llvm/lib/CodeGen/SpillPlacement.h

   1 //===- SpillPlacement.h - Optimal Spill Code Placement ---------*- C++ -*--===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This analysis computes the optimal spill code placement between basic blocks.
  10 //
  11 // The runOnMachineFunction() method only precomputes some profiling information
  12 // about the CFG. The real work is done by prepare(), addConstraints(), and
  13 // finish() which are called by the register allocator.
  14 //
  15 // Given a variable that is live across multiple basic blocks, and given
  16 // constraints on the basic blocks where the variable is live, determine which
  17 // edge bundles should have the variable in a register and which edge bundles
  18 // should have the variable in a stack slot.
  19 //
  20 // The returned bit vector can be used to place optimal spill code at basic
  21 // block entries and exits. Spill code placement inside a basic block is not
  22 // considered.
  23 //
  24 //===----------------------------------------------------------------------===//
  25
  26 #ifndef LLVM_LIB_CODEGEN_SPILLPLACEMENT_H
  27 #define LLVM_LIB_CODEGEN_SPILLPLACEMENT_H
  28
  29 #include "llvm/ADT/ArrayRef.h"
  30 #include "llvm/ADT/SmallVector.h"
  31 #include "llvm/ADT/SparseSet.h"
  32 #include "llvm/CodeGen/MachineFunctionPass.h"
  33 #include "llvm/Support/BlockFrequency.h"
  34
  35 namespace llvm {
  36
  37 class BitVector;
  38 class EdgeBundles;
  39 class MachineBlockFrequencyInfo;
  40 class MachineFunction;
  41
  42 class SpillPlacement : public MachineFunctionPass {
  43   struct Node;
  44   const MachineFunction *MF = nullptr;
  45   const EdgeBundles *bundles = nullptr;
  46   const MachineBlockFrequencyInfo *MBFI = nullptr;
  47   Node *nodes = nullptr;
  48
  49   // Nodes that are active in the current computation. Owned by the prepare()
  50   // caller.
  51   BitVector *ActiveNodes = nullptr;
  52
  53   // Nodes with active links. Populated by scanActiveBundles.
  54   SmallVector<unsigned, 8> Linked;
  55
  56   // Nodes that went positive during the last call to scanActiveBundles or
  57   // iterate.
  58   SmallVector<unsigned, 8> RecentPositive;
  59
  60   // Block frequencies are computed once. Indexed by block number.
  61   SmallVector<BlockFrequency, 8> BlockFrequencies;
  62
  63   /// Decision threshold. A node gets the output value 0 if the weighted sum of
  64   /// its inputs falls in the open interval (-Threshold;Threshold).
  65   BlockFrequency Threshold;
  66
  67   /// List of nodes that need to be updated in ::iterate.
  68   SparseSet<unsigned> TodoList;
  69
  70 public:
  71   static char ID; // Pass identification, replacement for typeid.
  72
  73   SpillPlacement() : MachineFunctionPass(ID) {}
  74   ~SpillPlacement() override { releaseMemory(); }
  75
  76   /// BorderConstraint - A basic block has separate constraints for entry and
  77   /// exit.
  78   enum BorderConstraint {
  79     DontCare,  ///< Block doesn't care / variable not live.
  80     PrefReg,   ///< Block entry/exit prefers a register.
  81     PrefSpill, ///< Block entry/exit prefers a stack slot.
  82     PrefBoth,  ///< Block entry prefers both register and stack.
  83     MustSpill  ///< A register is impossible, variable must be spilled.
  84   };
  85
  86   /// BlockConstraint - Entry and exit constraints for a basic block.
  87   struct BlockConstraint {
  88     unsigned Number;            ///< Basic block number (from MBB::getNumber()).
  89     BorderConstraint Entry : 8; ///< Constraint on block entry.
  90     BorderConstraint Exit : 8;  ///< Constraint on block exit.
  91
  92     /// True when this block changes the value of the live range. This means
  93     /// the block has a non-PHI def.  When this is false, a live-in value on
  94     /// the stack can be live-out on the stack without inserting a spill.
  95     bool ChangesValue;
  96
  97     void print(raw_ostream &OS) const;
  98     void dump() const;
  99   };
 100
 101   /// prepare - Reset state and prepare for a new spill placement computation.
 102   /// @param RegBundles Bit vector to receive the edge bundles where the
 103   ///                   variable should be kept in a register. Each bit
 104   ///                   corresponds to an edge bundle, a set bit means the
 105   ///                   variable should be kept in a register through the
 106   ///                   bundle. A clear bit means the variable should be
 107   ///                   spilled. This vector is retained.
 108   void prepare(BitVector &RegBundles);
 109
 110   /// addConstraints - Add constraints and biases. This method may be called
 111   /// more than once to accumulate constraints.
 112   /// @param LiveBlocks Constraints for blocks that have the variable live in or
 113   ///                   live out.
 114   void addConstraints(ArrayRef<BlockConstraint> LiveBlocks);
 115
 116   /// addPrefSpill - Add PrefSpill constraints to all blocks listed.  This is
 117   /// equivalent to calling addConstraint with identical BlockConstraints with
 118   /// Entry = Exit = PrefSpill, and ChangesValue = false.
 119   ///
 120   /// @param Blocks Array of block numbers that prefer to spill in and out.
 121   /// @param Strong When true, double the negative bias for these blocks.
 122   void addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong);
 123
 124   /// addLinks - Add transparent blocks with the given numbers.
 125   void addLinks(ArrayRef<unsigned> Links);
 126
 127   /// scanActiveBundles - Perform an initial scan of all bundles activated by
 128   /// addConstraints and addLinks, updating their state. Add all the bundles
 129   /// that now prefer a register to RecentPositive.
 130   /// Prepare internal data structures for iterate.
 131   /// Return true is there are any positive nodes.
 132   bool scanActiveBundles();
 133
 134   /// iterate - Update the network iteratively until convergence, or new bundles
 135   /// are found.
 136   void iterate();
 137
 138   /// getRecentPositive - Return an array of bundles that became positive during
 139   /// the previous call to scanActiveBundles or iterate.
 140   ArrayRef<unsigned> getRecentPositive() { return RecentPositive; }
 141
 142   /// finish - Compute the optimal spill code placement given the
 143   /// constraints. No MustSpill constraints will be violated, and the smallest
 144   /// possible number of PrefX constraints will be violated, weighted by
 145   /// expected execution frequencies.
 146   /// The selected bundles are returned in the bitvector passed to prepare().
 147   /// @return True if a perfect solution was found, allowing the variable to be
 148   ///         in a register through all relevant bundles.
 149   bool finish();
 150
 151   /// getBlockFrequency - Return the estimated block execution frequency per
 152   /// function invocation.
 153   BlockFrequency getBlockFrequency(unsigned Number) const {
 154     return BlockFrequencies[Number];
 155   }
 156
 157 private:
 158   bool runOnMachineFunction(MachineFunction &mf) override;
 159   void getAnalysisUsage(AnalysisUsage &AU) const override;
 160   void releaseMemory() override;
 161
 162   void activate(unsigned n);
 163   void setThreshold(BlockFrequency Entry);
 164
 165   bool update(unsigned n);
 166 };
 167
 168 } // end namespace llvm
 169
 170 #endif // LLVM_LIB_CODEGEN_SPILLPLACEMENT_H