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 class MachineLoopInfo;
  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 = nullptr;
  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   /// List of nodes that need to be updated in ::iterate.
  70   SparseSet<unsigned> TodoList;
  71
  72 public:
  73   static char ID; // Pass identification, replacement for typeid.
  74
  75   SpillPlacement() : MachineFunctionPass(ID) {}
  76   ~SpillPlacement() override { releaseMemory(); }
  77
  78   /// BorderConstraint - A basic block has separate constraints for entry and
  79   /// exit.
  80   enum BorderConstraint {
  81     DontCare,  ///< Block doesn't care / variable not live.
  82     PrefReg,   ///< Block entry/exit prefers a register.
  83     PrefSpill, ///< Block entry/exit prefers a stack slot.
  84     PrefBoth,  ///< Block entry prefers both register and stack.
  85     MustSpill  ///< A register is impossible, variable must be spilled.
  86   };
  87
  88   /// BlockConstraint - Entry and exit constraints for a basic block.
  89   struct BlockConstraint {
  90     unsigned Number;            ///< Basic block number (from MBB::getNumber()).
  91     BorderConstraint Entry : 8; ///< Constraint on block entry.
  92     BorderConstraint Exit : 8;  ///< Constraint on block exit.
  93
  94     /// True when this block changes the value of the live range. This means
  95     /// the block has a non-PHI def.  When this is false, a live-in value on
  96     /// the stack can be live-out on the stack without inserting a spill.
  97     bool ChangesValue;
  98   };
  99
 100   /// prepare - Reset state and prepare for a new spill placement computation.
 101   /// @param RegBundles Bit vector to receive the edge bundles where the
 102   ///                   variable should be kept in a register. Each bit
 103   ///                   corresponds to an edge bundle, a set bit means the
 104   ///                   variable should be kept in a register through the
 105   ///                   bundle. A clear bit means the variable should be
 106   ///                   spilled. This vector is retained.
 107   void prepare(BitVector &RegBundles);
 108
 109   /// addConstraints - Add constraints and biases. This method may be called
 110   /// more than once to accumulate constraints.
 111   /// @param LiveBlocks Constraints for blocks that have the variable live in or
 112   ///                   live out.
 113   void addConstraints(ArrayRef<BlockConstraint> LiveBlocks);
 114
 115   /// addPrefSpill - Add PrefSpill constraints to all blocks listed.  This is
 116   /// equivalent to calling addConstraint with identical BlockConstraints with
 117   /// Entry = Exit = PrefSpill, and ChangesValue = false.
 118   ///
 119   /// @param Blocks Array of block numbers that prefer to spill in and out.
 120   /// @param Strong When true, double the negative bias for these blocks.
 121   void addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong);
 122
 123   /// addLinks - Add transparent blocks with the given numbers.
 124   void addLinks(ArrayRef<unsigned> Links);
 125
 126   /// scanActiveBundles - Perform an initial scan of all bundles activated by
 127   /// addConstraints and addLinks, updating their state. Add all the bundles
 128   /// that now prefer a register to RecentPositive.
 129   /// Prepare internal data structures for iterate.
 130   /// Return true is there are any positive nodes.
 131   bool scanActiveBundles();
 132
 133   /// iterate - Update the network iteratively until convergence, or new bundles
 134   /// are found.
 135   void iterate();
 136
 137   /// getRecentPositive - Return an array of bundles that became positive during
 138   /// the previous call to scanActiveBundles or iterate.
 139   ArrayRef<unsigned> getRecentPositive() { return RecentPositive; }
 140
 141   /// finish - Compute the optimal spill code placement given the
 142   /// constraints. No MustSpill constraints will be violated, and the smallest
 143   /// possible number of PrefX constraints will be violated, weighted by
 144   /// expected execution frequencies.
 145   /// The selected bundles are returned in the bitvector passed to prepare().
 146   /// @return True if a perfect solution was found, allowing the variable to be
 147   ///         in a register through all relevant bundles.
 148   bool finish();
 149
 150   /// getBlockFrequency - Return the estimated block execution frequency per
 151   /// function invocation.
 152   BlockFrequency getBlockFrequency(unsigned Number) const {
 153     return BlockFrequencies[Number];
 154   }
 155
 156 private:
 157   bool runOnMachineFunction(MachineFunction &mf) override;
 158   void getAnalysisUsage(AnalysisUsage &AU) const override;
 159   void releaseMemory() override;
 160
 161   void activate(unsigned n);
 162   void setThreshold(const BlockFrequency &Entry);
 163
 164   bool update(unsigned n);
 165 };
 166
 167 } // end namespace llvm
 168
 169 #endif // LLVM_LIB_CODEGEN_SPILLPLACEMENT_H