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_CODEGEN_SPILLPLACEMENT_H
28#define LLVM_CODEGEN_SPILLPLACEMENT_H
29
30#include "llvm/ADT/ArrayRef.h"
31#include "llvm/ADT/SmallVector.h"
32#include "llvm/CodeGen/MachineFunctionPass.h"
33
34namespace llvm {
35
36class BitVector;
37class EdgeBundles;
38class MachineBasicBlock;
39class MachineLoopInfo;
40
41class SpillPlacement  : public MachineFunctionPass {
42  struct Node;
43  const MachineFunction *MF;
44  const EdgeBundles *bundles;
45  const MachineLoopInfo *loops;
46  Node *nodes;
47
48  // Nodes that are active in the current computation. Owned by the prepare()
49  // caller.
50  BitVector *ActiveNodes;
51
52  // Nodes with active links. Populated by scanActiveBundles.
53  SmallVector<unsigned, 8> Linked;
54
55  // Nodes that went positive during the last call to scanActiveBundles or
56  // iterate.
57  SmallVector<unsigned, 8> RecentPositive;
58
59  // Block frequencies are computed once. Indexed by block number.
60  SmallVector<float, 4> BlockFrequency;
61
62public:
63  static char ID; // Pass identification, replacement for typeid.
64
65  SpillPlacement() : MachineFunctionPass(ID), nodes(0) {}
66  ~SpillPlacement() { releaseMemory(); }
67
68  /// BorderConstraint - A basic block has separate constraints for entry and
69  /// exit.
70  enum BorderConstraint {
71    DontCare,  ///< Block doesn't care / variable not live.
72    PrefReg,   ///< Block entry/exit prefers a register.
73    PrefSpill, ///< Block entry/exit prefers a stack slot.
74    PrefBoth,  ///< Block entry prefers both register and stack.
75    MustSpill  ///< A register is impossible, variable must be spilled.
76  };
77
78  /// BlockConstraint - Entry and exit constraints for a basic block.
79  struct BlockConstraint {
80    unsigned Number;            ///< Basic block number (from MBB::getNumber()).
81    BorderConstraint Entry : 8; ///< Constraint on block entry.
82    BorderConstraint Exit : 8;  ///< Constraint on block exit.
83
84    /// True when this block changes the value of the live range. This means
85    /// the block has a non-PHI def.  When this is false, a live-in value on
86    /// the stack can be live-out on the stack without inserting a spill.
87    bool ChangesValue;
88  };
89
90  /// prepare - Reset state and prepare for a new spill placement computation.
91  /// @param RegBundles Bit vector to receive the edge bundles where the
92  ///                   variable should be kept in a register. Each bit
93  ///                   corresponds to an edge bundle, a set bit means the
94  ///                   variable should be kept in a register through the
95  ///                   bundle. A clear bit means the variable should be
96  ///                   spilled. This vector is retained.
97  void prepare(BitVector &RegBundles);
98
99  /// addConstraints - Add constraints and biases. This method may be called
100  /// more than once to accumulate constraints.
101  /// @param LiveBlocks Constraints for blocks that have the variable live in or
102  ///                   live out.
103  void addConstraints(ArrayRef<BlockConstraint> LiveBlocks);
104
105  /// addPrefSpill - Add PrefSpill constraints to all blocks listed.  This is
106  /// equivalent to calling addConstraint with identical BlockConstraints with
107  /// Entry = Exit = PrefSpill, and ChangesValue = false.
108  ///
109  /// @param Blocks Array of block numbers that prefer to spill in and out.
110  /// @param Strong When true, double the negative bias for these blocks.
111  void addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong);
112
113  /// addLinks - Add transparent blocks with the given numbers.
114  void addLinks(ArrayRef<unsigned> Links);
115
116  /// scanActiveBundles - Perform an initial scan of all bundles activated by
117  /// addConstraints and addLinks, updating their state. Add all the bundles
118  /// that now prefer a register to RecentPositive.
119  /// Prepare internal data structures for iterate.
120  /// Return true is there are any positive nodes.
121  bool scanActiveBundles();
122
123  /// iterate - Update the network iteratively until convergence, or new bundles
124  /// are found.
125  void iterate();
126
127  /// getRecentPositive - Return an array of bundles that became positive during
128  /// the previous call to scanActiveBundles or iterate.
129  ArrayRef<unsigned> getRecentPositive() { return RecentPositive; }
130
131  /// finish - Compute the optimal spill code placement given the
132  /// constraints. No MustSpill constraints will be violated, and the smallest
133  /// possible number of PrefX constraints will be violated, weighted by
134  /// expected execution frequencies.
135  /// The selected bundles are returned in the bitvector passed to prepare().
136  /// @return True if a perfect solution was found, allowing the variable to be
137  ///         in a register through all relevant bundles.
138  bool finish();
139
140  /// getBlockFrequency - Return the estimated block execution frequency per
141  /// function invocation.
142  float getBlockFrequency(unsigned Number) const {
143    return BlockFrequency[Number];
144  }
145
146private:
147  virtual bool runOnMachineFunction(MachineFunction&);
148  virtual void getAnalysisUsage(AnalysisUsage&) const;
149  virtual void releaseMemory();
150
151  void activate(unsigned);
152};
153
154} // end namespace llvm
155
156#endif
157