1 //===- LoopInfo.cpp - Natural Loop Calculator -----------------------------===//
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 defines the LoopInfo class that is used to identify natural loops
11 // and determine the loop depth of various nodes of the CFG. Note that the
12 // loops identified may actually be several natural loops that share the same
13 // header node... not just a single natural loop.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Analysis/LoopInfo.h"
18 #include "llvm/Constants.h"
19 #include "llvm/Instructions.h"
20 #include "llvm/Analysis/Dominators.h"
21 #include "llvm/Assembly/Writer.h"
22 #include "llvm/Support/CFG.h"
23 #include "llvm/Support/CommandLine.h"
24 #include "llvm/ADT/DepthFirstIterator.h"
25 #include "llvm/ADT/SmallPtrSet.h"
29 // Always verify loopinfo if expensive checking is enabled.
31 bool VerifyLoopInfo = true;
33 bool VerifyLoopInfo = false;
35 static cl::opt<bool,true>
36 VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
37 cl::desc("Verify loop info (time consuming)"));
39 char LoopInfo::ID = 0;
40 static RegisterPass<LoopInfo>
41 X("loops", "Natural Loop Information", true, true);
43 //===----------------------------------------------------------------------===//
44 // Loop implementation
47 /// isLoopInvariant - Return true if the specified value is loop invariant
49 bool Loop::isLoopInvariant(Value *V) const {
50 if (Instruction *I = dyn_cast<Instruction>(V))
51 return isLoopInvariant(I);
52 return true; // All non-instructions are loop invariant
55 /// isLoopInvariant - Return true if the specified instruction is
58 bool Loop::isLoopInvariant(Instruction *I) const {
59 return !contains(I->getParent());
62 /// makeLoopInvariant - If the given value is an instruciton inside of the
63 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
64 /// Return true if the value after any hoisting is loop invariant. This
65 /// function can be used as a slightly more aggressive replacement for
68 /// If InsertPt is specified, it is the point to hoist instructions to.
69 /// If null, the terminator of the loop preheader is used.
71 bool Loop::makeLoopInvariant(Value *V, bool &Changed,
72 Instruction *InsertPt) const {
73 if (Instruction *I = dyn_cast<Instruction>(V))
74 return makeLoopInvariant(I, Changed, InsertPt);
75 return true; // All non-instructions are loop-invariant.
78 /// makeLoopInvariant - If the given instruction is inside of the
79 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
80 /// Return true if the instruction after any hoisting is loop invariant. This
81 /// function can be used as a slightly more aggressive replacement for
84 /// If InsertPt is specified, it is the point to hoist instructions to.
85 /// If null, the terminator of the loop preheader is used.
87 bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
88 Instruction *InsertPt) const {
89 // Test if the value is already loop-invariant.
90 if (isLoopInvariant(I))
92 if (!I->isSafeToSpeculativelyExecute())
94 if (I->mayReadFromMemory())
96 // Determine the insertion point, unless one was given.
98 BasicBlock *Preheader = getLoopPreheader();
99 // Without a preheader, hoisting is not feasible.
102 InsertPt = Preheader->getTerminator();
104 // Don't hoist instructions with loop-variant operands.
105 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
106 if (!makeLoopInvariant(I->getOperand(i), Changed, InsertPt))
109 I->moveBefore(InsertPt);
114 /// getCanonicalInductionVariable - Check to see if the loop has a canonical
115 /// induction variable: an integer recurrence that starts at 0 and increments
116 /// by one each time through the loop. If so, return the phi node that
117 /// corresponds to it.
119 /// The IndVarSimplify pass transforms loops to have a canonical induction
122 PHINode *Loop::getCanonicalInductionVariable() const {
123 BasicBlock *H = getHeader();
125 BasicBlock *Incoming = 0, *Backedge = 0;
126 typedef GraphTraits<Inverse<BasicBlock*> > InvBlockTraits;
127 InvBlockTraits::ChildIteratorType PI = InvBlockTraits::child_begin(H);
128 assert(PI != InvBlockTraits::child_end(H) &&
129 "Loop must have at least one backedge!");
131 if (PI == InvBlockTraits::child_end(H)) return 0; // dead loop
133 if (PI != InvBlockTraits::child_end(H)) return 0; // multiple backedges?
135 if (contains(Incoming)) {
136 if (contains(Backedge))
138 std::swap(Incoming, Backedge);
139 } else if (!contains(Backedge))
142 // Loop over all of the PHI nodes, looking for a canonical indvar.
143 for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
144 PHINode *PN = cast<PHINode>(I);
145 if (ConstantInt *CI =
146 dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
147 if (CI->isNullValue())
148 if (Instruction *Inc =
149 dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
150 if (Inc->getOpcode() == Instruction::Add &&
151 Inc->getOperand(0) == PN)
152 if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
153 if (CI->equalsInt(1))
159 /// getCanonicalInductionVariableIncrement - Return the LLVM value that holds
160 /// the canonical induction variable value for the "next" iteration of the
161 /// loop. This always succeeds if getCanonicalInductionVariable succeeds.
163 Instruction *Loop::getCanonicalInductionVariableIncrement() const {
164 if (PHINode *PN = getCanonicalInductionVariable()) {
165 bool P1InLoop = contains(PN->getIncomingBlock(1));
166 return cast<Instruction>(PN->getIncomingValue(P1InLoop));
171 /// getTripCount - Return a loop-invariant LLVM value indicating the number of
172 /// times the loop will be executed. Note that this means that the backedge
173 /// of the loop executes N-1 times. If the trip-count cannot be determined,
174 /// this returns null.
176 /// The IndVarSimplify pass transforms loops to have a form that this
177 /// function easily understands.
179 Value *Loop::getTripCount() const {
180 // Canonical loops will end with a 'cmp ne I, V', where I is the incremented
181 // canonical induction variable and V is the trip count of the loop.
182 Instruction *Inc = getCanonicalInductionVariableIncrement();
183 if (Inc == 0) return 0;
184 PHINode *IV = cast<PHINode>(Inc->getOperand(0));
186 BasicBlock *BackedgeBlock =
187 IV->getIncomingBlock(contains(IV->getIncomingBlock(1)));
189 if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator()))
190 if (BI->isConditional()) {
191 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
192 if (ICI->getOperand(0) == Inc) {
193 if (BI->getSuccessor(0) == getHeader()) {
194 if (ICI->getPredicate() == ICmpInst::ICMP_NE)
195 return ICI->getOperand(1);
196 } else if (ICI->getPredicate() == ICmpInst::ICMP_EQ) {
197 return ICI->getOperand(1);
206 /// getSmallConstantTripCount - Returns the trip count of this loop as a
207 /// normal unsigned value, if possible. Returns 0 if the trip count is unknown
208 /// of not constant. Will also return 0 if the trip count is very large
210 unsigned Loop::getSmallConstantTripCount() const {
211 Value* TripCount = this->getTripCount();
213 if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCount)) {
214 // Guard against huge trip counts.
215 if (TripCountC->getValue().getActiveBits() <= 32) {
216 return (unsigned)TripCountC->getZExtValue();
223 /// getSmallConstantTripMultiple - Returns the largest constant divisor of the
224 /// trip count of this loop as a normal unsigned value, if possible. This
225 /// means that the actual trip count is always a multiple of the returned
226 /// value (don't forget the trip count could very well be zero as well!).
228 /// Returns 1 if the trip count is unknown or not guaranteed to be the
229 /// multiple of a constant (which is also the case if the trip count is simply
230 /// constant, use getSmallConstantTripCount for that case), Will also return 1
231 /// if the trip count is very large (>= 2^32).
232 unsigned Loop::getSmallConstantTripMultiple() const {
233 Value* TripCount = this->getTripCount();
234 // This will hold the ConstantInt result, if any
235 ConstantInt *Result = NULL;
237 // See if the trip count is constant itself
238 Result = dyn_cast<ConstantInt>(TripCount);
239 // if not, see if it is a multiplication
241 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCount)) {
242 switch (BO->getOpcode()) {
243 case BinaryOperator::Mul:
244 Result = dyn_cast<ConstantInt>(BO->getOperand(1));
251 // Guard against huge trip counts.
252 if (Result && Result->getValue().getActiveBits() <= 32) {
253 return (unsigned)Result->getZExtValue();
259 /// isLCSSAForm - Return true if the Loop is in LCSSA form
260 bool Loop::isLCSSAForm() const {
261 // Sort the blocks vector so that we can use binary search to do quick
263 SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
265 for (block_iterator BI = block_begin(), E = block_end(); BI != E; ++BI) {
266 BasicBlock *BB = *BI;
267 for (BasicBlock ::iterator I = BB->begin(), E = BB->end(); I != E;++I)
268 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
270 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
271 if (PHINode *P = dyn_cast<PHINode>(*UI)) {
272 UserBB = P->getIncomingBlock(UI);
275 // Check the current block, as a fast-path. Most values are used in
276 // the same block they are defined in.
277 if (UserBB != BB && !LoopBBs.count(UserBB))
285 /// isLoopSimplifyForm - Return true if the Loop is in the form that
286 /// the LoopSimplify form transforms loops to, which is sometimes called
288 bool Loop::isLoopSimplifyForm() const {
289 // Normal-form loops have a preheader, a single backedge, and all of their
290 // exits have all their predecessors inside the loop.
291 return getLoopPreheader() && getLoopLatch() && hasDedicatedExits();
294 /// hasDedicatedExits - Return true if no exit block for the loop
295 /// has a predecessor that is outside the loop.
296 bool Loop::hasDedicatedExits() const {
297 // Sort the blocks vector so that we can use binary search to do quick
299 SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
300 // Each predecessor of each exit block of a normal loop is contained
302 SmallVector<BasicBlock *, 4> ExitBlocks;
303 getExitBlocks(ExitBlocks);
304 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
305 for (pred_iterator PI = pred_begin(ExitBlocks[i]),
306 PE = pred_end(ExitBlocks[i]); PI != PE; ++PI)
307 if (!LoopBBs.count(*PI))
309 // All the requirements are met.
313 /// getUniqueExitBlocks - Return all unique successor blocks of this loop.
314 /// These are the blocks _outside of the current loop_ which are branched to.
315 /// This assumes that loop is in canonical form.
318 Loop::getUniqueExitBlocks(SmallVectorImpl<BasicBlock *> &ExitBlocks) const {
319 assert(isLoopSimplifyForm() &&
320 "getUniqueExitBlocks assumes the loop is in canonical form!");
322 // Sort the blocks vector so that we can use binary search to do quick
324 SmallVector<BasicBlock *, 128> LoopBBs(block_begin(), block_end());
325 std::sort(LoopBBs.begin(), LoopBBs.end());
327 SmallVector<BasicBlock *, 32> switchExitBlocks;
329 for (block_iterator BI = block_begin(), BE = block_end(); BI != BE; ++BI) {
331 BasicBlock *current = *BI;
332 switchExitBlocks.clear();
334 typedef GraphTraits<BasicBlock *> BlockTraits;
335 typedef GraphTraits<Inverse<BasicBlock *> > InvBlockTraits;
336 for (BlockTraits::ChildIteratorType I =
337 BlockTraits::child_begin(*BI), E = BlockTraits::child_end(*BI);
339 // If block is inside the loop then it is not a exit block.
340 if (std::binary_search(LoopBBs.begin(), LoopBBs.end(), *I))
343 InvBlockTraits::ChildIteratorType PI = InvBlockTraits::child_begin(*I);
344 BasicBlock *firstPred = *PI;
346 // If current basic block is this exit block's first predecessor
347 // then only insert exit block in to the output ExitBlocks vector.
348 // This ensures that same exit block is not inserted twice into
349 // ExitBlocks vector.
350 if (current != firstPred)
353 // If a terminator has more then two successors, for example SwitchInst,
354 // then it is possible that there are multiple edges from current block
355 // to one exit block.
356 if (std::distance(BlockTraits::child_begin(current),
357 BlockTraits::child_end(current)) <= 2) {
358 ExitBlocks.push_back(*I);
362 // In case of multiple edges from current block to exit block, collect
363 // only one edge in ExitBlocks. Use switchExitBlocks to keep track of
365 if (std::find(switchExitBlocks.begin(), switchExitBlocks.end(), *I)
366 == switchExitBlocks.end()) {
367 switchExitBlocks.push_back(*I);
368 ExitBlocks.push_back(*I);
374 /// getUniqueExitBlock - If getUniqueExitBlocks would return exactly one
375 /// block, return that block. Otherwise return null.
376 BasicBlock *Loop::getUniqueExitBlock() const {
377 SmallVector<BasicBlock *, 8> UniqueExitBlocks;
378 getUniqueExitBlocks(UniqueExitBlocks);
379 if (UniqueExitBlocks.size() == 1)
380 return UniqueExitBlocks[0];
384 //===----------------------------------------------------------------------===//
385 // LoopInfo implementation
387 bool LoopInfo::runOnFunction(Function &) {
389 LI.Calculate(getAnalysis<DominatorTree>().getBase()); // Update
393 void LoopInfo::verifyAnalysis() const {
394 // LoopInfo is a FunctionPass, but verifying every loop in the function
395 // each time verifyAnalysis is called is very expensive. The
396 // -verify-loop-info option can enable this. In order to perform some
397 // checking by default, LoopPass has been taught to call verifyLoop
398 // manually during loop pass sequences.
400 if (!VerifyLoopInfo) return;
402 for (iterator I = begin(), E = end(); I != E; ++I) {
403 assert(!(*I)->getParentLoop() && "Top-level loop has a parent!");
404 (*I)->verifyLoopNest();
407 // TODO: check BBMap consistency.
410 void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
411 AU.setPreservesAll();
412 AU.addRequired<DominatorTree>();
415 void LoopInfo::print(raw_ostream &OS, const Module*) const {