LLVM  10.0.0svn
SpillPlacement.h
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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"
34 
35 namespace llvm {
36 
37 class BitVector;
38 class EdgeBundles;
39 class MachineBlockFrequencyInfo;
40 class MachineFunction;
41 class MachineLoopInfo;
42 
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.
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  /// 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 
76  ~SpillPlacement() override { releaseMemory(); }
77 
78  /// BorderConstraint - A basic block has separate constraints for entry and
79  /// exit.
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.
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.
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
This class represents lattice values for constants.
Definition: AllocatorList.h:23
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:32
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.
static cl::opt< unsigned > Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), cl::init(100), cl::Hidden)
Node - Each edge bundle corresponds to a Hopfield node.
BorderConstraint
BorderConstraint - A basic block has separate constraints for entry and exit.
loops
Definition: LoopInfo.cpp:1054
bool ChangesValue
True when this block changes the value of the live range.