1 // Copyright 2014 the V8 project authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 #ifndef V8_COMPILER_CONTROL_EQUIVALENCE_H_
6 #define V8_COMPILER_CONTROL_EQUIVALENCE_H_
8 #include "src/compiler/graph.h"
9 #include "src/compiler/node.h"
10 #include "src/compiler/node-properties.h"
11 #include "src/zone-containers.h"
17 // Determines control dependence equivalence classes for control nodes. Any two
18 // nodes having the same set of control dependences land in one class. These
19 // classes can in turn be used to:
20 // - Build a program structure tree (PST) for controls in the graph.
21 // - Determine single-entry single-exit (SESE) regions within the graph.
23 // Note that this implementation actually uses cycle equivalence to establish
24 // class numbers. Any two nodes are cycle equivalent if they occur in the same
25 // set of cycles. It can be shown that control dependence equivalence reduces
26 // to undirected cycle equivalence for strongly connected control flow graphs.
28 // The algorithm is based on the paper, "The program structure tree: computing
29 // control regions in linear time" by Johnson, Pearson & Pingali (PLDI94) which
30 // also contains proofs for the aforementioned equivalence. References to line
31 // numbers in the algorithm from figure 4 have been added [line:x].
32 class ControlEquivalence : public ZoneObject {
34 ControlEquivalence(Zone* zone, Graph* graph)
39 node_data_(graph->NodeCount(), EmptyData(), zone) {}
41 // Run the main algorithm starting from the {exit} control node. This causes
42 // the following iterations over control edges of the graph:
43 // 1) A breadth-first backwards traversal to determine the set of nodes that
44 // participate in the next step. Takes O(E) time and O(N) space.
45 // 2) An undirected depth-first backwards traversal that determines class
46 // numbers for all participating nodes. Takes O(E) time and O(N) space.
47 void Run(Node* exit) {
48 if (GetClass(exit) != kInvalidClass) return;
49 DetermineParticipation(exit);
50 RunUndirectedDFS(exit);
53 // Retrieves a previously computed class number.
54 size_t ClassOf(Node* node) {
55 DCHECK(GetClass(node) != kInvalidClass);
56 return GetClass(node);
60 static const size_t kInvalidClass = static_cast<size_t>(-1);
61 typedef enum { kInputDirection, kUseDirection } DFSDirection;
64 DFSDirection direction; // Direction in which this bracket was added.
65 size_t recent_class; // Cached class when bracket was topmost.
66 size_t recent_size; // Cached set-size when bracket was topmost.
67 Node* from; // Node that this bracket originates from.
68 Node* to; // Node that this bracket points to.
71 // The set of brackets for each node during the DFS walk.
72 typedef ZoneLinkedList<Bracket> BracketList;
74 struct DFSStackEntry {
75 DFSDirection direction; // Direction currently used in DFS walk.
76 Node::InputEdges::iterator input; // Iterator used for "input" direction.
77 Node::UseEdges::iterator use; // Iterator used for "use" direction.
78 Node* parent_node; // Parent node of entry during DFS walk.
79 Node* node; // Node that this stack entry belongs to.
82 // The stack is used during the undirected DFS walk.
83 typedef ZoneStack<DFSStackEntry> DFSStack;
86 size_t class_number; // Equivalence class number assigned to node.
87 size_t dfs_number; // Pre-order DFS number assigned to node.
88 bool visited; // Indicates node has already been visited.
89 bool on_stack; // Indicates node is on DFS stack during walk.
90 bool participates; // Indicates node participates in DFS walk.
91 BracketList blist; // List of brackets per node.
94 // The per-node data computed during the DFS walk.
95 typedef ZoneVector<NodeData> Data;
97 // Called at pre-visit during DFS walk.
98 void VisitPre(Node* node) {
99 Trace("CEQ: Pre-visit of #%d:%s\n", node->id(), node->op()->mnemonic());
101 // Dispense a new pre-order number.
102 SetNumber(node, NewDFSNumber());
103 Trace(" Assigned DFS number is %d\n", GetNumber(node));
106 // Called at mid-visit during DFS walk.
107 void VisitMid(Node* node, DFSDirection direction) {
108 Trace("CEQ: Mid-visit of #%d:%s\n", node->id(), node->op()->mnemonic());
109 BracketList& blist = GetBracketList(node);
111 // Remove brackets pointing to this node [line:19].
112 BracketListDelete(blist, node, direction);
114 // Potentially introduce artificial dependency from start to end.
116 DCHECK_EQ(kInputDirection, direction);
117 VisitBackedge(node, graph_->end(), kInputDirection);
120 // Potentially start a new equivalence class [line:37].
121 BracketListTrace(blist);
122 Bracket* recent = &blist.back();
123 if (recent->recent_size != blist.size()) {
124 recent->recent_size = blist.size();
125 recent->recent_class = NewClassNumber();
128 // Assign equivalence class to node.
129 SetClass(node, recent->recent_class);
130 Trace(" Assigned class number is %d\n", GetClass(node));
133 // Called at post-visit during DFS walk.
134 void VisitPost(Node* node, Node* parent_node, DFSDirection direction) {
135 Trace("CEQ: Post-visit of #%d:%s\n", node->id(), node->op()->mnemonic());
136 BracketList& blist = GetBracketList(node);
138 // Remove brackets pointing to this node [line:19].
139 BracketListDelete(blist, node, direction);
141 // Propagate bracket list up the DFS tree [line:13].
142 if (parent_node != NULL) {
143 BracketList& parent_blist = GetBracketList(parent_node);
144 parent_blist.splice(parent_blist.end(), blist);
148 // Called when hitting a back edge in the DFS walk.
149 void VisitBackedge(Node* from, Node* to, DFSDirection direction) {
150 Trace("CEQ: Backedge from #%d:%s to #%d:%s\n", from->id(),
151 from->op()->mnemonic(), to->id(), to->op()->mnemonic());
153 // Push backedge onto the bracket list [line:25].
154 Bracket bracket = {direction, kInvalidClass, 0, from, to};
155 GetBracketList(from).push_back(bracket);
158 // Performs and undirected DFS walk of the graph. Conceptually all nodes are
159 // expanded, splitting "input" and "use" out into separate nodes. During the
160 // traversal, edges towards the representative nodes are preferred.
162 // \ / - Pre-visit: When N1 is visited in direction D the preferred
163 // x N1 edge towards N is taken next, calling VisitPre(N).
164 // | - Mid-visit: After all edges out of N2 in direction D have
165 // | N been visited, we switch the direction and start considering
166 // | edges out of N1 now, and we call VisitMid(N).
167 // x N2 - Post-visit: After all edges out of N1 in direction opposite
168 // / \ to D have been visited, we pop N and call VisitPost(N).
170 // This will yield a true spanning tree (without cross or forward edges) and
171 // also discover proper back edges in both directions.
172 void RunUndirectedDFS(Node* exit) {
173 ZoneStack<DFSStackEntry> stack(zone_);
174 DFSPush(stack, exit, NULL, kInputDirection);
177 while (!stack.empty()) { // Undirected depth-first backwards traversal.
178 DFSStackEntry& entry = stack.top();
179 Node* node = entry.node;
181 if (entry.direction == kInputDirection) {
182 if (entry.input != node->input_edges().end()) {
183 Edge edge = *entry.input;
184 Node* input = edge.to();
186 if (NodeProperties::IsControlEdge(edge)) {
187 // Visit next control input.
188 if (!GetData(input)->participates) continue;
189 if (GetData(input)->visited) continue;
190 if (GetData(input)->on_stack) {
191 // Found backedge if input is on stack.
192 if (input != entry.parent_node) {
193 VisitBackedge(node, input, kInputDirection);
196 // Push input onto stack.
197 DFSPush(stack, input, node, kInputDirection);
203 if (entry.use != node->use_edges().end()) {
204 // Switch direction to uses.
205 entry.direction = kUseDirection;
206 VisitMid(node, kInputDirection);
211 if (entry.direction == kUseDirection) {
212 if (entry.use != node->use_edges().end()) {
213 Edge edge = *entry.use;
214 Node* use = edge.from();
216 if (NodeProperties::IsControlEdge(edge)) {
217 // Visit next control use.
218 if (!GetData(use)->participates) continue;
219 if (GetData(use)->visited) continue;
220 if (GetData(use)->on_stack) {
221 // Found backedge if use is on stack.
222 if (use != entry.parent_node) {
223 VisitBackedge(node, use, kUseDirection);
226 // Push use onto stack.
227 DFSPush(stack, use, node, kUseDirection);
233 if (entry.input != node->input_edges().end()) {
234 // Switch direction to inputs.
235 entry.direction = kInputDirection;
236 VisitMid(node, kUseDirection);
241 // Pop node from stack when done with all inputs and uses.
242 DCHECK(entry.input == node->input_edges().end());
243 DCHECK(entry.use == node->use_edges().end());
245 VisitPost(node, entry.parent_node, entry.direction);
249 void DetermineParticipationEnqueue(ZoneQueue<Node*>& queue, Node* node) {
250 if (!GetData(node)->participates) {
251 GetData(node)->participates = true;
256 void DetermineParticipation(Node* exit) {
257 ZoneQueue<Node*> queue(zone_);
258 DetermineParticipationEnqueue(queue, exit);
259 while (!queue.empty()) { // Breadth-first backwards traversal.
260 Node* node = queue.front();
262 int max = NodeProperties::PastControlIndex(node);
263 for (int i = NodeProperties::FirstControlIndex(node); i < max; i++) {
264 DetermineParticipationEnqueue(queue, node->InputAt(i));
270 NodeData* GetData(Node* node) { return &node_data_[node->id()]; }
271 int NewClassNumber() { return class_number_++; }
272 int NewDFSNumber() { return dfs_number_++; }
274 // Template used to initialize per-node data.
275 NodeData EmptyData() {
276 return {kInvalidClass, 0, false, false, false, BracketList(zone_)};
279 // Accessors for the DFS number stored within the per-node data.
280 size_t GetNumber(Node* node) { return GetData(node)->dfs_number; }
281 void SetNumber(Node* node, size_t number) {
282 GetData(node)->dfs_number = number;
285 // Accessors for the equivalence class stored within the per-node data.
286 size_t GetClass(Node* node) { return GetData(node)->class_number; }
287 void SetClass(Node* node, size_t number) {
288 GetData(node)->class_number = number;
291 // Accessors for the bracket list stored within the per-node data.
292 BracketList& GetBracketList(Node* node) { return GetData(node)->blist; }
293 void SetBracketList(Node* node, BracketList& list) {
294 GetData(node)->blist = list;
297 // Mutates the DFS stack by pushing an entry.
298 void DFSPush(DFSStack& stack, Node* node, Node* from, DFSDirection dir) {
299 DCHECK(GetData(node)->participates);
300 DCHECK(!GetData(node)->visited);
301 GetData(node)->on_stack = true;
302 Node::InputEdges::iterator input = node->input_edges().begin();
303 Node::UseEdges::iterator use = node->use_edges().begin();
304 stack.push({dir, input, use, from, node});
307 // Mutates the DFS stack by popping an entry.
308 void DFSPop(DFSStack& stack, Node* node) {
309 DCHECK_EQ(stack.top().node, node);
310 GetData(node)->on_stack = false;
311 GetData(node)->visited = true;
315 // TODO(mstarzinger): Optimize this to avoid linear search.
316 void BracketListDelete(BracketList& blist, Node* to, DFSDirection direction) {
317 for (BracketList::iterator i = blist.begin(); i != blist.end(); /*nop*/) {
318 if (i->to == to && i->direction != direction) {
319 Trace(" BList erased: {%d->%d}\n", i->from->id(), i->to->id());
327 void BracketListTrace(BracketList& blist) {
328 if (FLAG_trace_turbo_scheduler) {
330 for (Bracket bracket : blist) {
331 Trace("{%d->%d} ", bracket.from->id(), bracket.to->id());
337 void Trace(const char* msg, ...) {
338 if (FLAG_trace_turbo_scheduler) {
340 va_start(arguments, msg);
341 base::OS::VPrint(msg, arguments);
348 int dfs_number_; // Generates new DFS pre-order numbers on demand.
349 int class_number_; // Generates new equivalence class numbers on demand.
350 Data node_data_; // Per-node data stored as a side-table.
353 } // namespace compiler
354 } // namespace internal
357 #endif // V8_COMPILER_CONTROL_EQUIVALENCE_H_