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1 //===---- LiveRangeCalc.h - Calculate live ranges ---------------*- 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 // The LiveRangeCalc class can be used to compute live ranges from scratch.  It
11 // caches information about values in the CFG to speed up repeated operations
12 // on the same live range.  The cache can be shared by non-overlapping live
13 // ranges.  SplitKit uses that when computing the live range of split products.
14 //
15 // A low-level interface is available to clients that know where a variable is
16 // live, but don't know which value it has as every point.  LiveRangeCalc will
17 // propagate values down the dominator tree, and even insert PHI-defs where
18 // needed.  SplitKit uses this faster interface when possible.
19 //
20 //===----------------------------------------------------------------------===//
21 
22 #ifndef LLVM_LIB_CODEGEN_LIVERANGECALC_H
23 #define LLVM_LIB_CODEGEN_LIVERANGECALC_H
24 
25 #include "llvm/ADT/BitVector.h"
26 #include "llvm/ADT/IndexedMap.h"
27 #include "llvm/CodeGen/LiveInterval.h"
28 
29 namespace llvm {
30 
31 /// Forward declarations for MachineDominators.h:
32 class MachineDominatorTree;
33 template <class NodeT> class DomTreeNodeBase;
34 typedef DomTreeNodeBase<MachineBasicBlock> MachineDomTreeNode;
35 
36 class LiveRangeCalc {
37   const MachineFunction *MF;
38   const MachineRegisterInfo *MRI;
39   SlotIndexes *Indexes;
40   MachineDominatorTree *DomTree;
41   VNInfo::Allocator *Alloc;
42 
43   /// LiveOutPair - A value and the block that defined it.  The domtree node is
44   /// redundant, it can be computed as: MDT[Indexes.getMBBFromIndex(VNI->def)].
45   typedef std::pair<VNInfo*, MachineDomTreeNode*> LiveOutPair;
46 
47   /// LiveOutMap - Map basic blocks to the value leaving the block.
48   typedef IndexedMap<LiveOutPair, MBB2NumberFunctor> LiveOutMap;
49 
50   /// Bit vector of active entries in LiveOut, also used as a visited set by
51   /// findReachingDefs.  One entry per basic block, indexed by block number.
52   /// This is kept as a separate bit vector because it can be cleared quickly
53   /// when switching live ranges.
54   BitVector Seen;
55 
56   /// Map each basic block where a live range is live out to the live-out value
57   /// and its defining block.
58   ///
59   /// For every basic block, MBB, one of these conditions shall be true:
60   ///
61   ///  1. !Seen.count(MBB->getNumber())
62   ///     Blocks without a Seen bit are ignored.
63   ///  2. LiveOut[MBB].second.getNode() == MBB
64   ///     The live-out value is defined in MBB.
65   ///  3. forall P in preds(MBB): LiveOut[P] == LiveOut[MBB]
66   ///     The live-out value passses through MBB. All predecessors must carry
67   ///     the same value.
68   ///
69   /// The domtree node may be null, it can be computed.
70   ///
71   /// The map can be shared by multiple live ranges as long as no two are
72   /// live-out of the same block.
73   LiveOutMap Map;
74 
75   /// LiveInBlock - Information about a basic block where a live range is known
76   /// to be live-in, but the value has not yet been determined.
77   struct LiveInBlock {
78     // The live range set that is live-in to this block.  The algorithms can
79     // handle multiple non-overlapping live ranges simultaneously.
80     LiveRange &LR;
81 
82     // DomNode - Dominator tree node for the block.
83     // Cleared when the final value has been determined and LI has been updated.
84     MachineDomTreeNode *DomNode;
85 
86     // Position in block where the live-in range ends, or SlotIndex() if the
87     // range passes through the block.  When the final value has been
88     // determined, the range from the block start to Kill will be added to LI.
89     SlotIndex Kill;
90 
91     // Live-in value filled in by updateSSA once it is known.
92     VNInfo *Value;
93 
LiveInBlockLiveInBlock94     LiveInBlock(LiveRange &LR, MachineDomTreeNode *node, SlotIndex kill)
95       : LR(LR), DomNode(node), Kill(kill), Value(nullptr) {}
96   };
97 
98   /// LiveIn - Work list of blocks where the live-in value has yet to be
99   /// determined.  This list is typically computed by findReachingDefs() and
100   /// used as a work list by updateSSA().  The low-level interface may also be
101   /// used to add entries directly.
102   SmallVector<LiveInBlock, 16> LiveIn;
103 
104   /// Assuming that @p LR is live-in to @p UseMBB, find the set of defs that can
105   /// reach it.
106   ///
107   /// If only one def can reach @p UseMBB, all paths from the def to @p UseMBB
108   /// are added to @p LR, and the function returns true.
109   ///
110   /// If multiple values can reach @p UseMBB, the blocks that need @p LR to be
111   /// live in are added to the LiveIn array, and the function returns false.
112   ///
113   /// PhysReg, when set, is used to verify live-in lists on basic blocks.
114   bool findReachingDefs(LiveRange &LR, MachineBasicBlock &UseMBB,
115                         SlotIndex Kill, unsigned PhysReg);
116 
117   /// updateSSA - Compute the values that will be live in to all requested
118   /// blocks in LiveIn.  Create PHI-def values as required to preserve SSA form.
119   ///
120   /// Every live-in block must be jointly dominated by the added live-out
121   /// blocks.  No values are read from the live ranges.
122   void updateSSA();
123 
124   /// Transfer information from the LiveIn vector to the live ranges and update
125   /// the given @p LiveOuts.
126   void updateFromLiveIns();
127 
128   /// Extend the live range of @p LR to reach all uses of Reg.
129   ///
130   /// All uses must be jointly dominated by existing liveness.  PHI-defs are
131   /// inserted as needed to preserve SSA form.
132   void extendToUses(LiveRange &LR, unsigned Reg, LaneBitmask LaneMask);
133 
134   /// Reset Map and Seen fields.
135   void resetLiveOutMap();
136 
137 public:
LiveRangeCalc()138   LiveRangeCalc() : MF(nullptr), MRI(nullptr), Indexes(nullptr),
139                     DomTree(nullptr), Alloc(nullptr) {}
140 
141   //===--------------------------------------------------------------------===//
142   // High-level interface.
143   //===--------------------------------------------------------------------===//
144   //
145   // Calculate live ranges from scratch.
146   //
147 
148   /// reset - Prepare caches for a new set of non-overlapping live ranges.  The
149   /// caches must be reset before attempting calculations with a live range
150   /// that may overlap a previously computed live range, and before the first
151   /// live range in a function.  If live ranges are not known to be
152   /// non-overlapping, call reset before each.
153   void reset(const MachineFunction *MF,
154              SlotIndexes*,
155              MachineDominatorTree*,
156              VNInfo::Allocator*);
157 
158   //===--------------------------------------------------------------------===//
159   // Mid-level interface.
160   //===--------------------------------------------------------------------===//
161   //
162   // Modify existing live ranges.
163   //
164 
165   /// Extend the live range of @p LR to reach @p Use.
166   ///
167   /// The existing values in @p LR must be live so they jointly dominate @p Use.
168   /// If @p Use is not dominated by a single existing value, PHI-defs are
169   /// inserted as required to preserve SSA form.
170   ///
171   /// PhysReg, when set, is used to verify live-in lists on basic blocks.
172   void extend(LiveRange &LR, SlotIndex Use, unsigned PhysReg = 0);
173 
174   /// createDeadDefs - Create a dead def in LI for every def operand of Reg.
175   /// Each instruction defining Reg gets a new VNInfo with a corresponding
176   /// minimal live range.
177   void createDeadDefs(LiveRange &LR, unsigned Reg);
178 
179   /// Extend the live range of @p LR to reach all uses of Reg.
180   ///
181   /// All uses must be jointly dominated by existing liveness.  PHI-defs are
182   /// inserted as needed to preserve SSA form.
extendToUses(LiveRange & LR,unsigned PhysReg)183   void extendToUses(LiveRange &LR, unsigned PhysReg) {
184     extendToUses(LR, PhysReg, ~0u);
185   }
186 
187   /// Calculates liveness for the register specified in live interval @p LI.
188   /// Creates subregister live ranges as needed if subreg liveness tracking is
189   /// enabled.
190   void calculate(LiveInterval &LI, bool TrackSubRegs);
191 
192   /// For live interval \p LI with correct SubRanges construct matching
193   /// information for the main live range. Expects the main live range to not
194   /// have any segments or value numbers.
195   void constructMainRangeFromSubranges(LiveInterval &LI);
196 
197   //===--------------------------------------------------------------------===//
198   // Low-level interface.
199   //===--------------------------------------------------------------------===//
200   //
201   // These functions can be used to compute live ranges where the live-in and
202   // live-out blocks are already known, but the SSA value in each block is
203   // unknown.
204   //
205   // After calling reset(), add known live-out values and known live-in blocks.
206   // Then call calculateValues() to compute the actual value that is
207   // live-in to each block, and add liveness to the live ranges.
208   //
209 
210   /// setLiveOutValue - Indicate that VNI is live out from MBB.  The
211   /// calculateValues() function will not add liveness for MBB, the caller
212   /// should take care of that.
213   ///
214   /// VNI may be null only if MBB is a live-through block also passed to
215   /// addLiveInBlock().
setLiveOutValue(MachineBasicBlock * MBB,VNInfo * VNI)216   void setLiveOutValue(MachineBasicBlock *MBB, VNInfo *VNI) {
217     Seen.set(MBB->getNumber());
218     Map[MBB] = LiveOutPair(VNI, nullptr);
219   }
220 
221   /// addLiveInBlock - Add a block with an unknown live-in value.  This
222   /// function can only be called once per basic block.  Once the live-in value
223   /// has been determined, calculateValues() will add liveness to LI.
224   ///
225   /// @param LR      The live range that is live-in to the block.
226   /// @param DomNode The domtree node for the block.
227   /// @param Kill    Index in block where LI is killed.  If the value is
228   ///                live-through, set Kill = SLotIndex() and also call
229   ///                setLiveOutValue(MBB, 0).
230   void addLiveInBlock(LiveRange &LR,
231                       MachineDomTreeNode *DomNode,
232                       SlotIndex Kill = SlotIndex()) {
233     LiveIn.push_back(LiveInBlock(LR, DomNode, Kill));
234   }
235 
236   /// calculateValues - Calculate the value that will be live-in to each block
237   /// added with addLiveInBlock.  Add PHI-def values as needed to preserve SSA
238   /// form.  Add liveness to all live-in blocks up to the Kill point, or the
239   /// whole block for live-through blocks.
240   ///
241   /// Every predecessor of a live-in block must have been given a value with
242   /// setLiveOutValue, the value may be null for live-trough blocks.
243   void calculateValues();
244 };
245 
246 } // end namespace llvm
247 
248 #endif
249