lib/CodeGen/MachineDominators.cpp

   1 //===- MachineDominators.cpp - Machine Dominator Calculation --------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements simple dominator construction algorithms for finding
  10 // forward dominators on machine functions.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/CodeGen/MachineDominators.h"
  15 #include "llvm/ADT/SmallBitVector.h"
  16 #include "llvm/CodeGen/Passes.h"
  17 #include "llvm/Support/CommandLine.h"
  18
  19 using namespace llvm;
  20
  21 namespace llvm {
  22 // Always verify dominfo if expensive checking is enabled.
  23 #ifdef EXPENSIVE_CHECKS
  24 bool VerifyMachineDomInfo = true;
  25 #else
  26 bool VerifyMachineDomInfo = false;
  27 #endif
  28 } // namespace llvm
  29
  30 static cl::opt<bool, true> VerifyMachineDomInfoX(
  31     "verify-machine-dom-info", cl::location(VerifyMachineDomInfo), cl::Hidden,
  32     cl::desc("Verify machine dominator info (time consuming)"));
  33
  34 namespace llvm {
  35 template class DomTreeNodeBase<MachineBasicBlock>;
  36 template class DominatorTreeBase<MachineBasicBlock, false>; // DomTreeBase
  37 }
  38
  39 char MachineDominatorTree::ID = 0;
  40
  41 INITIALIZE_PASS(MachineDominatorTree, "machinedomtree",
  42                 "MachineDominator Tree Construction", true, true)
  43
  44 char &llvm::MachineDominatorsID = MachineDominatorTree::ID;
  45
  46 void MachineDominatorTree::getAnalysisUsage(AnalysisUsage &AU) const {
  47   AU.setPreservesAll();
  48   MachineFunctionPass::getAnalysisUsage(AU);
  49 }
  50
  51 bool MachineDominatorTree::runOnMachineFunction(MachineFunction &F) {
  52   CriticalEdgesToSplit.clear();
  53   NewBBs.clear();
  54   DT.reset(new DomTreeBase<MachineBasicBlock>());
  55   DT->recalculate(F);
  56   return false;
  57 }
  58
  59 MachineDominatorTree::MachineDominatorTree()
  60     : MachineFunctionPass(ID) {
  61   initializeMachineDominatorTreePass(*PassRegistry::getPassRegistry());
  62 }
  63
  64 void MachineDominatorTree::releaseMemory() {
  65   CriticalEdgesToSplit.clear();
  66   DT.reset(nullptr);
  67 }
  68
  69 void MachineDominatorTree::verifyAnalysis() const {
  70   if (DT && VerifyMachineDomInfo)
  71     if (!DT->verify(DomTreeT::VerificationLevel::Basic)) {
  72       errs() << "MachineDominatorTree verification failed\n";
  73       abort();
  74     }
  75 }
  76
  77 void MachineDominatorTree::print(raw_ostream &OS, const Module*) const {
  78   if (DT)
  79     DT->print(OS);
  80 }
  81
  82 void MachineDominatorTree::applySplitCriticalEdges() const {
  83   // Bail out early if there is nothing to do.
  84   if (CriticalEdgesToSplit.empty())
  85     return;
  86
  87   // For each element in CriticalEdgesToSplit, remember whether or not element
  88   // is the new immediate domminator of its successor. The mapping is done by
  89   // index, i.e., the information for the ith element of CriticalEdgesToSplit is
  90   // the ith element of IsNewIDom.
  91   SmallBitVector IsNewIDom(CriticalEdgesToSplit.size(), true);
  92   size_t Idx = 0;
  93
  94   // Collect all the dominance properties info, before invalidating
  95   // the underlying DT.
  96   for (CriticalEdge &Edge : CriticalEdgesToSplit) {
  97     // Update dominator information.
  98     MachineBasicBlock *Succ = Edge.ToBB;
  99     MachineDomTreeNode *SuccDTNode = DT->getNode(Succ);
 100
 101     for (MachineBasicBlock *PredBB : Succ->predecessors()) {
 102       if (PredBB == Edge.NewBB)
 103         continue;
 104       // If we are in this situation:
 105       // FromBB1        FromBB2
 106       //    +              +
 107       //   + +            + +
 108       //  +   +          +   +
 109       // ...  Split1  Split2 ...
 110       //           +   +
 111       //            + +
 112       //             +
 113       //            Succ
 114       // Instead of checking the domiance property with Split2, we check it with
 115       // FromBB2 since Split2 is still unknown of the underlying DT structure.
 116       if (NewBBs.count(PredBB)) {
 117         assert(PredBB->pred_size() == 1 && "A basic block resulting from a "
 118                                            "critical edge split has more "
 119                                            "than one predecessor!");
 120         PredBB = *PredBB->pred_begin();
 121       }
 122       if (!DT->dominates(SuccDTNode, DT->getNode(PredBB))) {
 123         IsNewIDom[Idx] = false;
 124         break;
 125       }
 126     }
 127     ++Idx;
 128   }
 129
 130   // Now, update DT with the collected dominance properties info.
 131   Idx = 0;
 132   for (CriticalEdge &Edge : CriticalEdgesToSplit) {
 133     // We know FromBB dominates NewBB.
 134     MachineDomTreeNode *NewDTNode = DT->addNewBlock(Edge.NewBB, Edge.FromBB);
 135
 136     // If all the other predecessors of "Succ" are dominated by "Succ" itself
 137     // then the new block is the new immediate dominator of "Succ". Otherwise,
 138     // the new block doesn't dominate anything.
 139     if (IsNewIDom[Idx])
 140       DT->changeImmediateDominator(DT->getNode(Edge.ToBB), NewDTNode);
 141     ++Idx;
 142   }
 143   NewBBs.clear();
 144   CriticalEdgesToSplit.clear();
 145 }