LLVM  7.0.0svn
SpillPlacement.h
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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/ADT/SparseSet.h"
35 
36 namespace llvm {
37 
38 class BitVector;
39 class EdgeBundles;
40 class MachineBlockFrequencyInfo;
41 class MachineFunction;
42 class MachineLoopInfo;
43 
45  struct Node;
46  const MachineFunction *MF;
47  const EdgeBundles *bundles;
48  const MachineLoopInfo *loops;
49  const MachineBlockFrequencyInfo *MBFI;
50  Node *nodes = nullptr;
51 
52  // Nodes that are active in the current computation. Owned by the prepare()
53  // caller.
54  BitVector *ActiveNodes;
55 
56  // Nodes with active links. Populated by scanActiveBundles.
58 
59  // Nodes that went positive during the last call to scanActiveBundles or
60  // iterate.
61  SmallVector<unsigned, 8> RecentPositive;
62 
63  // Block frequencies are computed once. Indexed by block number.
64  SmallVector<BlockFrequency, 8> BlockFrequencies;
65 
66  /// Decision threshold. A node gets the output value 0 if the weighted sum of
67  /// its inputs falls in the open interval (-Threshold;Threshold).
69 
70  /// List of nodes that need to be updated in ::iterate.
71  SparseSet<unsigned> TodoList;
72 
73 public:
74  static char ID; // Pass identification, replacement for typeid.
75 
77  ~SpillPlacement() override { releaseMemory(); }
78 
79  /// BorderConstraint - A basic block has separate constraints for entry and
80  /// exit.
82  DontCare, ///< Block doesn't care / variable not live.
83  PrefReg, ///< Block entry/exit prefers a register.
84  PrefSpill, ///< Block entry/exit prefers a stack slot.
85  PrefBoth, ///< Block entry prefers both register and stack.
86  MustSpill ///< A register is impossible, variable must be spilled.
87  };
88 
89  /// BlockConstraint - Entry and exit constraints for a basic block.
90  struct BlockConstraint {
91  unsigned Number; ///< Basic block number (from MBB::getNumber()).
92  BorderConstraint Entry : 8; ///< Constraint on block entry.
93  BorderConstraint Exit : 8; ///< Constraint on block exit.
94 
95  /// True when this block changes the value of the live range. This means
96  /// the block has a non-PHI def. When this is false, a live-in value on
97  /// the stack can be live-out on the stack without inserting a spill.
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.
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(const BlockFrequency &Entry);
164 
165  bool update(unsigned n);
166 };
167 
168 } // end namespace llvm
169 
170 #endif // LLVM_LIB_CODEGEN_SPILLPLACEMENT_H
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
A register is impossible, variable must be spilled.
~SpillPlacement() override
MachineBlockFrequencyInfo pass uses BlockFrequencyInfoImpl implementation to estimate machine basic b...
void addPrefSpill(ArrayRef< unsigned > Blocks, bool Strong)
addPrefSpill - Add PrefSpill constraints to all blocks listed.
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
bool scanActiveBundles()
scanActiveBundles - Perform an initial scan of all bundles activated by addConstraints and addLinks...
ArrayRef< unsigned > getRecentPositive()
getRecentPositive - Return an array of bundles that became positive during the previous call to scanA...
void iterate()
iterate - Update the network iteratively until convergence, or new bundles are found.
BorderConstraint Exit
Constraint on block exit.
ArrayRef - Represent a constant reference to an array (0 or more elements consecutively in memory)...
Definition: APInt.h:33
BlockConstraint - Entry and exit constraints for a basic block.
void addLinks(ArrayRef< unsigned > Links)
addLinks - Add transparent blocks with the given numbers.
Unify divergent function exit nodes
Block doesn&#39;t care / variable not live.
BlockFrequency getBlockFrequency(unsigned Number) const
getBlockFrequency - Return the estimated block execution frequency per function invocation.
Represent the analysis usage information of a pass.
unsigned Number
Basic block number (from MBB::getNumber()).
BorderConstraint Entry
Constraint on block entry.
bool finish()
finish - Compute the optimal spill code placement given the constraints.
Block entry/exit prefers a register.
void addConstraints(ArrayRef< BlockConstraint > LiveBlocks)
addConstraints - Add constraints and biases.
Block entry/exit prefers a stack slot.
Block entry prefers both register and stack.
void prepare(BitVector &RegBundles)
prepare - Reset state and prepare for a new spill placement computation.
Node - Each edge bundle corresponds to a Hopfield node.
BorderConstraint
BorderConstraint - A basic block has separate constraints for entry and exit.
static int const Threshold
TODO: Write a new FunctionPass AliasAnalysis so that it can keep a cache.
loops
Definition: LoopInfo.cpp:782
bool ChangesValue
True when this block changes the value of the live range.