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"
32#include "llvm/CodeGen/MachineFunctionPass.h"
33#include "llvm/Support/BlockFrequency.h"
34
35namespace llvm {
36
37class BitVector;
38class EdgeBundles;
39class MachineBlockFrequencyInfo;
40class MachineFunction;
41class MachineLoopInfo;
42
43class SpillPlacement : public MachineFunctionPass {
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.
56  SmallVector<unsigned, 8> Linked;
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).
67  BlockFrequency Threshold;
68
69  /// List of nodes that need to be updated in ::iterate.
70  SparseSet<unsigned> TodoList;
71
72public:
73  static char ID; // Pass identification, replacement for typeid.
74
75  SpillPlacement() : MachineFunctionPass(ID) {}
76  ~SpillPlacement() override { releaseMemory(); }
77
78  /// BorderConstraint - A basic block has separate constraints for entry and
79  /// exit.
80  enum BorderConstraint {
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.
97    bool ChangesValue;
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.
152  BlockFrequency getBlockFrequency(unsigned Number) const {
153    return BlockFrequencies[Number];
154  }
155
156private:
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
170