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/CodeGen/Passes.h"
17 #include "llvm/ADT/SmallBitVector.h"
18
19 using namespace llvm;
20
21 namespace llvm {
22 template class DomTreeNodeBase<MachineBasicBlock>;
23 template class DominatorTreeBase<MachineBasicBlock>;
24 }
25
26 char MachineDominatorTree::ID = 0;
27
28 INITIALIZE_PASS(MachineDominatorTree, "machinedomtree",
29 "MachineDominator Tree Construction", true, true)
30
31 char &llvm::MachineDominatorsID = MachineDominatorTree::ID;
32
getAnalysisUsage(AnalysisUsage & AU) const33 void MachineDominatorTree::getAnalysisUsage(AnalysisUsage &AU) const {
34 AU.setPreservesAll();
35 MachineFunctionPass::getAnalysisUsage(AU);
36 }
37
runOnMachineFunction(MachineFunction & F)38 bool MachineDominatorTree::runOnMachineFunction(MachineFunction &F) {
39 CriticalEdgesToSplit.clear();
40 NewBBs.clear();
41 DT->recalculate(F);
42
43 return false;
44 }
45
MachineDominatorTree()46 MachineDominatorTree::MachineDominatorTree()
47 : MachineFunctionPass(ID) {
48 initializeMachineDominatorTreePass(*PassRegistry::getPassRegistry());
49 DT = new DominatorTreeBase<MachineBasicBlock>(false);
50 }
51
~MachineDominatorTree()52 MachineDominatorTree::~MachineDominatorTree() {
53 delete DT;
54 }
55
releaseMemory()56 void MachineDominatorTree::releaseMemory() {
57 DT->releaseMemory();
58 }
59
print(raw_ostream & OS,const Module *) const60 void MachineDominatorTree::print(raw_ostream &OS, const Module*) const {
61 DT->print(OS);
62 }
63
applySplitCriticalEdges() const64 void MachineDominatorTree::applySplitCriticalEdges() const {
65 // Bail out early if there is nothing to do.
66 if (CriticalEdgesToSplit.empty())
67 return;
68
69 // For each element in CriticalEdgesToSplit, remember whether or not element
70 // is the new immediate domminator of its successor. The mapping is done by
71 // index, i.e., the information for the ith element of CriticalEdgesToSplit is
72 // the ith element of IsNewIDom.
73 SmallBitVector IsNewIDom(CriticalEdgesToSplit.size(), true);
74 size_t Idx = 0;
75
76 // Collect all the dominance properties info, before invalidating
77 // the underlying DT.
78 for (CriticalEdge &Edge : CriticalEdgesToSplit) {
79 // Update dominator information.
80 MachineBasicBlock *Succ = Edge.ToBB;
81 MachineDomTreeNode *SuccDTNode = DT->getNode(Succ);
82
83 for (MachineBasicBlock *PredBB : Succ->predecessors()) {
84 if (PredBB == Edge.NewBB)
85 continue;
86 // If we are in this situation:
87 // FromBB1 FromBB2
88 // + +
89 // + + + +
90 // + + + +
91 // ... Split1 Split2 ...
92 // + +
93 // + +
94 // +
95 // Succ
96 // Instead of checking the domiance property with Split2, we check it with
97 // FromBB2 since Split2 is still unknown of the underlying DT structure.
98 if (NewBBs.count(PredBB)) {
99 assert(PredBB->pred_size() == 1 && "A basic block resulting from a "
100 "critical edge split has more "
101 "than one predecessor!");
102 PredBB = *PredBB->pred_begin();
103 }
104 if (!DT->dominates(SuccDTNode, DT->getNode(PredBB))) {
105 IsNewIDom[Idx] = false;
106 break;
107 }
108 }
109 ++Idx;
110 }
111
112 // Now, update DT with the collected dominance properties info.
113 Idx = 0;
114 for (CriticalEdge &Edge : CriticalEdgesToSplit) {
115 // We know FromBB dominates NewBB.
116 MachineDomTreeNode *NewDTNode = DT->addNewBlock(Edge.NewBB, Edge.FromBB);
117
118 // If all the other predecessors of "Succ" are dominated by "Succ" itself
119 // then the new block is the new immediate dominator of "Succ". Otherwise,
120 // the new block doesn't dominate anything.
121 if (IsNewIDom[Idx])
122 DT->changeImmediateDominator(DT->getNode(Edge.ToBB), NewDTNode);
123 ++Idx;
124 }
125 NewBBs.clear();
126 CriticalEdgesToSplit.clear();
127 }
128