SpillPlacement.h revision 8bfe50871f9cb1b022483e0e1307ab5b8c9e5650 
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 placeSpills() which is called by the
14// 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/CodeGen/MachineFunctionPass.h"
31
32namespace llvm {
33
34class BitVector;
35class EdgeBundles;
36class MachineBasicBlock;
37class MachineLoopInfo;
38template <typename> class SmallVectorImpl;
39
40class SpillPlacement  : public MachineFunctionPass {
41  struct Node;
42  const MachineFunction *MF;
43  const EdgeBundles *bundles;
44  const MachineLoopInfo *loops;
45  Node *nodes;
46
47  // Nodes that are active in the current computation. Owned by the placeSpills
48  // caller.
49  BitVector *ActiveNodes;
50
51public:
52  static char ID; // Pass identification, replacement for typeid.
53
54  SpillPlacement() : MachineFunctionPass(ID), nodes(0) {}
55  ~SpillPlacement() { releaseMemory(); }
56
57  /// BorderConstraint - A basic block has separate constraints for entry and
58  /// exit.
59  enum BorderConstraint {
60    DontCare,  ///< Block doesn't care / variable not live.
61    PrefReg,   ///< Block entry/exit prefers a register.
62    PrefSpill, ///< Block entry/exit prefers a stack slot.
63    MustSpill  ///< A register is impossible, variable must be spilled.
64  };
65
66  /// BlockConstraint - Entry and exit constraints for a basic block.
67  struct BlockConstraint {
68    unsigned Number;            ///< Basic block number (from MBB::getNumber()).
69    BorderConstraint Entry : 8; ///< Constraint on block entry.
70    BorderConstraint Exit : 8;  ///< Constraint on block exit.
71  };
72
73  /// placeSpills - Compute the optimal spill code placement given the
74  /// constraints. No MustSpill constraints will be violated, and the smallest
75  /// possible number of PrefX constraints will be violated, weighted by
76  /// expected execution frequencies.
77  /// @param LiveBlocks Constraints for blocks that have the variable live in or
78  ///                   live out. DontCare/DontCare means the variable is live
79  ///                   through the block. DontCare/X means the variable is live
80  ///                   out, but not live in.
81  /// @param RegBundles Bit vector to receive the edge bundles where the
82  ///                   variable should be kept in a register. Each bit
83  ///                   corresponds to an edge bundle, a set bit means the
84  ///                   variable should be kept in a register through the
85  ///                   bundle. A clear bit means the variable should be
86  ///                   spilled.
87  /// @return True if a perfect solution was found, allowing the variable to be
88  ///         in a register through all relevant bundles.
89  bool placeSpills(const SmallVectorImpl<BlockConstraint> &LiveBlocks,
90                   BitVector &RegBundles);
91
92private:
93  virtual bool runOnMachineFunction(MachineFunction&);
94  virtual void getAnalysisUsage(AnalysisUsage&) const;
95  virtual void releaseMemory();
96
97  void activate(unsigned);
98  float getBlockFrequency(const MachineBasicBlock*);
99  void prepareNodes(const SmallVectorImpl<BlockConstraint>&);
100  void iterate(const SmallVectorImpl<unsigned>&);
101};
102
103} // end namespace llvm
104
105#endif
106