interval (-Threshold;Threshold).
   BlockFrequency Threshold;
 
   /// List of nodes that need to be updated in ::iterate.
   SparseSet<unsigned> TodoList;
 
 public:
   /// BorderConstraint - A basic block has separate constraints for entry and
   /// exit.
   enum BorderConstraint {
     DontCare,  ///< Block doesn't care / variable not live.
     PrefReg,   ///< Block entry/exit prefers a register.
     PrefSpill, ///< Block entry/exit prefers a stack slot.
     PrefBoth,  ///< Block entry prefers both register and stack.
     MustSpill  ///< A register is impossible, variable must be spilled.
   };
 
   /// BlockConstraint - Entry and exit constraints for a basic block.
   struct BlockConstraint {
     unsigned Number;            ///< Basic block number (from MBB::getNumber()).
     BorderConstraint Entry : 8; ///< Constraint on block entry.
     BorderConstraint Exit : 8;  ///< Constraint on block exit.
 
     /// True when this block changes the value of the live range. This means
     /// the block has a non-PHI def.  When this is false, a live-in value on
     /// the stack can be live-out on the stack without inserting a spill.
     bool ChangesValue;
 
     void print(raw_ostream &OS) const;
     void dump() const;
   };
 
   /// prepare - Reset state and prepare for a new spill placement computation.
   /// @param RegBundles Bit vector to receive the edge bundles where the
   ///                   variable should be kept in a register. Each bit
   ///                   corresponds to an edge bundle, a set bit means the
   ///                   variable should be kept in a register through the
   ///                   bundle. A clear bit means the variable should be
   ///                   spilled. This vector is retained.
   void prepare(BitVector &RegBundles);
 
   /// addConstraints - Add constraints and biases. This method may be called
   /// more than once to accumulate constraints.
   /// @param LiveBlocks Constraints for blocks that have the variable live in or
   ///                   live out.
   void addConstraints(ArrayRef<BlockConstraint> LiveBlocks);
 
   /// addPrefSpill - Add PrefSpill constraints to all blocks listed.  This is
   /// equivalent to calling addConstraint with identical BlockConstraints with
   /// Entry = Exit = PrefSpill, and ChangesValue = false.
   ///
   /// @param Blocks Array of block numbers that prefer to spill in and out.
   /// @param Strong When true, double the negative bias for these blocks.
   void addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong);
 
   /// addLinks - Add transparent blocks with the given numbers.
   void addLinks(ArrayRef<unsigned> Links);
 
   /// scanActiveBundles - Perform an initial scan of all bundles activated by
   /// addConstraints and addLinks, updating their state. Add all the bundles
   /// that now prefer a register to RecentPositive.
   /// Prepare internal data structures for iterate.
   /// Return true is there are any positive nodes.
   bool scanActiveBundles();
 
   /// iterate - Update the network iteratively until convergence, or new bundles
   /// are found.
   void iterate();
 
   /// getRecentPositive - Return an array of bundles that became positive during
   /// the previous call to scanActiveBundles or iterate.
   ArrayRef<unsigned> getRecentPositive() { return RecentPositive; }
 
   /// finish - Compute the optimal spill code placement given the
   /// constraints. No MustSpill constraints will be violated, and the smallest
   /// possible number of PrefX constraints will be violated, weighted by
   /// expected execution frequencies.
   /// The selected bundles are returned in the bitvector passed to prepare().
   /// @return True if a perfect solution was found, allowing the variable to be
   ///         in a register through all relevant bundles.
   bool finish();
 
   /// getBlockFrequency - Return the estimated block execution frequency per
   /// function invocation.
   BlockFrequency getBlockFrequency(unsigned Number) const {
     return BlockFrequencies[Number];
   }
 
   bool invalidate(MachineFunction &MF, const PreservedAnalyses &PA,
                   MachineFunctionAnalysisManager::Invalidator &Inv);
 
   SpillPlacement(SpillPlacement &&);
   ~SpillPlacement();
 
 private:
   SpillPlacement();
 
   void releaseMemory();
 
   void run(MachineFunction &MF, EdgeBundles *Bundles,
            MachineBlockFrequencyInfo *MBFI);
   void activate(unsigned n);
   void setThreshold(BlockFrequency Entry);
 
   bool update(unsigned n);
 };
 
 class SpillPlacementWrapperLegacy : public MachineFunctionPass {
 public:
   static char ID;
   SpillPlacementWrapperLegacy() : MachineFunctionPass(ID) {}
 
   SpillPlacement &getResult() { return Impl; }
   const SpillPlacement &getResult() const { return Impl; }
 
 private:
   SpillPlacement Impl;
   bool runOnMachineFunction(MachineFunction &MF) override;
   void getAnalysisUsage(AnalysisUsage &AU) const override;
   void releaseMemory() override { Impl.releaseMemory(); }
 };
 
 class SpillPlacementAnalysis
     : public AnalysisInfoMixin<SpillPlacementAnalysis> {
   friend AnalysisInfoMixin<SpillPlacementAnalysis>;
   static AnalysisKey Key;
 
 public:
   using Result = SpillPlacement;
   SpillPlacement run(MachineFunction &, MachineFunctionAnalysisManager &);
 };
 
 } // end namespace llvm
 
 #endif // LLVM_LIB_CODEGEN_SPILLPLACEMENT_H

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