LLVM 18.0.0git
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"
31#include "llvm/ADT/SparseSet.h"
34
35namespace llvm {
36
37class BitVector;
38class EdgeBundles;
39class MachineBlockFrequencyInfo;
40class MachineFunction;
41
43 struct Node;
44 const MachineFunction *MF = nullptr;
45 const EdgeBundles *bundles = nullptr;
46 const MachineBlockFrequencyInfo *MBFI = nullptr;
47 Node *nodes = nullptr;
48
49 // Nodes that are active in the current computation. Owned by the prepare()
50 // caller.
51 BitVector *ActiveNodes = nullptr;
52
53 // Nodes with active links. Populated by scanActiveBundles.
55
56 // Nodes that went positive during the last call to scanActiveBundles or
57 // iterate.
58 SmallVector<unsigned, 8> RecentPositive;
59
60 // Block frequencies are computed once. Indexed by block number.
61 SmallVector<BlockFrequency, 8> BlockFrequencies;
62
63 /// Decision threshold. A node gets the output value 0 if the weighted sum of
64 /// its inputs falls in the open interval (-Threshold;Threshold).
65 BlockFrequency Threshold;
66
67 /// List of nodes that need to be updated in ::iterate.
68 SparseSet<unsigned> TodoList;
69
70public:
71 static char ID; // Pass identification, replacement for typeid.
72
74 ~SpillPlacement() override { releaseMemory(); }
75
76 /// BorderConstraint - A basic block has separate constraints for entry and
77 /// exit.
79 DontCare, ///< Block doesn't care / variable not live.
80 PrefReg, ///< Block entry/exit prefers a register.
81 PrefSpill, ///< Block entry/exit prefers a stack slot.
82 PrefBoth, ///< Block entry prefers both register and stack.
83 MustSpill ///< A register is impossible, variable must be spilled.
84 };
85
86 /// BlockConstraint - Entry and exit constraints for a basic block.
88 unsigned Number; ///< Basic block number (from MBB::getNumber()).
89 BorderConstraint Entry : 8; ///< Constraint on block entry.
90 BorderConstraint Exit : 8; ///< Constraint on block exit.
91
92 /// True when this block changes the value of the live range. This means
93 /// the block has a non-PHI def. When this is false, a live-in value on
94 /// the stack can be live-out on the stack without inserting a spill.
96
97 void print(raw_ostream &OS) const;
98 void dump() const;
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.
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
157private:
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(BlockFrequency Entry);
164
165 bool update(unsigned n);
166};
167
168} // end namespace llvm
169
170#endif // LLVM_LIB_CODEGEN_SPILLPLACEMENT_H
Unify divergent function exit nodes
bbsections prepare
DenseMap< Block *, BlockRelaxAux > Blocks
Definition: ELF_riscv.cpp:505
raw_pwrite_stream & OS
This file defines the SmallVector class.
This file defines the SparseSet class derived from the version described in Briggs,...
Represent the analysis usage information of a pass.
ArrayRef - Represent a constant reference to an array (0 or more elements consecutively in memory),...
Definition: ArrayRef.h:41
MachineBlockFrequencyInfo pass uses BlockFrequencyInfoImpl implementation to estimate machine basic b...
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1200
SparseSet - Fast set implementation for objects that can be identified by small unsigned keys.
Definition: SparseSet.h:124
~SpillPlacement() override
void addConstraints(ArrayRef< BlockConstraint > LiveBlocks)
addConstraints - Add constraints and biases.
bool finish()
finish - Compute the optimal spill code placement given the constraints.
void addPrefSpill(ArrayRef< unsigned > Blocks, bool Strong)
addPrefSpill - Add PrefSpill constraints to all blocks listed.
bool scanActiveBundles()
scanActiveBundles - Perform an initial scan of all bundles activated by addConstraints and addLinks,...
void addLinks(ArrayRef< unsigned > Links)
addLinks - Add transparent blocks with the given numbers.
void iterate()
iterate - Update the network iteratively until convergence, or new bundles are found.
BorderConstraint
BorderConstraint - A basic block has separate constraints for entry and exit.
@ MustSpill
A register is impossible, variable must be spilled.
@ DontCare
Block doesn't care / variable not live.
@ PrefBoth
Block entry prefers both register and stack.
@ PrefReg
Block entry/exit prefers a register.
@ PrefSpill
Block entry/exit prefers a stack slot.
ArrayRef< unsigned > getRecentPositive()
getRecentPositive - Return an array of bundles that became positive during the previous call to scanA...
BlockFrequency getBlockFrequency(unsigned Number) const
getBlockFrequency - Return the estimated block execution frequency per function invocation.
This class implements an extremely fast bulk output stream that can only output to a stream.
Definition: raw_ostream.h:52
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
Node - Each edge bundle corresponds to a Hopfield node.
BlockConstraint - Entry and exit constraints for a basic block.
BorderConstraint Exit
Constraint on block exit.
void print(raw_ostream &OS) const
bool ChangesValue
True when this block changes the value of the live range.
BorderConstraint Entry
Constraint on block entry.
unsigned Number
Basic block number (from MBB::getNumber()).