lib/CodeGen/SpillPlacement.h

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