LLVM  12.0.0git
LiveRangeCalc.h
Go to the documentation of this file.
1 //===- LiveRangeCalc.h - Calculate live ranges -----------------*- 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 // The LiveRangeCalc class can be used to implement the computation of
10 // live ranges from scratch.
11 // It caches information about values in the CFG to speed up repeated
12 // operations on the same live range. The cache can be shared by
13 // non-overlapping live ranges. SplitKit uses that when computing the live
14 // range of split products.
15 //
16 // A low-level interface is available to clients that know where a variable is
17 // live, but don't know which value it has as every point. LiveRangeCalc will
18 // propagate values down the dominator tree, and even insert PHI-defs where
19 // needed. SplitKit uses this faster interface when possible.
20 //
21 //===----------------------------------------------------------------------===//
22 
23 #ifndef LLVM_LIB_CODEGEN_LIVERANGECALC_H
24 #define LLVM_LIB_CODEGEN_LIVERANGECALC_H
25 
26 #include "llvm/ADT/ArrayRef.h"
27 #include "llvm/ADT/BitVector.h"
28 #include "llvm/ADT/DenseMap.h"
29 #include "llvm/ADT/IndexedMap.h"
30 #include "llvm/ADT/SmallVector.h"
34 #include "llvm/MC/LaneBitmask.h"
35 #include <utility>
36 
37 namespace llvm {
38 
39 template <class NodeT> class DomTreeNodeBase;
40 class MachineDominatorTree;
41 class MachineFunction;
42 class MachineRegisterInfo;
43 
44 using MachineDomTreeNode = DomTreeNodeBase<MachineBasicBlock>;
45 
47  const MachineFunction *MF = nullptr;
48  const MachineRegisterInfo *MRI = nullptr;
49  SlotIndexes *Indexes = nullptr;
50  MachineDominatorTree *DomTree = nullptr;
51  VNInfo::Allocator *Alloc = nullptr;
52 
53  /// LiveOutPair - A value and the block that defined it. The domtree node is
54  /// redundant, it can be computed as: MDT[Indexes.getMBBFromIndex(VNI->def)].
55  using LiveOutPair = std::pair<VNInfo *, MachineDomTreeNode *>;
56 
57  /// LiveOutMap - Map basic blocks to the value leaving the block.
59 
60  /// Bit vector of active entries in LiveOut, also used as a visited set by
61  /// findReachingDefs. One entry per basic block, indexed by block number.
62  /// This is kept as a separate bit vector because it can be cleared quickly
63  /// when switching live ranges.
64  BitVector Seen;
65 
66  /// Map LiveRange to sets of blocks (represented by bit vectors) that
67  /// in the live range are defined on entry and undefined on entry.
68  /// A block is defined on entry if there is a path from at least one of
69  /// the defs in the live range to the entry of the block, and conversely,
70  /// a block is undefined on entry, if there is no such path (i.e. no
71  /// definition reaches the entry of the block). A single LiveRangeCalc
72  /// object is used to track live-out information for multiple registers
73  /// in live range splitting (which is ok, since the live ranges of these
74  /// registers do not overlap), but the defined/undefined information must
75  /// be kept separate for each individual range.
76  /// By convention, EntryInfoMap[&LR] = { Defined, Undefined }.
78  EntryInfoMap EntryInfos;
79 
80  /// Map each basic block where a live range is live out to the live-out value
81  /// and its defining block.
82  ///
83  /// For every basic block, MBB, one of these conditions shall be true:
84  ///
85  /// 1. !Seen.count(MBB->getNumber())
86  /// Blocks without a Seen bit are ignored.
87  /// 2. LiveOut[MBB].second.getNode() == MBB
88  /// The live-out value is defined in MBB.
89  /// 3. forall P in preds(MBB): LiveOut[P] == LiveOut[MBB]
90  /// The live-out value passses through MBB. All predecessors must carry
91  /// the same value.
92  ///
93  /// The domtree node may be null, it can be computed.
94  ///
95  /// The map can be shared by multiple live ranges as long as no two are
96  /// live-out of the same block.
97  LiveOutMap Map;
98 
99  /// LiveInBlock - Information about a basic block where a live range is known
100  /// to be live-in, but the value has not yet been determined.
101  struct LiveInBlock {
102  // The live range set that is live-in to this block. The algorithms can
103  // handle multiple non-overlapping live ranges simultaneously.
104  LiveRange &LR;
105 
106  // DomNode - Dominator tree node for the block.
107  // Cleared when the final value has been determined and LI has been updated.
108  MachineDomTreeNode *DomNode;
109 
110  // Position in block where the live-in range ends, or SlotIndex() if the
111  // range passes through the block. When the final value has been
112  // determined, the range from the block start to Kill will be added to LI.
113  SlotIndex Kill;
114 
115  // Live-in value filled in by updateSSA once it is known.
116  VNInfo *Value = nullptr;
117 
118  LiveInBlock(LiveRange &LR, MachineDomTreeNode *node, SlotIndex kill)
119  : LR(LR), DomNode(node), Kill(kill) {}
120  };
121 
122  /// LiveIn - Work list of blocks where the live-in value has yet to be
123  /// determined. This list is typically computed by findReachingDefs() and
124  /// used as a work list by updateSSA(). The low-level interface may also be
125  /// used to add entries directly.
127 
128  /// Check if the entry to block @p MBB can be reached by any of the defs
129  /// in @p LR. Return true if none of the defs reach the entry to @p MBB.
130  bool isDefOnEntry(LiveRange &LR, ArrayRef<SlotIndex> Undefs,
131  MachineBasicBlock &MBB, BitVector &DefOnEntry,
132  BitVector &UndefOnEntry);
133 
134  /// Find the set of defs that can reach @p Kill. @p Kill must belong to
135  /// @p UseMBB.
136  ///
137  /// If exactly one def can reach @p UseMBB, and the def dominates @p Kill,
138  /// all paths from the def to @p UseMBB are added to @p LR, and the function
139  /// returns true.
140  ///
141  /// If multiple values can reach @p UseMBB, the blocks that need @p LR to be
142  /// live in are added to the LiveIn array, and the function returns false.
143  ///
144  /// The array @p Undef provides the locations where the range @p LR becomes
145  /// undefined by <def,read-undef> operands on other subranges. If @p Undef
146  /// is non-empty and @p Kill is jointly dominated only by the entries of
147  /// @p Undef, the function returns false.
148  ///
149  /// PhysReg, when set, is used to verify live-in lists on basic blocks.
150  bool findReachingDefs(LiveRange &LR, MachineBasicBlock &UseMBB, SlotIndex Use,
151  unsigned PhysReg, ArrayRef<SlotIndex> Undefs);
152 
153  /// updateSSA - Compute the values that will be live in to all requested
154  /// blocks in LiveIn. Create PHI-def values as required to preserve SSA form.
155  ///
156  /// Every live-in block must be jointly dominated by the added live-out
157  /// blocks. No values are read from the live ranges.
158  void updateSSA();
159 
160  /// Transfer information from the LiveIn vector to the live ranges and update
161  /// the given @p LiveOuts.
162  void updateFromLiveIns();
163 
164 protected:
165  /// Some getters to expose in a read-only way some private fields to
166  /// subclasses.
167  const MachineFunction *getMachineFunction() { return MF; }
168  const MachineRegisterInfo *getRegInfo() const { return MRI; }
169  SlotIndexes *getIndexes() { return Indexes; }
170  MachineDominatorTree *getDomTree() { return DomTree; }
171  VNInfo::Allocator *getVNAlloc() { return Alloc; }
172 
173  /// Reset Map and Seen fields.
174  void resetLiveOutMap();
175 
176 public:
177  LiveRangeCalc() = default;
178 
179  //===--------------------------------------------------------------------===//
180  // High-level interface.
181  //===--------------------------------------------------------------------===//
182  //
183  // Calculate live ranges from scratch.
184  //
185 
186  /// reset - Prepare caches for a new set of non-overlapping live ranges. The
187  /// caches must be reset before attempting calculations with a live range
188  /// that may overlap a previously computed live range, and before the first
189  /// live range in a function. If live ranges are not known to be
190  /// non-overlapping, call reset before each.
191  void reset(const MachineFunction *mf, SlotIndexes *SI,
193 
194  //===--------------------------------------------------------------------===//
195  // Mid-level interface.
196  //===--------------------------------------------------------------------===//
197  //
198  // Modify existing live ranges.
199  //
200 
201  /// Extend the live range of @p LR to reach @p Use.
202  ///
203  /// The existing values in @p LR must be live so they jointly dominate @p Use.
204  /// If @p Use is not dominated by a single existing value, PHI-defs are
205  /// inserted as required to preserve SSA form.
206  ///
207  /// PhysReg, when set, is used to verify live-in lists on basic blocks.
208  void extend(LiveRange &LR, SlotIndex Use, unsigned PhysReg,
209  ArrayRef<SlotIndex> Undefs);
210 
211  //===--------------------------------------------------------------------===//
212  // Low-level interface.
213  //===--------------------------------------------------------------------===//
214  //
215  // These functions can be used to compute live ranges where the live-in and
216  // live-out blocks are already known, but the SSA value in each block is
217  // unknown.
218  //
219  // After calling reset(), add known live-out values and known live-in blocks.
220  // Then call calculateValues() to compute the actual value that is
221  // live-in to each block, and add liveness to the live ranges.
222  //
223 
224  /// setLiveOutValue - Indicate that VNI is live out from MBB. The
225  /// calculateValues() function will not add liveness for MBB, the caller
226  /// should take care of that.
227  ///
228  /// VNI may be null only if MBB is a live-through block also passed to
229  /// addLiveInBlock().
231  Seen.set(MBB->getNumber());
232  Map[MBB] = LiveOutPair(VNI, nullptr);
233  }
234 
235  /// addLiveInBlock - Add a block with an unknown live-in value. This
236  /// function can only be called once per basic block. Once the live-in value
237  /// has been determined, calculateValues() will add liveness to LI.
238  ///
239  /// @param LR The live range that is live-in to the block.
240  /// @param DomNode The domtree node for the block.
241  /// @param Kill Index in block where LI is killed. If the value is
242  /// live-through, set Kill = SLotIndex() and also call
243  /// setLiveOutValue(MBB, 0).
245  SlotIndex Kill = SlotIndex()) {
246  LiveIn.push_back(LiveInBlock(LR, DomNode, Kill));
247  }
248 
249  /// calculateValues - Calculate the value that will be live-in to each block
250  /// added with addLiveInBlock. Add PHI-def values as needed to preserve SSA
251  /// form. Add liveness to all live-in blocks up to the Kill point, or the
252  /// whole block for live-through blocks.
253  ///
254  /// Every predecessor of a live-in block must have been given a value with
255  /// setLiveOutValue, the value may be null for live-trough blocks.
256  void calculateValues();
257 
258  /// A diagnostic function to check if the end of the block @p MBB is
259  /// jointly dominated by the blocks corresponding to the slot indices
260  /// in @p Defs. This function is mainly for use in self-verification
261  /// checks.
263  static bool isJointlyDominated(const MachineBasicBlock *MBB,
264  ArrayRef<SlotIndex> Defs,
265  const SlotIndexes &Indexes);
266 };
267 
268 } // end namespace llvm
269 
270 #endif // LLVM_LIB_CODEGEN_LIVERANGECALC_H
BitVector & set()
Definition: BitVector.h:398
A common definition of LaneBitmask for use in TableGen and CodeGen.
This class represents lattice values for constants.
Definition: AllocatorList.h:23
void setLiveOutValue(MachineBasicBlock *MBB, VNInfo *VNI)
setLiveOutValue - Indicate that VNI is live out from MBB.
void push_back(const T &Elt)
Definition: SmallVector.h:246
VNInfo - Value Number Information.
Definition: LiveInterval.h:52
This class represents the liveness of a register, stack slot, etc.
Definition: LiveInterval.h:156
void resetLiveOutMap()
Reset Map and Seen fields.
MachineBasicBlock & MBB
DomTreeNodeBase< MachineBasicBlock > MachineDomTreeNode
A Use represents the edge between a Value definition and its users.
Definition: Use.h:44
LiveRangeCalc()=default
SlotIndexes pass.
Definition: SlotIndexes.h:314
void addLiveInBlock(LiveRange &LR, MachineDomTreeNode *DomNode, SlotIndex Kill=SlotIndex())
addLiveInBlock - Add a block with an unknown live-in value.
Base class for the actual dominator tree node.
ArrayRef - Represent a constant reference to an array (0 or more elements consecutively in memory)...
Definition: APInt.h:32
void extend(LiveRange &LR, SlotIndex Use, unsigned PhysReg, ArrayRef< SlotIndex > Undefs)
Extend the live range of LR to reach Use.
int getNumber() const
MachineBasicBlocks are uniquely numbered at the function level, unless they&#39;re not in a MachineFuncti...
Allocate memory in an ever growing pool, as if by bump-pointer.
Definition: Allocator.h:67
#define LLVM_ATTRIBUTE_UNUSED
Definition: Compiler.h:187
SlotIndexes * getIndexes()
const MachineFunction * getMachineFunction()
Some getters to expose in a read-only way some private fields to subclasses.
VNInfo::Allocator * getVNAlloc()
void reset(const MachineFunction *mf, SlotIndexes *SI, MachineDominatorTree *MDT, VNInfo::Allocator *VNIA)
reset - Prepare caches for a new set of non-overlapping live ranges.
MachineDominatorTree * getDomTree()
MachineRegisterInfo - Keep track of information for virtual and physical registers, including vreg register classes, use/def chains for registers, etc.
const MachineRegisterInfo * getRegInfo() const
LLVM Value Representation.
Definition: Value.h:74
SlotIndex - An opaque wrapper around machine indexes.
Definition: SlotIndexes.h:83
void calculateValues()
calculateValues - Calculate the value that will be live-in to each block added with addLiveInBlock...
DominatorTree Class - Concrete subclass of DominatorTreeBase that is used to compute a normal dominat...
static LLVM_ATTRIBUTE_UNUSED bool isJointlyDominated(const MachineBasicBlock *MBB, ArrayRef< SlotIndex > Defs, const SlotIndexes &Indexes)
A diagnostic function to check if the end of the block MBB is jointly dominated by the blocks corresp...