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