LiveRangeCalc.cpp revision b5a457c4cbc71db6ae313ef1bf8eadac65767ab0 
1//===---- LiveRangeCalc.cpp - Calculate live ranges -----------------------===//
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// Implementation of the LiveRangeCalc class.
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "regalloc"
15#include "LiveRangeCalc.h"
16#include "llvm/CodeGen/MachineDominators.h"
17
18using namespace llvm;
19
20void LiveRangeCalc::reset(const MachineFunction *MF) {
21  unsigned N = MF->getNumBlockIDs();
22  Seen.clear();
23  Seen.resize(N);
24  LiveOut.resize(N);
25  LiveIn.clear();
26}
27
28
29// Transfer information from the LiveIn vector to the live ranges.
30void LiveRangeCalc::updateLiveIns(VNInfo *OverrideVNI, SlotIndexes *Indexes) {
31  for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(),
32         E = LiveIn.end(); I != E; ++I) {
33    if (!I->DomNode)
34      continue;
35    MachineBasicBlock *MBB = I->DomNode->getBlock();
36
37    VNInfo *VNI = OverrideVNI ? OverrideVNI : I->Value;
38    assert(VNI && "No live-in value found");
39
40    SlotIndex Start, End;
41    tie(Start, End) = Indexes->getMBBRange(MBB);
42
43    if (I->Kill.isValid())
44      I->LI->addRange(LiveRange(Start, I->Kill, VNI));
45    else {
46      I->LI->addRange(LiveRange(Start, End, VNI));
47      // The value is live-through, update LiveOut as well.  Defer the Domtree
48      // lookup until it is needed.
49      assert(Seen.test(MBB->getNumber()));
50      LiveOut[MBB] = LiveOutPair(VNI, 0);
51    }
52  }
53  LiveIn.clear();
54}
55
56
57void LiveRangeCalc::extend(LiveInterval *LI,
58                           SlotIndex Kill,
59                           SlotIndexes *Indexes,
60                           MachineDominatorTree *DomTree,
61                           VNInfo::Allocator *Alloc) {
62  assert(LI && "Missing live range");
63  assert(Kill.isValid() && "Invalid SlotIndex");
64  assert(Indexes && "Missing SlotIndexes");
65  assert(DomTree && "Missing dominator tree");
66  SlotIndex LastUse = Kill.getPrevSlot();
67
68  MachineBasicBlock *KillMBB = Indexes->getMBBFromIndex(LastUse);
69  assert(Kill && "No MBB at Kill");
70
71  // Is there a def in the same MBB we can extend?
72  if (LI->extendInBlock(Indexes->getMBBStartIdx(KillMBB), LastUse))
73    return;
74
75  // Find the single reaching def, or determine if Kill is jointly dominated by
76  // multiple values, and we may need to create even more phi-defs to preserve
77  // VNInfo SSA form.  Perform a search for all predecessor blocks where we
78  // know the dominating VNInfo.
79  VNInfo *VNI = findReachingDefs(LI, KillMBB, Kill, Indexes, DomTree);
80
81  // When there were multiple different values, we may need new PHIs.
82  if (!VNI)
83    updateSSA(Indexes, DomTree, Alloc);
84
85  updateLiveIns(VNI, Indexes);
86}
87
88
89// This function is called by a client after using the low-level API to add
90// live-out and live-in blocks.  The unique value optimization is not
91// available, SplitEditor::transferValues handles that case directly anyway.
92void LiveRangeCalc::calculateValues(SlotIndexes *Indexes,
93                                    MachineDominatorTree *DomTree,
94                                    VNInfo::Allocator *Alloc) {
95  assert(Indexes && "Missing SlotIndexes");
96  assert(DomTree && "Missing dominator tree");
97  updateSSA(Indexes, DomTree, Alloc);
98  updateLiveIns(0, Indexes);
99}
100
101
102VNInfo *LiveRangeCalc::findReachingDefs(LiveInterval *LI,
103                                        MachineBasicBlock *KillMBB,
104                                        SlotIndex Kill,
105                                        SlotIndexes *Indexes,
106                                        MachineDominatorTree *DomTree) {
107  // Blocks where LI should be live-in.
108  SmallVector<MachineBasicBlock*, 16> WorkList(1, KillMBB);
109
110  // Remember if we have seen more than one value.
111  bool UniqueVNI = true;
112  VNInfo *TheVNI = 0;
113
114  // Using Seen as a visited set, perform a BFS for all reaching defs.
115  for (unsigned i = 0; i != WorkList.size(); ++i) {
116    MachineBasicBlock *MBB = WorkList[i];
117    assert(!MBB->pred_empty() && "Value live-in to entry block?");
118    for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
119           PE = MBB->pred_end(); PI != PE; ++PI) {
120       MachineBasicBlock *Pred = *PI;
121
122       // Is this a known live-out block?
123       if (Seen.test(Pred->getNumber())) {
124         if (VNInfo *VNI = LiveOut[Pred].first) {
125           if (TheVNI && TheVNI != VNI)
126             UniqueVNI = false;
127           TheVNI = VNI;
128         }
129         continue;
130       }
131
132       SlotIndex Start, End;
133       tie(Start, End) = Indexes->getMBBRange(Pred);
134
135       // First time we see Pred.  Try to determine the live-out value, but set
136       // it as null if Pred is live-through with an unknown value.
137       VNInfo *VNI = LI->extendInBlock(Start, End.getPrevSlot());
138       setLiveOutValue(Pred, VNI);
139       if (VNI) {
140         if (TheVNI && TheVNI != VNI)
141           UniqueVNI = false;
142         TheVNI = VNI;
143         continue;
144       }
145
146       // No, we need a live-in value for Pred as well
147       if (Pred != KillMBB)
148          WorkList.push_back(Pred);
149       else
150          // Loopback to KillMBB, so value is really live through.
151         Kill = SlotIndex();
152    }
153  }
154
155  // Transfer WorkList to LiveInBlocks in reverse order.
156  // This ordering works best with updateSSA().
157  LiveIn.clear();
158  LiveIn.reserve(WorkList.size());
159  while(!WorkList.empty())
160    addLiveInBlock(LI, DomTree->getNode(WorkList.pop_back_val()));
161
162  // The kill block may not be live-through.
163  assert(LiveIn.back().DomNode->getBlock() == KillMBB);
164  LiveIn.back().Kill = Kill;
165
166  return UniqueVNI ? TheVNI : 0;
167}
168
169
170// This is essentially the same iterative algorithm that SSAUpdater uses,
171// except we already have a dominator tree, so we don't have to recompute it.
172void LiveRangeCalc::updateSSA(SlotIndexes *Indexes,
173                              MachineDominatorTree *DomTree,
174                              VNInfo::Allocator *Alloc) {
175  assert(Indexes && "Missing SlotIndexes");
176  assert(DomTree && "Missing dominator tree");
177
178  // Interate until convergence.
179  unsigned Changes;
180  do {
181    Changes = 0;
182    // Propagate live-out values down the dominator tree, inserting phi-defs
183    // when necessary.
184    for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(),
185           E = LiveIn.end(); I != E; ++I) {
186      MachineDomTreeNode *Node = I->DomNode;
187      // Skip block if the live-in value has already been determined.
188      if (!Node)
189        continue;
190      MachineBasicBlock *MBB = Node->getBlock();
191      MachineDomTreeNode *IDom = Node->getIDom();
192      LiveOutPair IDomValue;
193
194      // We need a live-in value to a block with no immediate dominator?
195      // This is probably an unreachable block that has survived somehow.
196      bool needPHI = !IDom || !Seen.test(IDom->getBlock()->getNumber());
197
198      // IDom dominates all of our predecessors, but it may not be their
199      // immediate dominator. Check if any of them have live-out values that are
200      // properly dominated by IDom. If so, we need a phi-def here.
201      if (!needPHI) {
202        IDomValue = LiveOut[IDom->getBlock()];
203
204        // Cache the DomTree node that defined the value.
205        if (IDomValue.first && !IDomValue.second)
206          LiveOut[IDom->getBlock()].second = IDomValue.second =
207            DomTree->getNode(Indexes->getMBBFromIndex(IDomValue.first->def));
208
209        for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
210               PE = MBB->pred_end(); PI != PE; ++PI) {
211          LiveOutPair &Value = LiveOut[*PI];
212          if (!Value.first || Value.first == IDomValue.first)
213            continue;
214
215          // Cache the DomTree node that defined the value.
216          if (!Value.second)
217            Value.second =
218              DomTree->getNode(Indexes->getMBBFromIndex(Value.first->def));
219
220          // This predecessor is carrying something other than IDomValue.
221          // It could be because IDomValue hasn't propagated yet, or it could be
222          // because MBB is in the dominance frontier of that value.
223          if (DomTree->dominates(IDom, Value.second)) {
224            needPHI = true;
225            break;
226          }
227        }
228      }
229
230      // The value may be live-through even if Kill is set, as can happen when
231      // we are called from extendRange. In that case LiveOutSeen is true, and
232      // LiveOut indicates a foreign or missing value.
233      LiveOutPair &LOP = LiveOut[MBB];
234
235      // Create a phi-def if required.
236      if (needPHI) {
237        ++Changes;
238        assert(Alloc && "Need VNInfo allocator to create PHI-defs");
239        SlotIndex Start, End;
240        tie(Start, End) = Indexes->getMBBRange(MBB);
241        VNInfo *VNI = I->LI->getNextValue(Start, 0, *Alloc);
242        VNI->setIsPHIDef(true);
243        I->Value = VNI;
244        // This block is done, we know the final value.
245        I->DomNode = 0;
246
247        // Add liveness since updateLiveIns now skips this node.
248        if (I->Kill.isValid())
249          I->LI->addRange(LiveRange(Start, I->Kill, VNI));
250        else {
251          I->LI->addRange(LiveRange(Start, End, VNI));
252          LOP = LiveOutPair(VNI, Node);
253        }
254      } else if (IDomValue.first) {
255        // No phi-def here. Remember incoming value.
256        I->Value = IDomValue.first;
257
258        // If the IDomValue is killed in the block, don't propagate through.
259        if (I->Kill.isValid())
260          continue;
261
262        // Propagate IDomValue if it isn't killed:
263        // MBB is live-out and doesn't define its own value.
264        if (LOP.first == IDomValue.first)
265          continue;
266        ++Changes;
267        LOP = IDomValue;
268      }
269    }
270  } while (Changes);
271}
272