/3rd_party/llvm/lib/CodeGen/LiveRangeCalc.cpp

  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#include "llvm/CodeGen/MachineRegisterInfo.h"
 18
 19using namespace llvm;
 20
 21void LiveRangeCalc::reset(const MachineFunction *mf,
 22                          SlotIndexes *SI,
 23                          MachineDominatorTree *MDT,
 24                          VNInfo::Allocator *VNIA) {
 25  MF = mf;
 26  MRI = &MF->getRegInfo();
 27  Indexes = SI;
 28  DomTree = MDT;
 29  Alloc = VNIA;
 30
 31  unsigned N = MF->getNumBlockIDs();
 32  Seen.clear();
 33  Seen.resize(N);
 34  LiveOut.resize(N);
 35  LiveIn.clear();
 36}
 37
 38
 39void LiveRangeCalc::createDeadDefs(LiveRange &LR, unsigned Reg) {
 40  assert(MRI && Indexes && "call reset() first");
 41
 42  // Visit all def operands. If the same instruction has multiple defs of Reg,
 43  // LR.createDeadDef() will deduplicate.
 44  for (MachineRegisterInfo::def_iterator
 45       I = MRI->def_begin(Reg), E = MRI->def_end(); I != E; ++I) {
 46    const MachineInstr *MI = &*I;
 47    // Find the corresponding slot index.
 48    SlotIndex Idx;
 49    if (MI->isPHI())
 50      // PHI defs begin at the basic block start index.
 51      Idx = Indexes->getMBBStartIdx(MI->getParent());
 52    else
 53      // Instructions are either normal 'r', or early clobber 'e'.
 54      Idx = Indexes->getInstructionIndex(MI)
 55        .getRegSlot(I.getOperand().isEarlyClobber());
 56
 57    // Create the def in LR. This may find an existing def.
 58    LR.createDeadDef(Idx, *Alloc);
 59  }
 60}
 61
 62
 63void LiveRangeCalc::extendToUses(LiveRange &LR, unsigned Reg) {
 64  assert(MRI && Indexes && "call reset() first");
 65
 66  // Visit all operands that read Reg. This may include partial defs.
 67  for (MachineRegisterInfo::reg_nodbg_iterator I = MRI->reg_nodbg_begin(Reg),
 68       E = MRI->reg_nodbg_end(); I != E; ++I) {
 69    MachineOperand &MO = I.getOperand();
 70    // Clear all kill flags. They will be reinserted after register allocation
 71    // by LiveIntervalAnalysis::addKillFlags().
 72    if (MO.isUse())
 73      MO.setIsKill(false);
 74    if (!MO.readsReg())
 75      continue;
 76    // MI is reading Reg. We may have visited MI before if it happens to be
 77    // reading Reg multiple times. That is OK, extend() is idempotent.
 78    const MachineInstr *MI = &*I;
 79
 80    // Find the SlotIndex being read.
 81    SlotIndex Idx;
 82    if (MI->isPHI()) {
 83      assert(!MO.isDef() && "Cannot handle PHI def of partial register.");
 84      // PHI operands are paired: (Reg, PredMBB).
 85      // Extend the live range to be live-out from PredMBB.
 86      Idx = Indexes->getMBBEndIdx(MI->getOperand(I.getOperandNo()+1).getMBB());
 87    } else {
 88      // This is a normal instruction.
 89      Idx = Indexes->getInstructionIndex(MI).getRegSlot();
 90      // Check for early-clobber redefs.
 91      unsigned DefIdx;
 92      if (MO.isDef()) {
 93        if (MO.isEarlyClobber())
 94          Idx = Idx.getRegSlot(true);
 95      } else if (MI->isRegTiedToDefOperand(I.getOperandNo(), &DefIdx)) {
 96        // FIXME: This would be a lot easier if tied early-clobber uses also
 97        // had an early-clobber flag.
 98        if (MI->getOperand(DefIdx).isEarlyClobber())
 99          Idx = Idx.getRegSlot(true);
100      }
101    }
102    extend(LR, Idx, Reg);
103  }
104}
105
106
107// Transfer information from the LiveIn vector to the live ranges.
108void LiveRangeCalc::updateLiveIns() {
109  LiveRangeUpdater Updater;
110  for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(),
111         E = LiveIn.end(); I != E; ++I) {
112    if (!I->DomNode)
113      continue;
114    MachineBasicBlock *MBB = I->DomNode->getBlock();
115    assert(I->Value && "No live-in value found");
116    SlotIndex Start, End;
117    tie(Start, End) = Indexes->getMBBRange(MBB);
118
119    if (I->Kill.isValid())
120      // Value is killed inside this block.
121      End = I->Kill;
122    else {
123      // The value is live-through, update LiveOut as well.
124      // Defer the Domtree lookup until it is needed.
125      assert(Seen.test(MBB->getNumber()));
126      LiveOut[MBB] = LiveOutPair(I->Value, (MachineDomTreeNode *)0);
127    }
128    Updater.setDest(&I->LR);
129    Updater.add(Start, End, I->Value);
130  }
131  LiveIn.clear();
132}
133
134
135void LiveRangeCalc::extend(LiveRange &LR, SlotIndex Kill, unsigned PhysReg) {
136  assert(Kill.isValid() && "Invalid SlotIndex");
137  assert(Indexes && "Missing SlotIndexes");
138  assert(DomTree && "Missing dominator tree");
139
140  MachineBasicBlock *KillMBB = Indexes->getMBBFromIndex(Kill.getPrevSlot());
141  assert(KillMBB && "No MBB at Kill");
142
143  // Is there a def in the same MBB we can extend?
144  if (LR.extendInBlock(Indexes->getMBBStartIdx(KillMBB), Kill))
145    return;
146
147  // Find the single reaching def, or determine if Kill is jointly dominated by
148  // multiple values, and we may need to create even more phi-defs to preserve
149  // VNInfo SSA form.  Perform a search for all predecessor blocks where we
150  // know the dominating VNInfo.
151  if (findReachingDefs(LR, *KillMBB, Kill, PhysReg))
152    return;
153
154  // When there were multiple different values, we may need new PHIs.
155  calculateValues();
156}
157
158
159// This function is called by a client after using the low-level API to add
160// live-out and live-in blocks.  The unique value optimization is not
161// available, SplitEditor::transferValues handles that case directly anyway.
162void LiveRangeCalc::calculateValues() {
163  assert(Indexes && "Missing SlotIndexes");
164  assert(DomTree && "Missing dominator tree");
165  updateSSA();
166  updateLiveIns();
167}
168
169
170bool LiveRangeCalc::findReachingDefs(LiveRange &LR, MachineBasicBlock &KillMBB,
171                                     SlotIndex Kill, unsigned PhysReg) {
172  unsigned KillMBBNum = KillMBB.getNumber();
173
174  // Block numbers where LR should be live-in.
175  SmallVector<unsigned, 16> WorkList(1, KillMBBNum);
176
177  // Remember if we have seen more than one value.
178  bool UniqueVNI = true;
179  VNInfo *TheVNI = 0;
180
181  // Using Seen as a visited set, perform a BFS for all reaching defs.
182  for (unsigned i = 0; i != WorkList.size(); ++i) {
183    MachineBasicBlock *MBB = MF->getBlockNumbered(WorkList[i]);
184
185#ifndef NDEBUG
186    if (MBB->pred_empty()) {
187      MBB->getParent()->verify();
188      llvm_unreachable("Use not jointly dominated by defs.");
189    }
190
191    if (TargetRegisterInfo::isPhysicalRegister(PhysReg) &&
192        !MBB->isLiveIn(PhysReg)) {
193      MBB->getParent()->verify();
194      errs() << "The register needs to be live in to BB#" << MBB->getNumber()
195             << ", but is missing from the live-in list.\n";
196      llvm_unreachable("Invalid global physical register");
197    }
198#endif
199
200    for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
201         PE = MBB->pred_end(); PI != PE; ++PI) {
202       MachineBasicBlock *Pred = *PI;
203
204       // Is this a known live-out block?
205       if (Seen.test(Pred->getNumber())) {
206         if (VNInfo *VNI = LiveOut[Pred].first) {
207           if (TheVNI && TheVNI != VNI)
208             UniqueVNI = false;
209           TheVNI = VNI;
210         }
211         continue;
212       }
213
214       SlotIndex Start, End;
215       tie(Start, End) = Indexes->getMBBRange(Pred);
216
217       // First time we see Pred.  Try to determine the live-out value, but set
218       // it as null if Pred is live-through with an unknown value.
219       VNInfo *VNI = LR.extendInBlock(Start, End);
220       setLiveOutValue(Pred, VNI);
221       if (VNI) {
222         if (TheVNI && TheVNI != VNI)
223           UniqueVNI = false;
224         TheVNI = VNI;
225         continue;
226       }
227
228       // No, we need a live-in value for Pred as well
229       if (Pred != &KillMBB)
230          WorkList.push_back(Pred->getNumber());
231       else
232          // Loopback to KillMBB, so value is really live through.
233         Kill = SlotIndex();
234    }
235  }
236
237  LiveIn.clear();
238
239  // Both updateSSA() and LiveRangeUpdater benefit from ordered blocks, but
240  // neither require it. Skip the sorting overhead for small updates.
241  if (WorkList.size() > 4)
242    array_pod_sort(WorkList.begin(), WorkList.end());
243
244  // If a unique reaching def was found, blit in the live ranges immediately.
245  if (UniqueVNI) {
246    LiveRangeUpdater Updater(&LR);
247    for (SmallVectorImpl<unsigned>::const_iterator I = WorkList.begin(),
248         E = WorkList.end(); I != E; ++I) {
249       SlotIndex Start, End;
250       tie(Start, End) = Indexes->getMBBRange(*I);
251       // Trim the live range in KillMBB.
252       if (*I == KillMBBNum && Kill.isValid())
253         End = Kill;
254       else
255         LiveOut[MF->getBlockNumbered(*I)] =
256           LiveOutPair(TheVNI, (MachineDomTreeNode *)0);
257       Updater.add(Start, End, TheVNI);
258    }
259    return true;
260  }
261
262  // Multiple values were found, so transfer the work list to the LiveIn array
263  // where UpdateSSA will use it as a work list.
264  LiveIn.reserve(WorkList.size());
265  for (SmallVectorImpl<unsigned>::const_iterator
266       I = WorkList.begin(), E = WorkList.end(); I != E; ++I) {
267    MachineBasicBlock *MBB = MF->getBlockNumbered(*I);
268    addLiveInBlock(LR, DomTree->getNode(MBB));
269    if (MBB == &KillMBB)
270      LiveIn.back().Kill = Kill;
271  }
272
273  return false;
274}
275
276
277// This is essentially the same iterative algorithm that SSAUpdater uses,
278// except we already have a dominator tree, so we don't have to recompute it.
279void LiveRangeCalc::updateSSA() {
280  assert(Indexes && "Missing SlotIndexes");
281  assert(DomTree && "Missing dominator tree");
282
283  // Interate until convergence.
284  unsigned Changes;
285  do {
286    Changes = 0;
287    // Propagate live-out values down the dominator tree, inserting phi-defs
288    // when necessary.
289    for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(),
290           E = LiveIn.end(); I != E; ++I) {
291      MachineDomTreeNode *Node = I->DomNode;
292      // Skip block if the live-in value has already been determined.
293      if (!Node)
294        continue;
295      MachineBasicBlock *MBB = Node->getBlock();
296      MachineDomTreeNode *IDom = Node->getIDom();
297      LiveOutPair IDomValue;
298
299      // We need a live-in value to a block with no immediate dominator?
300      // This is probably an unreachable block that has survived somehow.
301      bool needPHI = !IDom || !Seen.test(IDom->getBlock()->getNumber());
302
303      // IDom dominates all of our predecessors, but it may not be their
304      // immediate dominator. Check if any of them have live-out values that are
305      // properly dominated by IDom. If so, we need a phi-def here.
306      if (!needPHI) {
307        IDomValue = LiveOut[IDom->getBlock()];
308
309        // Cache the DomTree node that defined the value.
310        if (IDomValue.first && !IDomValue.second)
311          LiveOut[IDom->getBlock()].second = IDomValue.second =
312            DomTree->getNode(Indexes->getMBBFromIndex(IDomValue.first->def));
313
314        for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
315               PE = MBB->pred_end(); PI != PE; ++PI) {
316          LiveOutPair &Value = LiveOut[*PI];
317          if (!Value.first || Value.first == IDomValue.first)
318            continue;
319
320          // Cache the DomTree node that defined the value.
321          if (!Value.second)
322            Value.second =
323              DomTree->getNode(Indexes->getMBBFromIndex(Value.first->def));
324
325          // This predecessor is carrying something other than IDomValue.
326          // It could be because IDomValue hasn't propagated yet, or it could be
327          // because MBB is in the dominance frontier of that value.
328          if (DomTree->dominates(IDom, Value.second)) {
329            needPHI = true;
330            break;
331          }
332        }
333      }
334
335      // The value may be live-through even if Kill is set, as can happen when
336      // we are called from extendRange. In that case LiveOutSeen is true, and
337      // LiveOut indicates a foreign or missing value.
338      LiveOutPair &LOP = LiveOut[MBB];
339
340      // Create a phi-def if required.
341      if (needPHI) {
342        ++Changes;
343        assert(Alloc && "Need VNInfo allocator to create PHI-defs");
344        SlotIndex Start, End;
345        tie(Start, End) = Indexes->getMBBRange(MBB);
346        LiveRange &LR = I->LR;
347        VNInfo *VNI = LR.getNextValue(Start, *Alloc);
348        I->Value = VNI;
349        // This block is done, we know the final value.
350        I->DomNode = 0;
351
352        // Add liveness since updateLiveIns now skips this node.
353        if (I->Kill.isValid())
354          LR.addSegment(LiveInterval::Segment(Start, I->Kill, VNI));
355        else {
356          LR.addSegment(LiveInterval::Segment(Start, End, VNI));
357          LOP = LiveOutPair(VNI, Node);
358        }
359      } else if (IDomValue.first) {
360        // No phi-def here. Remember incoming value.
361        I->Value = IDomValue.first;
362
363        // If the IDomValue is killed in the block, don't propagate through.
364        if (I->Kill.isValid())
365          continue;
366
367        // Propagate IDomValue if it isn't killed:
368        // MBB is live-out and doesn't define its own value.
369        if (LOP.first == IDomValue.first)
370          continue;
371        ++Changes;
372        LOP = IDomValue;
373      }
374    }
375  } while (Changes);
376}