1 //===- Dominators.cpp - Dominator Calculation -----------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements simple dominator construction algorithms for finding
11 // forward dominators. Postdominators are available in libanalysis, but are not
12 // included in libvmcore, because it's not needed. Forward dominators are
13 // needed to support the Verifier pass.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/IR/Dominators.h"
18 #include "llvm/ADT/DepthFirstIterator.h"
19 #include "llvm/ADT/SmallPtrSet.h"
20 #include "llvm/Config/llvm-config.h"
21 #include "llvm/IR/CFG.h"
22 #include "llvm/IR/Constants.h"
23 #include "llvm/IR/Instructions.h"
24 #include "llvm/IR/PassManager.h"
25 #include "llvm/Support/CommandLine.h"
26 #include "llvm/Support/Debug.h"
27 #include "llvm/Support/GenericDomTreeConstruction.h"
28 #include "llvm/Support/raw_ostream.h"
32 bool llvm::VerifyDomInfo = false;
33 static cl::opt<bool, true>
34 VerifyDomInfoX("verify-dom-info", cl::location(VerifyDomInfo), cl::Hidden,
35 cl::desc("Verify dominator info (time consuming)"));
37 #ifdef EXPENSIVE_CHECKS
38 static constexpr bool ExpensiveChecksEnabled = true;
40 static constexpr bool ExpensiveChecksEnabled = false;
43 bool BasicBlockEdge::isSingleEdge() const {
44 const TerminatorInst *TI = Start->getTerminator();
45 unsigned NumEdgesToEnd = 0;
46 for (unsigned int i = 0, n = TI->getNumSuccessors(); i < n; ++i) {
47 if (TI->getSuccessor(i) == End)
49 if (NumEdgesToEnd >= 2)
52 assert(NumEdgesToEnd == 1);
56 //===----------------------------------------------------------------------===//
57 // DominatorTree Implementation
58 //===----------------------------------------------------------------------===//
60 // Provide public access to DominatorTree information. Implementation details
61 // can be found in Dominators.h, GenericDomTree.h, and
62 // GenericDomTreeConstruction.h.
64 //===----------------------------------------------------------------------===//
66 template class llvm::DomTreeNodeBase<BasicBlock>;
67 template class llvm::DominatorTreeBase<BasicBlock, false>; // DomTreeBase
68 template class llvm::DominatorTreeBase<BasicBlock, true>; // PostDomTreeBase
70 template struct llvm::DomTreeBuilder::Update<BasicBlock *>;
72 template void llvm::DomTreeBuilder::Calculate<DomTreeBuilder::BBDomTree>(
73 DomTreeBuilder::BBDomTree &DT);
74 template void llvm::DomTreeBuilder::Calculate<DomTreeBuilder::BBPostDomTree>(
75 DomTreeBuilder::BBPostDomTree &DT);
77 template void llvm::DomTreeBuilder::InsertEdge<DomTreeBuilder::BBDomTree>(
78 DomTreeBuilder::BBDomTree &DT, BasicBlock *From, BasicBlock *To);
79 template void llvm::DomTreeBuilder::InsertEdge<DomTreeBuilder::BBPostDomTree>(
80 DomTreeBuilder::BBPostDomTree &DT, BasicBlock *From, BasicBlock *To);
82 template void llvm::DomTreeBuilder::DeleteEdge<DomTreeBuilder::BBDomTree>(
83 DomTreeBuilder::BBDomTree &DT, BasicBlock *From, BasicBlock *To);
84 template void llvm::DomTreeBuilder::DeleteEdge<DomTreeBuilder::BBPostDomTree>(
85 DomTreeBuilder::BBPostDomTree &DT, BasicBlock *From, BasicBlock *To);
87 template void llvm::DomTreeBuilder::ApplyUpdates<DomTreeBuilder::BBDomTree>(
88 DomTreeBuilder::BBDomTree &DT, DomTreeBuilder::BBUpdates);
89 template void llvm::DomTreeBuilder::ApplyUpdates<DomTreeBuilder::BBPostDomTree>(
90 DomTreeBuilder::BBPostDomTree &DT, DomTreeBuilder::BBUpdates);
92 template bool llvm::DomTreeBuilder::Verify<DomTreeBuilder::BBDomTree>(
93 const DomTreeBuilder::BBDomTree &DT,
94 DomTreeBuilder::BBDomTree::VerificationLevel VL);
95 template bool llvm::DomTreeBuilder::Verify<DomTreeBuilder::BBPostDomTree>(
96 const DomTreeBuilder::BBPostDomTree &DT,
97 DomTreeBuilder::BBPostDomTree::VerificationLevel VL);
99 bool DominatorTree::invalidate(Function &F, const PreservedAnalyses &PA,
100 FunctionAnalysisManager::Invalidator &) {
101 // Check whether the analysis, all analyses on functions, or the function's
102 // CFG have been preserved.
103 auto PAC = PA.getChecker<DominatorTreeAnalysis>();
104 return !(PAC.preserved() || PAC.preservedSet<AllAnalysesOn<Function>>() ||
105 PAC.preservedSet<CFGAnalyses>());
108 // dominates - Return true if Def dominates a use in User. This performs
109 // the special checks necessary if Def and User are in the same basic block.
110 // Note that Def doesn't dominate a use in Def itself!
111 bool DominatorTree::dominates(const Instruction *Def,
112 const Instruction *User) const {
113 const BasicBlock *UseBB = User->getParent();
114 const BasicBlock *DefBB = Def->getParent();
116 // Any unreachable use is dominated, even if Def == User.
117 if (!isReachableFromEntry(UseBB))
120 // Unreachable definitions don't dominate anything.
121 if (!isReachableFromEntry(DefBB))
124 // An instruction doesn't dominate a use in itself.
128 // The value defined by an invoke dominates an instruction only if it
129 // dominates every instruction in UseBB.
130 // A PHI is dominated only if the instruction dominates every possible use in
132 if (isa<InvokeInst>(Def) || isa<PHINode>(User))
133 return dominates(Def, UseBB);
136 return dominates(DefBB, UseBB);
138 // Loop through the basic block until we find Def or User.
139 BasicBlock::const_iterator I = DefBB->begin();
140 for (; &*I != Def && &*I != User; ++I)
146 // true if Def would dominate a use in any instruction in UseBB.
147 // note that dominates(Def, Def->getParent()) is false.
148 bool DominatorTree::dominates(const Instruction *Def,
149 const BasicBlock *UseBB) const {
150 const BasicBlock *DefBB = Def->getParent();
152 // Any unreachable use is dominated, even if DefBB == UseBB.
153 if (!isReachableFromEntry(UseBB))
156 // Unreachable definitions don't dominate anything.
157 if (!isReachableFromEntry(DefBB))
163 // Invoke results are only usable in the normal destination, not in the
164 // exceptional destination.
165 if (const auto *II = dyn_cast<InvokeInst>(Def)) {
166 BasicBlock *NormalDest = II->getNormalDest();
167 BasicBlockEdge E(DefBB, NormalDest);
168 return dominates(E, UseBB);
171 return dominates(DefBB, UseBB);
174 bool DominatorTree::dominates(const BasicBlockEdge &BBE,
175 const BasicBlock *UseBB) const {
176 // If the BB the edge ends in doesn't dominate the use BB, then the
177 // edge also doesn't.
178 const BasicBlock *Start = BBE.getStart();
179 const BasicBlock *End = BBE.getEnd();
180 if (!dominates(End, UseBB))
183 // Simple case: if the end BB has a single predecessor, the fact that it
184 // dominates the use block implies that the edge also does.
185 if (End->getSinglePredecessor())
188 // The normal edge from the invoke is critical. Conceptually, what we would
189 // like to do is split it and check if the new block dominates the use.
190 // With X being the new block, the graph would look like:
203 // Given the definition of dominance, NormalDest is dominated by X iff X
204 // dominates all of NormalDest's predecessors (X, B, C in the example). X
205 // trivially dominates itself, so we only have to find if it dominates the
206 // other predecessors. Since the only way out of X is via NormalDest, X can
207 // only properly dominate a node if NormalDest dominates that node too.
208 int IsDuplicateEdge = 0;
209 for (const_pred_iterator PI = pred_begin(End), E = pred_end(End);
211 const BasicBlock *BB = *PI;
213 // If there are multiple edges between Start and End, by definition they
214 // can't dominate anything.
215 if (IsDuplicateEdge++)
220 if (!dominates(End, BB))
226 bool DominatorTree::dominates(const BasicBlockEdge &BBE, const Use &U) const {
227 Instruction *UserInst = cast<Instruction>(U.getUser());
228 // A PHI in the end of the edge is dominated by it.
229 PHINode *PN = dyn_cast<PHINode>(UserInst);
230 if (PN && PN->getParent() == BBE.getEnd() &&
231 PN->getIncomingBlock(U) == BBE.getStart())
234 // Otherwise use the edge-dominates-block query, which
235 // handles the crazy critical edge cases properly.
236 const BasicBlock *UseBB;
238 UseBB = PN->getIncomingBlock(U);
240 UseBB = UserInst->getParent();
241 return dominates(BBE, UseBB);
244 bool DominatorTree::dominates(const Instruction *Def, const Use &U) const {
245 Instruction *UserInst = cast<Instruction>(U.getUser());
246 const BasicBlock *DefBB = Def->getParent();
248 // Determine the block in which the use happens. PHI nodes use
249 // their operands on edges; simulate this by thinking of the use
250 // happening at the end of the predecessor block.
251 const BasicBlock *UseBB;
252 if (PHINode *PN = dyn_cast<PHINode>(UserInst))
253 UseBB = PN->getIncomingBlock(U);
255 UseBB = UserInst->getParent();
257 // Any unreachable use is dominated, even if Def == User.
258 if (!isReachableFromEntry(UseBB))
261 // Unreachable definitions don't dominate anything.
262 if (!isReachableFromEntry(DefBB))
265 // Invoke instructions define their return values on the edges to their normal
266 // successors, so we have to handle them specially.
267 // Among other things, this means they don't dominate anything in
268 // their own block, except possibly a phi, so we don't need to
269 // walk the block in any case.
270 if (const InvokeInst *II = dyn_cast<InvokeInst>(Def)) {
271 BasicBlock *NormalDest = II->getNormalDest();
272 BasicBlockEdge E(DefBB, NormalDest);
273 return dominates(E, U);
276 // If the def and use are in different blocks, do a simple CFG dominator
279 return dominates(DefBB, UseBB);
281 // Ok, def and use are in the same block. If the def is an invoke, it
282 // doesn't dominate anything in the block. If it's a PHI, it dominates
283 // everything in the block.
284 if (isa<PHINode>(UserInst))
287 // Otherwise, just loop through the basic block until we find Def or User.
288 BasicBlock::const_iterator I = DefBB->begin();
289 for (; &*I != Def && &*I != UserInst; ++I)
292 return &*I != UserInst;
295 bool DominatorTree::isReachableFromEntry(const Use &U) const {
296 Instruction *I = dyn_cast<Instruction>(U.getUser());
298 // ConstantExprs aren't really reachable from the entry block, but they
299 // don't need to be treated like unreachable code either.
302 // PHI nodes use their operands on their incoming edges.
303 if (PHINode *PN = dyn_cast<PHINode>(I))
304 return isReachableFromEntry(PN->getIncomingBlock(U));
306 // Everything else uses their operands in their own block.
307 return isReachableFromEntry(I->getParent());
310 //===----------------------------------------------------------------------===//
311 // DominatorTreeAnalysis and related pass implementations
312 //===----------------------------------------------------------------------===//
314 // This implements the DominatorTreeAnalysis which is used with the new pass
315 // manager. It also implements some methods from utility passes.
317 //===----------------------------------------------------------------------===//
319 DominatorTree DominatorTreeAnalysis::run(Function &F,
320 FunctionAnalysisManager &) {
326 AnalysisKey DominatorTreeAnalysis::Key;
328 DominatorTreePrinterPass::DominatorTreePrinterPass(raw_ostream &OS) : OS(OS) {}
330 PreservedAnalyses DominatorTreePrinterPass::run(Function &F,
331 FunctionAnalysisManager &AM) {
332 OS << "DominatorTree for function: " << F.getName() << "\n";
333 AM.getResult<DominatorTreeAnalysis>(F).print(OS);
335 return PreservedAnalyses::all();
338 PreservedAnalyses DominatorTreeVerifierPass::run(Function &F,
339 FunctionAnalysisManager &AM) {
340 auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
343 return PreservedAnalyses::all();
346 //===----------------------------------------------------------------------===//
347 // DominatorTreeWrapperPass Implementation
348 //===----------------------------------------------------------------------===//
350 // The implementation details of the wrapper pass that holds a DominatorTree
351 // suitable for use with the legacy pass manager.
353 //===----------------------------------------------------------------------===//
355 char DominatorTreeWrapperPass::ID = 0;
356 INITIALIZE_PASS(DominatorTreeWrapperPass, "domtree",
357 "Dominator Tree Construction", true, true)
359 bool DominatorTreeWrapperPass::runOnFunction(Function &F) {
364 void DominatorTreeWrapperPass::verifyAnalysis() const {
366 assert(DT.verify(DominatorTree::VerificationLevel::Full));
367 else if (ExpensiveChecksEnabled)
368 assert(DT.verify(DominatorTree::VerificationLevel::Basic));
371 void DominatorTreeWrapperPass::print(raw_ostream &OS, const Module *) const {
375 //===----------------------------------------------------------------------===//
376 // DeferredDominance Implementation
377 //===----------------------------------------------------------------------===//
379 // The implementation details of the DeferredDominance class which allows
380 // one to queue updates to a DominatorTree.
382 //===----------------------------------------------------------------------===//
384 /// Queues multiple updates and discards duplicates.
385 void DeferredDominance::applyUpdates(
386 ArrayRef<DominatorTree::UpdateType> Updates) {
387 SmallVector<DominatorTree::UpdateType, 8> Seen;
388 for (auto U : Updates)
389 // Avoid duplicates to applyUpdate() to save on analysis.
390 if (std::none_of(Seen.begin(), Seen.end(),
391 [U](DominatorTree::UpdateType S) { return S == U; })) {
393 applyUpdate(U.getKind(), U.getFrom(), U.getTo());
397 /// Helper method for a single edge insertion. It's almost always better
398 /// to batch updates and call applyUpdates to quickly remove duplicate edges.
399 /// This is best used when there is only a single insertion needed to update
401 void DeferredDominance::insertEdge(BasicBlock *From, BasicBlock *To) {
402 applyUpdate(DominatorTree::Insert, From, To);
405 /// Helper method for a single edge deletion. It's almost always better
406 /// to batch updates and call applyUpdates to quickly remove duplicate edges.
407 /// This is best used when there is only a single deletion needed to update
409 void DeferredDominance::deleteEdge(BasicBlock *From, BasicBlock *To) {
410 applyUpdate(DominatorTree::Delete, From, To);
413 /// Delays the deletion of a basic block until a flush() event.
414 void DeferredDominance::deleteBB(BasicBlock *DelBB) {
415 assert(DelBB && "Invalid push_back of nullptr DelBB.");
416 assert(pred_empty(DelBB) && "DelBB has one or more predecessors.");
417 // DelBB is unreachable and all its instructions are dead.
418 while (!DelBB->empty()) {
419 Instruction &I = DelBB->back();
420 // Replace used instructions with an arbitrary value (undef).
422 I.replaceAllUsesWith(llvm::UndefValue::get(I.getType()));
423 DelBB->getInstList().pop_back();
425 // Make sure DelBB has a valid terminator instruction. As long as DelBB is a
426 // Child of Function F it must contain valid IR.
427 new UnreachableInst(DelBB->getContext(), DelBB);
428 DeletedBBs.insert(DelBB);
431 /// Returns true if DelBB is awaiting deletion at a flush() event.
432 bool DeferredDominance::pendingDeletedBB(BasicBlock *DelBB) {
433 if (DeletedBBs.empty())
435 return DeletedBBs.count(DelBB) != 0;
438 /// Returns true if pending DT updates are queued for a flush() event.
439 bool DeferredDominance::pending() { return !PendUpdates.empty(); }
441 /// Flushes all pending updates and block deletions. Returns a
442 /// correct DominatorTree reference to be used by the caller for analysis.
443 DominatorTree &DeferredDominance::flush() {
444 // Updates to DT must happen before blocks are deleted below. Otherwise the
445 // DT traversal will encounter badref blocks and assert.
446 if (!PendUpdates.empty()) {
447 DT.applyUpdates(PendUpdates);
454 /// Drops all internal state and forces a (slow) recalculation of the
455 /// DominatorTree based on the current state of the LLVM IR in F. This should
456 /// only be used in corner cases such as the Entry block of F being deleted.
457 void DeferredDominance::recalculate(Function &F) {
458 // flushDelBB must be flushed before the recalculation. The state of the IR
459 // must be consistent before the DT traversal algorithm determines the
461 if (flushDelBB() || !PendUpdates.empty()) {
467 /// Debug method to help view the state of pending updates.
468 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
469 LLVM_DUMP_METHOD void DeferredDominance::dump() const {
470 raw_ostream &OS = llvm::dbgs();
471 OS << "PendUpdates:\n";
473 for (auto U : PendUpdates) {
474 OS << " " << I << " : ";
476 if (U.getKind() == DominatorTree::Insert)
480 BasicBlock *From = U.getFrom();
482 auto S = From->getName();
483 if (!From->hasName())
485 OS << S << "(" << From << "), ";
489 BasicBlock *To = U.getTo();
491 auto S = To->getName();
494 OS << S << "(" << To << ")\n";
499 OS << "DeletedBBs:\n";
501 for (auto BB : DeletedBBs) {
502 OS << " " << I << " : ";
505 OS << BB->getName() << "(";
513 /// Apply an update (Kind, From, To) to the internal queued updates. The
514 /// update is only added when determined to be necessary. Checks for
515 /// self-domination, unnecessary updates, duplicate requests, and balanced
516 /// pairs of requests are all performed. Returns true if the update is
517 /// queued and false if it is discarded.
518 bool DeferredDominance::applyUpdate(DominatorTree::UpdateKind Kind,
519 BasicBlock *From, BasicBlock *To) {
521 return false; // Cannot dominate self; discard update.
523 // Discard updates by inspecting the current state of successors of From.
524 // Since applyUpdate() must be called *after* the Terminator of From is
525 // altered we can determine if the update is unnecessary.
526 bool HasEdge = std::any_of(succ_begin(From), succ_end(From),
527 [To](BasicBlock *B) { return B == To; });
528 if (Kind == DominatorTree::Insert && !HasEdge)
529 return false; // Unnecessary Insert: edge does not exist in IR.
530 if (Kind == DominatorTree::Delete && HasEdge)
531 return false; // Unnecessary Delete: edge still exists in IR.
533 // Analyze pending updates to determine if the update is unnecessary.
534 DominatorTree::UpdateType Update = {Kind, From, To};
535 DominatorTree::UpdateType Invert = {Kind != DominatorTree::Insert
536 ? DominatorTree::Insert
537 : DominatorTree::Delete,
539 for (auto I = PendUpdates.begin(), E = PendUpdates.end(); I != E; ++I) {
541 return false; // Discard duplicate updates.
543 // Update and Invert are both valid (equivalent to a no-op). Remove
544 // Invert from PendUpdates and discard the Update.
545 PendUpdates.erase(I);
549 PendUpdates.push_back(Update); // Save the valid update.
553 /// Performs all pending basic block deletions. We have to defer the deletion
554 /// of these blocks until after the DominatorTree updates are applied. The
555 /// internal workings of the DominatorTree code expect every update's From
556 /// and To blocks to exist and to be a member of the same Function.
557 bool DeferredDominance::flushDelBB() {
558 if (DeletedBBs.empty())
560 for (auto *BB : DeletedBBs)
561 BB->eraseFromParent();