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      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 
     18 using namespace llvm;
     19 
     20 void 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.
     30 void 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 
     57 void 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.
     91 void 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 
    101 VNInfo *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.
    171 void 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