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