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/Support/Debug.h"
25 #include "llvm/ADT/DepthFirstIterator.h"
26 #include "llvm/ADT/SmallPtrSet.h"
30 // Always verify loopinfo if expensive checking is enabled.
32 bool VerifyLoopInfo = true;
34 bool VerifyLoopInfo = false;
36 static cl::opt<bool,true>
37 VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
38 cl::desc("Verify loop info (time consuming)"));
40 char LoopInfo::ID = 0;
41 static RegisterPass<LoopInfo>
42 X("loops", "Natural Loop Information", true, true);
44 //===----------------------------------------------------------------------===//
45 // Loop implementation
48 /// isLoopInvariant - Return true if the specified value is loop invariant
50 bool Loop::isLoopInvariant(Value *V) const {
51 if (Instruction *I = dyn_cast<Instruction>(V))
52 return isLoopInvariant(I);
53 return true; // All non-instructions are loop invariant
56 /// isLoopInvariant - Return true if the specified instruction is
59 bool Loop::isLoopInvariant(Instruction *I) const {
63 /// makeLoopInvariant - If the given value is an instruciton inside of the
64 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
65 /// Return true if the value after any hoisting is loop invariant. This
66 /// function can be used as a slightly more aggressive replacement for
69 /// If InsertPt is specified, it is the point to hoist instructions to.
70 /// If null, the terminator of the loop preheader is used.
72 bool Loop::makeLoopInvariant(Value *V, bool &Changed,
73 Instruction *InsertPt) const {
74 if (Instruction *I = dyn_cast<Instruction>(V))
75 return makeLoopInvariant(I, Changed, InsertPt);
76 return true; // All non-instructions are loop-invariant.
79 /// makeLoopInvariant - If the given instruction is inside of the
80 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
81 /// Return true if the instruction after any hoisting is loop invariant. This
82 /// function can be used as a slightly more aggressive replacement for
85 /// If InsertPt is specified, it is the point to hoist instructions to.
86 /// If null, the terminator of the loop preheader is used.
88 bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
89 Instruction *InsertPt) const {
90 // Test if the value is already loop-invariant.
91 if (isLoopInvariant(I))
93 if (!I->isSafeToSpeculativelyExecute())
95 if (I->mayReadFromMemory())
97 // Determine the insertion point, unless one was given.
99 BasicBlock *Preheader = getLoopPreheader();
100 // Without a preheader, hoisting is not feasible.
103 InsertPt = Preheader->getTerminator();
105 // Don't hoist instructions with loop-variant operands.
106 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
107 if (!makeLoopInvariant(I->getOperand(i), Changed, InsertPt))
110 I->moveBefore(InsertPt);
115 /// getCanonicalInductionVariable - Check to see if the loop has a canonical
116 /// induction variable: an integer recurrence that starts at 0 and increments
117 /// by one each time through the loop. If so, return the phi node that
118 /// corresponds to it.
120 /// The IndVarSimplify pass transforms loops to have a canonical induction
123 PHINode *Loop::getCanonicalInductionVariable() const {
124 BasicBlock *H = getHeader();
126 BasicBlock *Incoming = 0, *Backedge = 0;
127 typedef GraphTraits<Inverse<BasicBlock*> > InvBlockTraits;
128 InvBlockTraits::ChildIteratorType PI = InvBlockTraits::child_begin(H);
129 assert(PI != InvBlockTraits::child_end(H) &&
130 "Loop must have at least one backedge!");
132 if (PI == InvBlockTraits::child_end(H)) return 0; // dead loop
134 if (PI != InvBlockTraits::child_end(H)) return 0; // multiple backedges?
136 if (contains(Incoming)) {
137 if (contains(Backedge))
139 std::swap(Incoming, Backedge);
140 } else if (!contains(Backedge))
143 // Loop over all of the PHI nodes, looking for a canonical indvar.
144 for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
145 PHINode *PN = cast<PHINode>(I);
146 if (ConstantInt *CI =
147 dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
148 if (CI->isNullValue())
149 if (Instruction *Inc =
150 dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
151 if (Inc->getOpcode() == Instruction::Add &&
152 Inc->getOperand(0) == PN)
153 if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
154 if (CI->equalsInt(1))
160 /// getCanonicalInductionVariableIncrement - Return the LLVM value that holds
161 /// the canonical induction variable value for the "next" iteration of the
162 /// loop. This always succeeds if getCanonicalInductionVariable succeeds.
164 Instruction *Loop::getCanonicalInductionVariableIncrement() const {
165 if (PHINode *PN = getCanonicalInductionVariable()) {
166 bool P1InLoop = contains(PN->getIncomingBlock(1));
167 return cast<Instruction>(PN->getIncomingValue(P1InLoop));
172 /// getTripCount - Return a loop-invariant LLVM value indicating the number of
173 /// times the loop will be executed. Note that this means that the backedge
174 /// of the loop executes N-1 times. If the trip-count cannot be determined,
175 /// this returns null.
177 /// The IndVarSimplify pass transforms loops to have a form that this
178 /// function easily understands.
180 Value *Loop::getTripCount() const {
181 // Canonical loops will end with a 'cmp ne I, V', where I is the incremented
182 // canonical induction variable and V is the trip count of the loop.
183 Instruction *Inc = getCanonicalInductionVariableIncrement();
184 if (Inc == 0) return 0;
185 PHINode *IV = cast<PHINode>(Inc->getOperand(0));
187 BasicBlock *BackedgeBlock =
188 IV->getIncomingBlock(contains(IV->getIncomingBlock(1)));
190 if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator()))
191 if (BI->isConditional()) {
192 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
193 if (ICI->getOperand(0) == Inc) {
194 if (BI->getSuccessor(0) == getHeader()) {
195 if (ICI->getPredicate() == ICmpInst::ICMP_NE)
196 return ICI->getOperand(1);
197 } else if (ICI->getPredicate() == ICmpInst::ICMP_EQ) {
198 return ICI->getOperand(1);
207 /// getSmallConstantTripCount - Returns the trip count of this loop as a
208 /// normal unsigned value, if possible. Returns 0 if the trip count is unknown
209 /// of not constant. Will also return 0 if the trip count is very large
211 unsigned Loop::getSmallConstantTripCount() const {
212 Value* TripCount = this->getTripCount();
214 if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCount)) {
215 // Guard against huge trip counts.
216 if (TripCountC->getValue().getActiveBits() <= 32) {
217 return (unsigned)TripCountC->getZExtValue();
224 /// getSmallConstantTripMultiple - Returns the largest constant divisor of the
225 /// trip count of this loop as a normal unsigned value, if possible. This
226 /// means that the actual trip count is always a multiple of the returned
227 /// value (don't forget the trip count could very well be zero as well!).
229 /// Returns 1 if the trip count is unknown or not guaranteed to be the
230 /// multiple of a constant (which is also the case if the trip count is simply
231 /// constant, use getSmallConstantTripCount for that case), Will also return 1
232 /// if the trip count is very large (>= 2^32).
233 unsigned Loop::getSmallConstantTripMultiple() const {
234 Value* TripCount = this->getTripCount();
235 // This will hold the ConstantInt result, if any
236 ConstantInt *Result = NULL;
238 // See if the trip count is constant itself
239 Result = dyn_cast<ConstantInt>(TripCount);
240 // if not, see if it is a multiplication
242 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCount)) {
243 switch (BO->getOpcode()) {
244 case BinaryOperator::Mul:
245 Result = dyn_cast<ConstantInt>(BO->getOperand(1));
247 case BinaryOperator::Shl:
248 if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1)))
249 if (CI->getValue().getActiveBits() <= 5)
250 return 1u << CI->getZExtValue();
257 // Guard against huge trip counts.
258 if (Result && Result->getValue().getActiveBits() <= 32) {
259 return (unsigned)Result->getZExtValue();
265 /// isLCSSAForm - Return true if the Loop is in LCSSA form
266 bool Loop::isLCSSAForm(DominatorTree &DT) const {
267 // Sort the blocks vector so that we can use binary search to do quick
269 SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
271 for (block_iterator BI = block_begin(), E = block_end(); BI != E; ++BI) {
272 BasicBlock *BB = *BI;
273 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;++I)
274 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
276 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
277 if (PHINode *P = dyn_cast<PHINode>(*UI))
278 UserBB = P->getIncomingBlock(UI);
280 // Check the current block, as a fast-path, before checking whether
281 // the use is anywhere in the loop. Most values are used in the same
282 // block they are defined in. Also, blocks not reachable from the
283 // entry are special; uses in them don't need to go through PHIs.
285 !LoopBBs.count(UserBB) &&
286 DT.isReachableFromEntry(UserBB))
294 /// isLoopSimplifyForm - Return true if the Loop is in the form that
295 /// the LoopSimplify form transforms loops to, which is sometimes called
297 bool Loop::isLoopSimplifyForm() const {
298 // Normal-form loops have a preheader, a single backedge, and all of their
299 // exits have all their predecessors inside the loop.
300 return getLoopPreheader() && getLoopLatch() && hasDedicatedExits();
303 /// hasDedicatedExits - Return true if no exit block for the loop
304 /// has a predecessor that is outside the loop.
305 bool Loop::hasDedicatedExits() const {
306 // Sort the blocks vector so that we can use binary search to do quick
308 SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
309 // Each predecessor of each exit block of a normal loop is contained
311 SmallVector<BasicBlock *, 4> ExitBlocks;
312 getExitBlocks(ExitBlocks);
313 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
314 for (pred_iterator PI = pred_begin(ExitBlocks[i]),
315 PE = pred_end(ExitBlocks[i]); PI != PE; ++PI)
316 if (!LoopBBs.count(*PI))
318 // All the requirements are met.
322 /// getUniqueExitBlocks - Return all unique successor blocks of this loop.
323 /// These are the blocks _outside of the current loop_ which are branched to.
324 /// This assumes that loop exits are in canonical form.
327 Loop::getUniqueExitBlocks(SmallVectorImpl<BasicBlock *> &ExitBlocks) const {
328 assert(hasDedicatedExits() &&
329 "getUniqueExitBlocks assumes the loop has canonical form exits!");
331 // Sort the blocks vector so that we can use binary search to do quick
333 SmallVector<BasicBlock *, 128> LoopBBs(block_begin(), block_end());
334 std::sort(LoopBBs.begin(), LoopBBs.end());
336 SmallVector<BasicBlock *, 32> switchExitBlocks;
338 for (block_iterator BI = block_begin(), BE = block_end(); BI != BE; ++BI) {
340 BasicBlock *current = *BI;
341 switchExitBlocks.clear();
343 typedef GraphTraits<BasicBlock *> BlockTraits;
344 typedef GraphTraits<Inverse<BasicBlock *> > InvBlockTraits;
345 for (BlockTraits::ChildIteratorType I =
346 BlockTraits::child_begin(*BI), E = BlockTraits::child_end(*BI);
348 // If block is inside the loop then it is not a exit block.
349 if (std::binary_search(LoopBBs.begin(), LoopBBs.end(), *I))
352 InvBlockTraits::ChildIteratorType PI = InvBlockTraits::child_begin(*I);
353 BasicBlock *firstPred = *PI;
355 // If current basic block is this exit block's first predecessor
356 // then only insert exit block in to the output ExitBlocks vector.
357 // This ensures that same exit block is not inserted twice into
358 // ExitBlocks vector.
359 if (current != firstPred)
362 // If a terminator has more then two successors, for example SwitchInst,
363 // then it is possible that there are multiple edges from current block
364 // to one exit block.
365 if (std::distance(BlockTraits::child_begin(current),
366 BlockTraits::child_end(current)) <= 2) {
367 ExitBlocks.push_back(*I);
371 // In case of multiple edges from current block to exit block, collect
372 // only one edge in ExitBlocks. Use switchExitBlocks to keep track of
374 if (std::find(switchExitBlocks.begin(), switchExitBlocks.end(), *I)
375 == switchExitBlocks.end()) {
376 switchExitBlocks.push_back(*I);
377 ExitBlocks.push_back(*I);
383 /// getUniqueExitBlock - If getUniqueExitBlocks would return exactly one
384 /// block, return that block. Otherwise return null.
385 BasicBlock *Loop::getUniqueExitBlock() const {
386 SmallVector<BasicBlock *, 8> UniqueExitBlocks;
387 getUniqueExitBlocks(UniqueExitBlocks);
388 if (UniqueExitBlocks.size() == 1)
389 return UniqueExitBlocks[0];
393 void Loop::dump() const {
397 //===----------------------------------------------------------------------===//
398 // LoopInfo implementation
400 bool LoopInfo::runOnFunction(Function &) {
402 LI.Calculate(getAnalysis<DominatorTree>().getBase()); // Update
406 void LoopInfo::verifyAnalysis() const {
407 // LoopInfo is a FunctionPass, but verifying every loop in the function
408 // each time verifyAnalysis is called is very expensive. The
409 // -verify-loop-info option can enable this. In order to perform some
410 // checking by default, LoopPass has been taught to call verifyLoop
411 // manually during loop pass sequences.
413 if (!VerifyLoopInfo) return;
415 for (iterator I = begin(), E = end(); I != E; ++I) {
416 assert(!(*I)->getParentLoop() && "Top-level loop has a parent!");
417 (*I)->verifyLoopNest();
420 // TODO: check BBMap consistency.
423 void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
424 AU.setPreservesAll();
425 AU.addRequired<DominatorTree>();
428 void LoopInfo::print(raw_ostream &OS, const Module*) const {