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
getAnalysisUsage(AnalysisUsage & AU) const49 void MachineDominatorTree::getAnalysisUsage(AnalysisUsage &AU) const {
50 AU.setPreservesAll();
51 MachineFunctionPass::getAnalysisUsage(AU);
52 }
53
runOnMachineFunction(MachineFunction & F)54 bool MachineDominatorTree::runOnMachineFunction(MachineFunction &F) {
55 calculate(F);
56 return false;
57 }
58
calculate(MachineFunction & F)59 void MachineDominatorTree::calculate(MachineFunction &F) {
60 CriticalEdgesToSplit.clear();
61 NewBBs.clear();
62 DT.reset(new DomTreeBase<MachineBasicBlock>());
63 DT->recalculate(F);
64 }
65
MachineDominatorTree()66 MachineDominatorTree::MachineDominatorTree()
67 : MachineFunctionPass(ID) {
68 initializeMachineDominatorTreePass(*PassRegistry::getPassRegistry());
69 }
70
releaseMemory()71 void MachineDominatorTree::releaseMemory() {
72 CriticalEdgesToSplit.clear();
73 DT.reset(nullptr);
74 }
75
verifyAnalysis() const76 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
print(raw_ostream & OS,const Module *) const84 void MachineDominatorTree::print(raw_ostream &OS, const Module*) const {
85 if (DT)
86 DT->print(OS);
87 }
88
applySplitCriticalEdges() const89 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 }
153