1//===- DependenceGraphBuilder.cpp ------------------------------------------==//
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// This file implements common steps of the build algorithm for construction
9// of dependence graphs such as DDG and PDG.
10//===----------------------------------------------------------------------===//
11
12#include "llvm/Analysis/DependenceGraphBuilder.h"
13#include "llvm/ADT/DepthFirstIterator.h"
14#include "llvm/ADT/EnumeratedArray.h"
15#include "llvm/ADT/PostOrderIterator.h"
16#include "llvm/ADT/SCCIterator.h"
17#include "llvm/ADT/Statistic.h"
18#include "llvm/Analysis/DDG.h"
19
20using namespace llvm;
21
22#define DEBUG_TYPE "dgb"
23
24STATISTIC(TotalGraphs, "Number of dependence graphs created.");
25STATISTIC(TotalDefUseEdges, "Number of def-use edges created.");
26STATISTIC(TotalMemoryEdges, "Number of memory dependence edges created.");
27STATISTIC(TotalFineGrainedNodes, "Number of fine-grained nodes created.");
28STATISTIC(TotalPiBlockNodes, "Number of pi-block nodes created.");
29STATISTIC(TotalConfusedEdges,
30 "Number of confused memory dependencies between two nodes.");
31STATISTIC(TotalEdgeReversals,
32 "Number of times the source and sink of dependence was reversed to "
33 "expose cycles in the graph.");
34
35using InstructionListType = SmallVector<Instruction *, 2>;
36
37//===--------------------------------------------------------------------===//
38// AbstractDependenceGraphBuilder implementation
39//===--------------------------------------------------------------------===//
40
41template <class G>
42void AbstractDependenceGraphBuilder<G>::computeInstructionOrdinals() {
43 // The BBList is expected to be in program order.
44 size_t NextOrdinal = 1;
45 for (auto *BB : BBList)
46 for (auto &I : *BB)
47 InstOrdinalMap.insert(KV: std::make_pair(x: &I, y: NextOrdinal++));
48}
49
50template <class G>
51void AbstractDependenceGraphBuilder<G>::createFineGrainedNodes() {
52 ++TotalGraphs;
53 assert(IMap.empty() && "Expected empty instruction map at start");
54 for (BasicBlock *BB : BBList)
55 for (Instruction &I : *BB) {
56 auto &NewNode = createFineGrainedNode(I);
57 IMap.insert(std::make_pair(&I, &NewNode));
58 NodeOrdinalMap.insert(std::make_pair(&NewNode, getOrdinal(I)));
59 ++TotalFineGrainedNodes;
60 }
61}
62
63template <class G>
64void AbstractDependenceGraphBuilder<G>::createAndConnectRootNode() {
65 // Create a root node that connects to every connected component of the graph.
66 // This is done to allow graph iterators to visit all the disjoint components
67 // of the graph, in a single walk.
68 //
69 // This algorithm works by going through each node of the graph and for each
70 // node N, do a DFS starting from N. A rooted edge is established between the
71 // root node and N (if N is not yet visited). All the nodes reachable from N
72 // are marked as visited and are skipped in the DFS of subsequent nodes.
73 //
74 // Note: This algorithm tries to limit the number of edges out of the root
75 // node to some extent, but there may be redundant edges created depending on
76 // the iteration order. For example for a graph {A -> B}, an edge from the
77 // root node is added to both nodes if B is visited before A. While it does
78 // not result in minimal number of edges, this approach saves compile-time
79 // while keeping the number of edges in check.
80 auto &RootNode = createRootNode();
81 df_iterator_default_set<const NodeType *, 4> Visited;
82 for (auto *N : Graph) {
83 if (*N == RootNode)
84 continue;
85 for (auto I : depth_first_ext(N, Visited))
86 if (I == N)
87 createRootedEdge(Src&: RootNode, Tgt&: *N);
88 }
89}
90
91template <class G> void AbstractDependenceGraphBuilder<G>::createPiBlocks() {
92 if (!shouldCreatePiBlocks())
93 return;
94
95 LLVM_DEBUG(dbgs() << "==== Start of Creation of Pi-Blocks ===\n");
96
97 // The overall algorithm is as follows:
98 // 1. Identify SCCs and for each SCC create a pi-block node containing all
99 // the nodes in that SCC.
100 // 2. Identify incoming edges incident to the nodes inside of the SCC and
101 // reconnect them to the pi-block node.
102 // 3. Identify outgoing edges from the nodes inside of the SCC to nodes
103 // outside of it and reconnect them so that the edges are coming out of the
104 // SCC node instead.
105
106 // Adding nodes as we iterate through the SCCs cause the SCC
107 // iterators to get invalidated. To prevent this invalidation, we first
108 // collect a list of nodes that are part of an SCC, and then iterate over
109 // those lists to create the pi-block nodes. Each element of the list is a
110 // list of nodes in an SCC. Note: trivial SCCs containing a single node are
111 // ignored.
112 SmallVector<NodeListType, 4> ListOfSCCs;
113 for (auto &SCC : make_range(scc_begin(&Graph), scc_end(&Graph))) {
114 if (SCC.size() > 1)
115 ListOfSCCs.emplace_back(SCC.begin(), SCC.end());
116 }
117
118 for (NodeListType &NL : ListOfSCCs) {
119 LLVM_DEBUG(dbgs() << "Creating pi-block node with " << NL.size()
120 << " nodes in it.\n");
121
122 // SCC iterator may put the nodes in an order that's different from the
123 // program order. To preserve original program order, we sort the list of
124 // nodes based on ordinal numbers computed earlier.
125 llvm::sort(NL, [&](NodeType *LHS, NodeType *RHS) {
126 return getOrdinal(*LHS) < getOrdinal(*RHS);
127 });
128
129 NodeType &PiNode = createPiBlock(L: NL);
130 ++TotalPiBlockNodes;
131
132 // Build a set to speed up the lookup for edges whose targets
133 // are inside the SCC.
134 SmallPtrSet<NodeType *, 4> NodesInSCC(NL.begin(), NL.end());
135
136 // We have the set of nodes in the SCC. We go through the set of nodes
137 // that are outside of the SCC and look for edges that cross the two sets.
138 for (NodeType *N : Graph) {
139
140 // Skip the SCC node and all the nodes inside of it.
141 if (*N == PiNode || NodesInSCC.count(N))
142 continue;
143
144 enum Direction {
145 Incoming, // Incoming edges to the SCC
146 Outgoing, // Edges going ot of the SCC
147 DirectionCount // To make the enum usable as an array index.
148 };
149
150 // Use these flags to help us avoid creating redundant edges. If there
151 // are more than one edges from an outside node to inside nodes, we only
152 // keep one edge from that node to the pi-block node. Similarly, if
153 // there are more than one edges from inside nodes to an outside node,
154 // we only keep one edge from the pi-block node to the outside node.
155 // There is a flag defined for each direction (incoming vs outgoing) and
156 // for each type of edge supported, using a two-dimensional boolean
157 // array.
158 using EdgeKind = typename EdgeType::EdgeKind;
159 EnumeratedArray<bool, EdgeKind> EdgeAlreadyCreated[DirectionCount]{false,
160 false};
161
162 auto createEdgeOfKind = [this](NodeType &Src, NodeType &Dst,
163 const EdgeKind K) {
164 switch (K) {
165 case EdgeKind::RegisterDefUse:
166 createDefUseEdge(Src, Tgt&: Dst);
167 break;
168 case EdgeKind::MemoryDependence:
169 createMemoryEdge(Src, Tgt&: Dst);
170 break;
171 case EdgeKind::Rooted:
172 createRootedEdge(Src, Tgt&: Dst);
173 break;
174 default:
175 llvm_unreachable("Unsupported type of edge.");
176 }
177 };
178
179 auto reconnectEdges = [&](NodeType *Src, NodeType *Dst, NodeType *New,
180 const Direction Dir) {
181 if (!Src->hasEdgeTo(*Dst))
182 return;
183 LLVM_DEBUG(
184 dbgs() << "reconnecting("
185 << (Dir == Direction::Incoming ? "incoming)" : "outgoing)")
186 << ":\nSrc:" << *Src << "\nDst:" << *Dst << "\nNew:" << *New
187 << "\n");
188 assert((Dir == Direction::Incoming || Dir == Direction::Outgoing) &&
189 "Invalid direction.");
190
191 SmallVector<EdgeType *, 10> EL;
192 Src->findEdgesTo(*Dst, EL);
193 for (EdgeType *OldEdge : EL) {
194 EdgeKind Kind = OldEdge->getKind();
195 if (!EdgeAlreadyCreated[Dir][Kind]) {
196 if (Dir == Direction::Incoming) {
197 createEdgeOfKind(*Src, *New, Kind);
198 LLVM_DEBUG(dbgs() << "created edge from Src to New.\n");
199 } else if (Dir == Direction::Outgoing) {
200 createEdgeOfKind(*New, *Dst, Kind);
201 LLVM_DEBUG(dbgs() << "created edge from New to Dst.\n");
202 }
203 EdgeAlreadyCreated[Dir][Kind] = true;
204 }
205 Src->removeEdge(*OldEdge);
206 destroyEdge(E&: *OldEdge);
207 LLVM_DEBUG(dbgs() << "removed old edge between Src and Dst.\n\n");
208 }
209 };
210
211 for (NodeType *SCCNode : NL) {
212 // Process incoming edges incident to the pi-block node.
213 reconnectEdges(N, SCCNode, &PiNode, Direction::Incoming);
214
215 // Process edges that are coming out of the pi-block node.
216 reconnectEdges(SCCNode, N, &PiNode, Direction::Outgoing);
217 }
218 }
219 }
220
221 // Ordinal maps are no longer needed.
222 InstOrdinalMap.clear();
223 NodeOrdinalMap.clear();
224
225 LLVM_DEBUG(dbgs() << "==== End of Creation of Pi-Blocks ===\n");
226}
227
228template <class G> void AbstractDependenceGraphBuilder<G>::createDefUseEdges() {
229 for (NodeType *N : Graph) {
230 InstructionListType SrcIList;
231 N->collectInstructions([](const Instruction *I) { return true; }, SrcIList);
232
233 // Use a set to mark the targets that we link to N, so we don't add
234 // duplicate def-use edges when more than one instruction in a target node
235 // use results of instructions that are contained in N.
236 SmallPtrSet<NodeType *, 4> VisitedTargets;
237
238 for (Instruction *II : SrcIList) {
239 for (User *U : II->users()) {
240 Instruction *UI = dyn_cast<Instruction>(Val: U);
241 if (!UI)
242 continue;
243 NodeType *DstNode = nullptr;
244 if (IMap.find(UI) != IMap.end())
245 DstNode = IMap.find(UI)->second;
246
247 // In the case of loops, the scope of the subgraph is all the
248 // basic blocks (and instructions within them) belonging to the loop. We
249 // simply ignore all the edges coming from (or going into) instructions
250 // or basic blocks outside of this range.
251 if (!DstNode) {
252 LLVM_DEBUG(
253 dbgs()
254 << "skipped def-use edge since the sink" << *UI
255 << " is outside the range of instructions being considered.\n");
256 continue;
257 }
258
259 // Self dependencies are ignored because they are redundant and
260 // uninteresting.
261 if (DstNode == N) {
262 LLVM_DEBUG(dbgs()
263 << "skipped def-use edge since the sink and the source ("
264 << N << ") are the same.\n");
265 continue;
266 }
267
268 if (VisitedTargets.insert(DstNode).second) {
269 createDefUseEdge(Src&: *N, Tgt&: *DstNode);
270 ++TotalDefUseEdges;
271 }
272 }
273 }
274 }
275}
276
277template <class G>
278void AbstractDependenceGraphBuilder<G>::createMemoryDependencyEdges() {
279 using DGIterator = typename G::iterator;
280 auto isMemoryAccess = [](const Instruction *I) {
281 return I->mayReadOrWriteMemory();
282 };
283 for (DGIterator SrcIt = Graph.begin(), E = Graph.end(); SrcIt != E; ++SrcIt) {
284 InstructionListType SrcIList;
285 (*SrcIt)->collectInstructions(isMemoryAccess, SrcIList);
286 if (SrcIList.empty())
287 continue;
288
289 for (DGIterator DstIt = SrcIt; DstIt != E; ++DstIt) {
290 if (**SrcIt == **DstIt)
291 continue;
292 InstructionListType DstIList;
293 (*DstIt)->collectInstructions(isMemoryAccess, DstIList);
294 if (DstIList.empty())
295 continue;
296 bool ForwardEdgeCreated = false;
297 bool BackwardEdgeCreated = false;
298 for (Instruction *ISrc : SrcIList) {
299 for (Instruction *IDst : DstIList) {
300 auto D = DI.depends(Src: ISrc, Dst: IDst, PossiblyLoopIndependent: true);
301 if (!D)
302 continue;
303
304 // If we have a dependence with its left-most non-'=' direction
305 // being '>' we need to reverse the direction of the edge, because
306 // the source of the dependence cannot occur after the sink. For
307 // confused dependencies, we will create edges in both directions to
308 // represent the possibility of a cycle.
309
310 auto createConfusedEdges = [&](NodeType &Src, NodeType &Dst) {
311 if (!ForwardEdgeCreated) {
312 createMemoryEdge(Src, Tgt&: Dst);
313 ++TotalMemoryEdges;
314 }
315 if (!BackwardEdgeCreated) {
316 createMemoryEdge(Src&: Dst, Tgt&: Src);
317 ++TotalMemoryEdges;
318 }
319 ForwardEdgeCreated = BackwardEdgeCreated = true;
320 ++TotalConfusedEdges;
321 };
322
323 auto createForwardEdge = [&](NodeType &Src, NodeType &Dst) {
324 if (!ForwardEdgeCreated) {
325 createMemoryEdge(Src, Tgt&: Dst);
326 ++TotalMemoryEdges;
327 }
328 ForwardEdgeCreated = true;
329 };
330
331 auto createBackwardEdge = [&](NodeType &Src, NodeType &Dst) {
332 if (!BackwardEdgeCreated) {
333 createMemoryEdge(Src&: Dst, Tgt&: Src);
334 ++TotalMemoryEdges;
335 }
336 BackwardEdgeCreated = true;
337 };
338
339 if (D->isConfused())
340 createConfusedEdges(**SrcIt, **DstIt);
341 else if (D->isOrdered() && !D->isLoopIndependent()) {
342 bool ReversedEdge = false;
343 for (unsigned Level = 1; Level <= D->getLevels(); ++Level) {
344 if (D->getDirection(Level) == Dependence::DVEntry::EQ)
345 continue;
346 else if (D->getDirection(Level) == Dependence::DVEntry::GT) {
347 createBackwardEdge(**SrcIt, **DstIt);
348 ReversedEdge = true;
349 ++TotalEdgeReversals;
350 break;
351 } else if (D->getDirection(Level) == Dependence::DVEntry::LT)
352 break;
353 else {
354 createConfusedEdges(**SrcIt, **DstIt);
355 break;
356 }
357 }
358 if (!ReversedEdge)
359 createForwardEdge(**SrcIt, **DstIt);
360 } else
361 createForwardEdge(**SrcIt, **DstIt);
362
363 // Avoid creating duplicate edges.
364 if (ForwardEdgeCreated && BackwardEdgeCreated)
365 break;
366 }
367
368 // If we've created edges in both directions, there is no more
369 // unique edge that we can create between these two nodes, so we
370 // can exit early.
371 if (ForwardEdgeCreated && BackwardEdgeCreated)
372 break;
373 }
374 }
375 }
376}
377
378template <class G> void AbstractDependenceGraphBuilder<G>::simplify() {
379 if (!shouldSimplify())
380 return;
381 LLVM_DEBUG(dbgs() << "==== Start of Graph Simplification ===\n");
382
383 // This algorithm works by first collecting a set of candidate nodes that have
384 // an out-degree of one (in terms of def-use edges), and then ignoring those
385 // whose targets have an in-degree more than one. Each node in the resulting
386 // set can then be merged with its corresponding target and put back into the
387 // worklist until no further merge candidates are available.
388 SmallPtrSet<NodeType *, 32> CandidateSourceNodes;
389
390 // A mapping between nodes and their in-degree. To save space, this map
391 // only contains nodes that are targets of nodes in the CandidateSourceNodes.
392 DenseMap<NodeType *, unsigned> TargetInDegreeMap;
393
394 for (NodeType *N : Graph) {
395 if (N->getEdges().size() != 1)
396 continue;
397 EdgeType &Edge = N->back();
398 if (!Edge.isDefUse())
399 continue;
400 CandidateSourceNodes.insert(N);
401
402 // Insert an element into the in-degree map and initialize to zero. The
403 // count will get updated in the next step.
404 TargetInDegreeMap.insert({&Edge.getTargetNode(), 0});
405 }
406
407 LLVM_DEBUG({
408 dbgs() << "Size of candidate src node list:" << CandidateSourceNodes.size()
409 << "\nNode with single outgoing def-use edge:\n";
410 for (NodeType *N : CandidateSourceNodes) {
411 dbgs() << N << "\n";
412 }
413 });
414
415 for (NodeType *N : Graph) {
416 for (EdgeType *E : *N) {
417 NodeType *Tgt = &E->getTargetNode();
418 auto TgtIT = TargetInDegreeMap.find(Tgt);
419 if (TgtIT != TargetInDegreeMap.end())
420 ++(TgtIT->second);
421 }
422 }
423
424 LLVM_DEBUG({
425 dbgs() << "Size of target in-degree map:" << TargetInDegreeMap.size()
426 << "\nContent of in-degree map:\n";
427 for (auto &I : TargetInDegreeMap) {
428 dbgs() << I.first << " --> " << I.second << "\n";
429 }
430 });
431
432 SmallVector<NodeType *, 32> Worklist(CandidateSourceNodes.begin(),
433 CandidateSourceNodes.end());
434 while (!Worklist.empty()) {
435 NodeType &Src = *Worklist.pop_back_val();
436 // As nodes get merged, we need to skip any node that has been removed from
437 // the candidate set (see below).
438 if (!CandidateSourceNodes.erase(&Src))
439 continue;
440
441 assert(Src.getEdges().size() == 1 &&
442 "Expected a single edge from the candidate src node.");
443 NodeType &Tgt = Src.back().getTargetNode();
444 assert(TargetInDegreeMap.find(&Tgt) != TargetInDegreeMap.end() &&
445 "Expected target to be in the in-degree map.");
446
447 if (TargetInDegreeMap[&Tgt] != 1)
448 continue;
449
450 if (!areNodesMergeable(A: Src, B: Tgt))
451 continue;
452
453 // Do not merge if there is also an edge from target to src (immediate
454 // cycle).
455 if (Tgt.hasEdgeTo(Src))
456 continue;
457
458 LLVM_DEBUG(dbgs() << "Merging:" << Src << "\nWith:" << Tgt << "\n");
459
460 mergeNodes(A&: Src, B&: Tgt);
461
462 // If the target node is in the candidate set itself, we need to put the
463 // src node back into the worklist again so it gives the target a chance
464 // to get merged into it. For example if we have:
465 // {(a)->(b), (b)->(c), (c)->(d), ...} and the worklist is initially {b, a},
466 // then after merging (a) and (b) together, we need to put (a,b) back in
467 // the worklist so that (c) can get merged in as well resulting in
468 // {(a,b,c) -> d}
469 // We also need to remove the old target (b), from the worklist. We first
470 // remove it from the candidate set here, and skip any item from the
471 // worklist that is not in the set.
472 if (CandidateSourceNodes.erase(&Tgt)) {
473 Worklist.push_back(&Src);
474 CandidateSourceNodes.insert(&Src);
475 LLVM_DEBUG(dbgs() << "Putting " << &Src << " back in the worklist.\n");
476 }
477 }
478 LLVM_DEBUG(dbgs() << "=== End of Graph Simplification ===\n");
479}
480
481template <class G>
482void AbstractDependenceGraphBuilder<G>::sortNodesTopologically() {
483
484 // If we don't create pi-blocks, then we may not have a DAG.
485 if (!shouldCreatePiBlocks())
486 return;
487
488 SmallVector<NodeType *, 64> NodesInPO;
489 using NodeKind = typename NodeType::NodeKind;
490 for (NodeType *N : post_order(&Graph)) {
491 if (N->getKind() == NodeKind::PiBlock) {
492 // Put members of the pi-block right after the pi-block itself, for
493 // convenience.
494 const NodeListType &PiBlockMembers = getNodesInPiBlock(N: *N);
495 llvm::append_range(NodesInPO, PiBlockMembers);
496 }
497 NodesInPO.push_back(N);
498 }
499
500 size_t OldSize = Graph.Nodes.size();
501 Graph.Nodes.clear();
502 append_range(Graph.Nodes, reverse(NodesInPO));
503 if (Graph.Nodes.size() != OldSize)
504 assert(false &&
505 "Expected the number of nodes to stay the same after the sort");
506}
507
508template class llvm::AbstractDependenceGraphBuilder<DataDependenceGraph>;
509template class llvm::DependenceGraphInfo<DDGNode>;
510