LLVM  3.7.0
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"
34 
35 namespace llvm {
36 
37 class BitVector;
38 class EdgeBundles;
39 class MachineBasicBlock;
40 class MachineLoopInfo;
41 class MachineBlockFrequencyInfo;
42 
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.
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).
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.
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.
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
BlockFrequency getBlockFrequency(unsigned Number) const
getBlockFrequency - Return the estimated block execution frequency per function invocation.
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: ArrayRef.h:31
BlockConstraint - Entry and exit constraints for a basic block.
void addLinks(ArrayRef< unsigned > Links)
addLinks - Add transparent blocks with the given numbers.
Block doesn't care / variable not live.
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:696
bool ChangesValue
True when this block changes the value of the live range.