1 //===- PostDominators.cpp - Post-Dominator Calculation --------------------===//
3 // This file implements the post-dominator construction algorithms.
5 //===----------------------------------------------------------------------===//
7 #include "llvm/Analysis/PostDominators.h"
8 #include "llvm/iTerminators.h"
9 #include "llvm/Support/CFG.h"
10 #include "Support/DepthFirstIterator.h"
11 #include "Support/SetOperations.h"
13 //===----------------------------------------------------------------------===//
14 // PostDominatorSet Implementation
15 //===----------------------------------------------------------------------===//
17 static RegisterAnalysis<PostDominatorSet>
18 B("postdomset", "Post-Dominator Set Construction", true);
20 // Postdominator set construction. This converts the specified function to only
21 // have a single exit node (return stmt), then calculates the post dominance
22 // sets for the function.
24 bool PostDominatorSet::runOnFunction(Function &F) {
25 Doms.clear(); // Reset from the last time we were run...
27 // Scan the function looking for the root nodes of the post-dominance
28 // relationships. These blocks end with return and unwind instructions.
29 // While we are iterating over the function, we also initialize all of the
32 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
33 Doms[I]; // Initialize to empty
35 if (isa<ReturnInst>(I->getTerminator()) ||
36 isa<UnwindInst>(I->getTerminator()))
40 // If there are no exit nodes for the function, postdomsets are all empty.
41 // This can happen if the function just contains an infinite loop, for
43 if (Roots.empty()) return false;
45 // If we have more than one root, we insert an artificial "null" exit, which
46 // has "virtual edges" to each of the real exit nodes.
54 std::set<const BasicBlock*> Visited;
55 DomSetType WorkingSet;
57 for (unsigned i = 0, e = Roots.size(); i != e; ++i)
58 for (idf_iterator<BasicBlock*> It = idf_begin(Roots[i]),
59 E = idf_end(Roots[i]); It != E; ++It) {
61 succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
62 if (SI != SE) { // Is there SOME successor?
63 // Loop until we get to a successor that has had it's dom set filled
64 // in at least once. We are guaranteed to have this because we are
65 // traversing the graph in DFO and have handled start nodes specially.
67 while (Doms[*SI].size() == 0) ++SI;
68 WorkingSet = Doms[*SI];
70 for (++SI; SI != SE; ++SI) { // Intersect all of the successor sets
71 DomSetType &SuccSet = Doms[*SI];
73 set_intersect(WorkingSet, SuccSet);
76 // If this node has no successors, it must be one of the root nodes.
77 // We will already take care of the notion that the node
78 // post-dominates itself. The only thing we have to add is that if
79 // there are multiple root nodes, we want to insert a special "null"
80 // exit node which dominates the roots as well.
85 WorkingSet.insert(BB); // A block always dominates itself
86 DomSetType &BBSet = Doms[BB];
87 if (BBSet != WorkingSet) {
88 BBSet.swap(WorkingSet); // Constant time operation!
89 Changed = true; // The sets changed.
91 WorkingSet.clear(); // Clear out the set for next iteration
97 //===----------------------------------------------------------------------===//
98 // ImmediatePostDominators Implementation
99 //===----------------------------------------------------------------------===//
101 static RegisterAnalysis<ImmediatePostDominators>
102 D("postidom", "Immediate Post-Dominators Construction", true);
104 //===----------------------------------------------------------------------===//
105 // PostDominatorTree Implementation
106 //===----------------------------------------------------------------------===//
108 static RegisterAnalysis<PostDominatorTree>
109 F("postdomtree", "Post-Dominator Tree Construction", true);
111 void PostDominatorTree::calculate(const PostDominatorSet &DS) {
112 if (Roots.empty()) return;
113 BasicBlock *Root = Roots.size() == 1 ? Roots[0] : 0;
115 Nodes[Root] = RootNode = new Node(Root, 0); // Add a node for the root...
117 // Iterate over all nodes in depth first order...
118 for (unsigned i = 0, e = Roots.size(); i != e; ++i)
119 for (idf_iterator<BasicBlock*> I = idf_begin(Roots[i]),
120 E = idf_end(Roots[i]); I != E; ++I) {
122 const DominatorSet::DomSetType &Dominators = DS.getDominators(BB);
123 unsigned DomSetSize = Dominators.size();
124 if (DomSetSize == 1) continue; // Root node... IDom = null
126 // If we have already computed the immediate dominator for this node,
127 // don't revisit. This can happen due to nodes reachable from multiple
128 // roots, but which the idf_iterator doesn't know about.
129 if (Nodes.find(BB) != Nodes.end()) continue;
131 // Loop over all dominators of this node. This corresponds to looping
132 // over nodes in the dominator chain, looking for a node whose dominator
133 // set is equal to the current nodes, except that the current node does
134 // not exist in it. This means that it is one level higher in the dom
135 // chain than the current node, and it is our idom! We know that we have
136 // already added a DominatorTree node for our idom, because the idom must
137 // be a predecessor in the depth first order that we are iterating through
140 DominatorSet::DomSetType::const_iterator I = Dominators.begin();
141 DominatorSet::DomSetType::const_iterator End = Dominators.end();
142 for (; I != End; ++I) { // Iterate over dominators...
143 // All of our dominators should form a chain, where the number
144 // of elements in the dominator set indicates what level the
145 // node is at in the chain. We want the node immediately
146 // above us, so it will have an identical dominator set,
147 // except that BB will not dominate it... therefore it's
148 // dominator set size will be one less than BB's...
150 if (DS.getDominators(*I).size() == DomSetSize - 1) {
151 // We know that the immediate dominator should already have a node,
152 // because we are traversing the CFG in depth first order!
154 Node *IDomNode = Nodes[*I];
155 assert(IDomNode && "No node for IDOM?");
157 // Add a new tree node for this BasicBlock, and link it as a child of
159 Nodes[BB] = IDomNode->addChild(new Node(BB, IDomNode));
166 //===----------------------------------------------------------------------===//
167 // PostDominanceFrontier Implementation
168 //===----------------------------------------------------------------------===//
170 static RegisterAnalysis<PostDominanceFrontier>
171 H("postdomfrontier", "Post-Dominance Frontier Construction", true);
173 const DominanceFrontier::DomSetType &
174 PostDominanceFrontier::calculate(const PostDominatorTree &DT,
175 const DominatorTree::Node *Node) {
176 // Loop over CFG successors to calculate DFlocal[Node]
177 BasicBlock *BB = Node->getNode();
178 DomSetType &S = Frontiers[BB]; // The new set to fill in...
179 if (getRoots().empty()) return S;
182 for (pred_iterator SI = pred_begin(BB), SE = pred_end(BB);
184 // Does Node immediately dominate this predeccessor?
185 if (DT[*SI]->getIDom() != Node)
188 // At this point, S is DFlocal. Now we union in DFup's of our children...
189 // Loop through and visit the nodes that Node immediately dominates (Node's
190 // children in the IDomTree)
192 for (PostDominatorTree::Node::const_iterator
193 NI = Node->begin(), NE = Node->end(); NI != NE; ++NI) {
194 DominatorTree::Node *IDominee = *NI;
195 const DomSetType &ChildDF = calculate(DT, IDominee);
197 DomSetType::const_iterator CDFI = ChildDF.begin(), CDFE = ChildDF.end();
198 for (; CDFI != CDFE; ++CDFI) {
199 if (!Node->dominates(DT[*CDFI]))
207 // stub - a dummy function to make linking work ok.
208 void PostDominanceFrontier::stub() {