GenericDomTreeConstruction.h revision 274968 
1//===- GenericDomTreeConstruction.h - Dominator Calculation ------*- C++ -*-==//
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/// \file
10///
11/// Generic dominator tree construction - This file provides routines to
12/// construct immediate dominator information for a flow-graph based on the
13/// algorithm described in this document:
14///
15///   A Fast Algorithm for Finding Dominators in a Flowgraph
16///   T. Lengauer & R. Tarjan, ACM TOPLAS July 1979, pgs 121-141.
17///
18/// This implements the O(n*log(n)) versions of EVAL and LINK, because it turns
19/// out that the theoretically slower O(n*log(n)) implementation is actually
20/// faster than the almost-linear O(n*alpha(n)) version, even for large CFGs.
21///
22//===----------------------------------------------------------------------===//
23
24
25#ifndef LLVM_SUPPORT_GENERIC_DOM_TREE_CONSTRUCTION_H
26#define LLVM_SUPPORT_GENERIC_DOM_TREE_CONSTRUCTION_H
27
28#include "llvm/ADT/SmallPtrSet.h"
29#include "llvm/Support/GenericDomTree.h"
30
31namespace llvm {
32
33template<class GraphT>
34unsigned DFSPass(DominatorTreeBase<typename GraphT::NodeType>& DT,
35                 typename GraphT::NodeType* V, unsigned N) {
36  // This is more understandable as a recursive algorithm, but we can't use the
37  // recursive algorithm due to stack depth issues.  Keep it here for
38  // documentation purposes.
39#if 0
40  InfoRec &VInfo = DT.Info[DT.Roots[i]];
41  VInfo.DFSNum = VInfo.Semi = ++N;
42  VInfo.Label = V;
43
44  Vertex.push_back(V);        // Vertex[n] = V;
45
46  for (succ_iterator SI = succ_begin(V), E = succ_end(V); SI != E; ++SI) {
47    InfoRec &SuccVInfo = DT.Info[*SI];
48    if (SuccVInfo.Semi == 0) {
49      SuccVInfo.Parent = V;
50      N = DTDFSPass(DT, *SI, N);
51    }
52  }
53#else
54  bool IsChildOfArtificialExit = (N != 0);
55
56  SmallVector<std::pair<typename GraphT::NodeType*,
57                        typename GraphT::ChildIteratorType>, 32> Worklist;
58  Worklist.push_back(std::make_pair(V, GraphT::child_begin(V)));
59  while (!Worklist.empty()) {
60    typename GraphT::NodeType* BB = Worklist.back().first;
61    typename GraphT::ChildIteratorType NextSucc = Worklist.back().second;
62
63    typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &BBInfo =
64                                                                    DT.Info[BB];
65
66    // First time we visited this BB?
67    if (NextSucc == GraphT::child_begin(BB)) {
68      BBInfo.DFSNum = BBInfo.Semi = ++N;
69      BBInfo.Label = BB;
70
71      DT.Vertex.push_back(BB);       // Vertex[n] = V;
72
73      if (IsChildOfArtificialExit)
74        BBInfo.Parent = 1;
75
76      IsChildOfArtificialExit = false;
77    }
78
79    // store the DFS number of the current BB - the reference to BBInfo might
80    // get invalidated when processing the successors.
81    unsigned BBDFSNum = BBInfo.DFSNum;
82
83    // If we are done with this block, remove it from the worklist.
84    if (NextSucc == GraphT::child_end(BB)) {
85      Worklist.pop_back();
86      continue;
87    }
88
89    // Increment the successor number for the next time we get to it.
90    ++Worklist.back().second;
91
92    // Visit the successor next, if it isn't already visited.
93    typename GraphT::NodeType* Succ = *NextSucc;
94
95    typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &SuccVInfo =
96                                                                  DT.Info[Succ];
97    if (SuccVInfo.Semi == 0) {
98      SuccVInfo.Parent = BBDFSNum;
99      Worklist.push_back(std::make_pair(Succ, GraphT::child_begin(Succ)));
100    }
101  }
102#endif
103    return N;
104}
105
106template<class GraphT>
107typename GraphT::NodeType*
108Eval(DominatorTreeBase<typename GraphT::NodeType>& DT,
109     typename GraphT::NodeType *VIn, unsigned LastLinked) {
110  typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &VInInfo =
111                                                                  DT.Info[VIn];
112  if (VInInfo.DFSNum < LastLinked)
113    return VIn;
114
115  SmallVector<typename GraphT::NodeType*, 32> Work;
116  SmallPtrSet<typename GraphT::NodeType*, 32> Visited;
117
118  if (VInInfo.Parent >= LastLinked)
119    Work.push_back(VIn);
120
121  while (!Work.empty()) {
122    typename GraphT::NodeType* V = Work.back();
123    typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &VInfo =
124                                                                     DT.Info[V];
125    typename GraphT::NodeType* VAncestor = DT.Vertex[VInfo.Parent];
126
127    // Process Ancestor first
128    if (Visited.insert(VAncestor) && VInfo.Parent >= LastLinked) {
129      Work.push_back(VAncestor);
130      continue;
131    }
132    Work.pop_back();
133
134    // Update VInfo based on Ancestor info
135    if (VInfo.Parent < LastLinked)
136      continue;
137
138    typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &VAInfo =
139                                                             DT.Info[VAncestor];
140    typename GraphT::NodeType* VAncestorLabel = VAInfo.Label;
141    typename GraphT::NodeType* VLabel = VInfo.Label;
142    if (DT.Info[VAncestorLabel].Semi < DT.Info[VLabel].Semi)
143      VInfo.Label = VAncestorLabel;
144    VInfo.Parent = VAInfo.Parent;
145  }
146
147  return VInInfo.Label;
148}
149
150template<class FuncT, class NodeT>
151void Calculate(DominatorTreeBase<typename GraphTraits<NodeT>::NodeType>& DT,
152               FuncT& F) {
153  typedef GraphTraits<NodeT> GraphT;
154
155  unsigned N = 0;
156  bool MultipleRoots = (DT.Roots.size() > 1);
157  if (MultipleRoots) {
158    typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &BBInfo =
159        DT.Info[nullptr];
160    BBInfo.DFSNum = BBInfo.Semi = ++N;
161    BBInfo.Label = nullptr;
162
163    DT.Vertex.push_back(nullptr);       // Vertex[n] = V;
164  }
165
166  // Step #1: Number blocks in depth-first order and initialize variables used
167  // in later stages of the algorithm.
168  for (unsigned i = 0, e = static_cast<unsigned>(DT.Roots.size());
169       i != e; ++i)
170    N = DFSPass<GraphT>(DT, DT.Roots[i], N);
171
172  // it might be that some blocks did not get a DFS number (e.g., blocks of
173  // infinite loops). In these cases an artificial exit node is required.
174  MultipleRoots |= (DT.isPostDominator() && N != GraphTraits<FuncT*>::size(&F));
175
176  // When naively implemented, the Lengauer-Tarjan algorithm requires a separate
177  // bucket for each vertex. However, this is unnecessary, because each vertex
178  // is only placed into a single bucket (that of its semidominator), and each
179  // vertex's bucket is processed before it is added to any bucket itself.
180  //
181  // Instead of using a bucket per vertex, we use a single array Buckets that
182  // has two purposes. Before the vertex V with preorder number i is processed,
183  // Buckets[i] stores the index of the first element in V's bucket. After V's
184  // bucket is processed, Buckets[i] stores the index of the next element in the
185  // bucket containing V, if any.
186  SmallVector<unsigned, 32> Buckets;
187  Buckets.resize(N + 1);
188  for (unsigned i = 1; i <= N; ++i)
189    Buckets[i] = i;
190
191  for (unsigned i = N; i >= 2; --i) {
192    typename GraphT::NodeType* W = DT.Vertex[i];
193    typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &WInfo =
194                                                                     DT.Info[W];
195
196    // Step #2: Implicitly define the immediate dominator of vertices
197    for (unsigned j = i; Buckets[j] != i; j = Buckets[j]) {
198      typename GraphT::NodeType* V = DT.Vertex[Buckets[j]];
199      typename GraphT::NodeType* U = Eval<GraphT>(DT, V, i + 1);
200      DT.IDoms[V] = DT.Info[U].Semi < i ? U : W;
201    }
202
203    // Step #3: Calculate the semidominators of all vertices
204
205    // initialize the semi dominator to point to the parent node
206    WInfo.Semi = WInfo.Parent;
207    typedef GraphTraits<Inverse<NodeT> > InvTraits;
208    for (typename InvTraits::ChildIteratorType CI =
209         InvTraits::child_begin(W),
210         E = InvTraits::child_end(W); CI != E; ++CI) {
211      typename InvTraits::NodeType *N = *CI;
212      if (DT.Info.count(N)) {  // Only if this predecessor is reachable!
213        unsigned SemiU = DT.Info[Eval<GraphT>(DT, N, i + 1)].Semi;
214        if (SemiU < WInfo.Semi)
215          WInfo.Semi = SemiU;
216      }
217    }
218
219    // If V is a non-root vertex and sdom(V) = parent(V), then idom(V) is
220    // necessarily parent(V). In this case, set idom(V) here and avoid placing
221    // V into a bucket.
222    if (WInfo.Semi == WInfo.Parent) {
223      DT.IDoms[W] = DT.Vertex[WInfo.Parent];
224    } else {
225      Buckets[i] = Buckets[WInfo.Semi];
226      Buckets[WInfo.Semi] = i;
227    }
228  }
229
230  if (N >= 1) {
231    typename GraphT::NodeType* Root = DT.Vertex[1];
232    for (unsigned j = 1; Buckets[j] != 1; j = Buckets[j]) {
233      typename GraphT::NodeType* V = DT.Vertex[Buckets[j]];
234      DT.IDoms[V] = Root;
235    }
236  }
237
238  // Step #4: Explicitly define the immediate dominator of each vertex
239  for (unsigned i = 2; i <= N; ++i) {
240    typename GraphT::NodeType* W = DT.Vertex[i];
241    typename GraphT::NodeType*& WIDom = DT.IDoms[W];
242    if (WIDom != DT.Vertex[DT.Info[W].Semi])
243      WIDom = DT.IDoms[WIDom];
244  }
245
246  if (DT.Roots.empty()) return;
247
248  // Add a node for the root.  This node might be the actual root, if there is
249  // one exit block, or it may be the virtual exit (denoted by (BasicBlock *)0)
250  // which postdominates all real exits if there are multiple exit blocks, or
251  // an infinite loop.
252  typename GraphT::NodeType* Root = !MultipleRoots ? DT.Roots[0] : nullptr;
253
254  DT.DomTreeNodes[Root] = DT.RootNode =
255                  new DomTreeNodeBase<typename GraphT::NodeType>(Root, nullptr);
256
257  // Loop over all of the reachable blocks in the function...
258  for (unsigned i = 2; i <= N; ++i) {
259    typename GraphT::NodeType* W = DT.Vertex[i];
260
261    DomTreeNodeBase<typename GraphT::NodeType> *BBNode = DT.DomTreeNodes[W];
262    if (BBNode) continue;  // Haven't calculated this node yet?
263
264    typename GraphT::NodeType* ImmDom = DT.getIDom(W);
265
266    assert(ImmDom || DT.DomTreeNodes[nullptr]);
267
268    // Get or calculate the node for the immediate dominator
269    DomTreeNodeBase<typename GraphT::NodeType> *IDomNode =
270                                                     DT.getNodeForBlock(ImmDom);
271
272    // Add a new tree node for this BasicBlock, and link it as a child of
273    // IDomNode
274    DomTreeNodeBase<typename GraphT::NodeType> *C =
275                    new DomTreeNodeBase<typename GraphT::NodeType>(W, IDomNode);
276    DT.DomTreeNodes[W] = IDomNode->addChild(C);
277  }
278
279  // Free temporary memory used to construct idom's
280  DT.IDoms.clear();
281  DT.Info.clear();
282  std::vector<typename GraphT::NodeType*>().swap(DT.Vertex);
283
284  DT.updateDFSNumbers();
285}
286
287}
288
289#endif
290