lib/CodeGen/MachineDominators.cpp

   1 //===- MachineDominators.cpp - Machine Dominator Calculation --------------===//
   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 // This file implements simple dominator construction algorithms for finding
  11 // forward dominators on machine functions.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "llvm/CodeGen/MachineDominators.h"
  16 #include "llvm/ADT/SmallBitVector.h"
  17 #include "llvm/CodeGen/Passes.h"
  18 #include "llvm/Support/CommandLine.h"
  19
  20 using namespace llvm;
  21
  22 // Always verify dominfo if expensive checking is enabled.
  23 #ifdef EXPENSIVE_CHECKS
  24 static bool VerifyMachineDomInfo = true;
  25 #else
  26 static bool VerifyMachineDomInfo = false;
  27 #endif
  28 static cl::opt<bool, true> VerifyMachineDomInfoX(
  29     "verify-machine-dom-info", cl::location(VerifyMachineDomInfo), cl::Hidden,
  30     cl::desc("Verify machine dominator info (time consuming)"));
  31
  32 namespace llvm {
  33 template class DomTreeNodeBase<MachineBasicBlock>;
  34 template class DominatorTreeBase<MachineBasicBlock, false>; // DomTreeBase
  35 }
  36
  37 char MachineDominatorTree::ID = 0;
  38
  39 INITIALIZE_PASS(MachineDominatorTree, "machinedomtree",
  40                 "MachineDominator Tree Construction", true, true)
  41
  42 char &llvm::MachineDominatorsID = MachineDominatorTree::ID;
  43
  44 void MachineDominatorTree::getAnalysisUsage(AnalysisUsage &AU) const {
  45   AU.setPreservesAll();
  46   MachineFunctionPass::getAnalysisUsage(AU);
  47 }
  48
  49 bool MachineDominatorTree::runOnMachineFunction(MachineFunction &F) {
  50   CriticalEdgesToSplit.clear();
  51   NewBBs.clear();
  52   DT.reset(new DomTreeBase<MachineBasicBlock>());
  53   DT->recalculate(F);
  54   return false;
  55 }
  56
  57 MachineDominatorTree::MachineDominatorTree()
  58     : MachineFunctionPass(ID) {
  59   initializeMachineDominatorTreePass(*PassRegistry::getPassRegistry());
  60 }
  61
  62 void MachineDominatorTree::releaseMemory() {
  63   CriticalEdgesToSplit.clear();
  64   DT.reset(nullptr);
  65 }
  66
  67 void MachineDominatorTree::verifyAnalysis() const {
  68   if (DT && VerifyMachineDomInfo) {
  69     MachineFunction &F = *getRoot()->getParent();
  70
  71     DomTreeBase<MachineBasicBlock> OtherDT;
  72     OtherDT.recalculate(F);
  73     if (getRootNode()->getBlock() != OtherDT.getRootNode()->getBlock() ||
  74         DT->compare(OtherDT)) {
  75       errs() << "MachineDominatorTree for function " << F.getName()
  76             << " is not up to date!\nComputed:\n";
  77       DT->print(errs());
  78       errs() << "\nActual:\n";
  79       OtherDT.print(errs());
  80       abort();
  81     }
  82   }
  83 }
  84
  85 void MachineDominatorTree::print(raw_ostream &OS, const Module*) const {
  86   if (DT)
  87     DT->print(OS);
  88 }
  89
  90 void MachineDominatorTree::applySplitCriticalEdges() const {
  91   // Bail out early if there is nothing to do.
  92   if (CriticalEdgesToSplit.empty())
  93     return;
  94
  95   // For each element in CriticalEdgesToSplit, remember whether or not element
  96   // is the new immediate domminator of its successor. The mapping is done by
  97   // index, i.e., the information for the ith element of CriticalEdgesToSplit is
  98   // the ith element of IsNewIDom.
  99   SmallBitVector IsNewIDom(CriticalEdgesToSplit.size(), true);
 100   size_t Idx = 0;
 101
 102   // Collect all the dominance properties info, before invalidating
 103   // the underlying DT.
 104   for (CriticalEdge &Edge : CriticalEdgesToSplit) {
 105     // Update dominator information.
 106     MachineBasicBlock *Succ = Edge.ToBB;
 107     MachineDomTreeNode *SuccDTNode = DT->getNode(Succ);
 108
 109     for (MachineBasicBlock *PredBB : Succ->predecessors()) {
 110       if (PredBB == Edge.NewBB)
 111         continue;
 112       // If we are in this situation:
 113       // FromBB1        FromBB2
 114       //    +              +
 115       //   + +            + +
 116       //  +   +          +   +
 117       // ...  Split1  Split2 ...
 118       //           +   +
 119       //            + +
 120       //             +
 121       //            Succ
 122       // Instead of checking the domiance property with Split2, we check it with
 123       // FromBB2 since Split2 is still unknown of the underlying DT structure.
 124       if (NewBBs.count(PredBB)) {
 125         assert(PredBB->pred_size() == 1 && "A basic block resulting from a "
 126                                            "critical edge split has more "
 127                                            "than one predecessor!");
 128         PredBB = *PredBB->pred_begin();
 129       }
 130       if (!DT->dominates(SuccDTNode, DT->getNode(PredBB))) {
 131         IsNewIDom[Idx] = false;
 132         break;
 133       }
 134     }
 135     ++Idx;
 136   }
 137
 138   // Now, update DT with the collected dominance properties info.
 139   Idx = 0;
 140   for (CriticalEdge &Edge : CriticalEdgesToSplit) {
 141     // We know FromBB dominates NewBB.
 142     MachineDomTreeNode *NewDTNode = DT->addNewBlock(Edge.NewBB, Edge.FromBB);
 143
 144     // If all the other predecessors of "Succ" are dominated by "Succ" itself
 145     // then the new block is the new immediate dominator of "Succ". Otherwise,
 146     // the new block doesn't dominate anything.
 147     if (IsNewIDom[Idx])
 148       DT->changeImmediateDominator(DT->getNode(Edge.ToBB), NewDTNode);
 149     ++Idx;
 150   }
 151   NewBBs.clear();
 152   CriticalEdgesToSplit.clear();
 153 }