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 static bool VerifyLoopInfo = true;
34 static 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 INITIALIZE_PASS(LoopInfo, "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))
52 return true; // All non-instructions are loop invariant
55 /// hasLoopInvariantOperands - Return true if all the operands of the
56 /// specified instruction are loop invariant.
57 bool Loop::hasLoopInvariantOperands(Instruction *I) const {
58 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
59 if (!isLoopInvariant(I->getOperand(i)))
65 /// makeLoopInvariant - If the given value is an instruciton inside of the
66 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
67 /// Return true if the value after any hoisting is loop invariant. This
68 /// function can be used as a slightly more aggressive replacement for
71 /// If InsertPt is specified, it is the point to hoist instructions to.
72 /// If null, the terminator of the loop preheader is used.
74 bool Loop::makeLoopInvariant(Value *V, bool &Changed,
75 Instruction *InsertPt) const {
76 if (Instruction *I = dyn_cast<Instruction>(V))
77 return makeLoopInvariant(I, Changed, InsertPt);
78 return true; // All non-instructions are loop-invariant.
81 /// makeLoopInvariant - If the given instruction is inside of the
82 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
83 /// Return true if the instruction after any hoisting is loop invariant. This
84 /// function can be used as a slightly more aggressive replacement for
87 /// If InsertPt is specified, it is the point to hoist instructions to.
88 /// If null, the terminator of the loop preheader is used.
90 bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
91 Instruction *InsertPt) const {
92 // Test if the value is already loop-invariant.
93 if (isLoopInvariant(I))
95 if (!I->isSafeToSpeculativelyExecute())
97 if (I->mayReadFromMemory())
99 // Determine the insertion point, unless one was given.
101 BasicBlock *Preheader = getLoopPreheader();
102 // Without a preheader, hoisting is not feasible.
105 InsertPt = Preheader->getTerminator();
107 // Don't hoist instructions with loop-variant operands.
108 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
109 if (!makeLoopInvariant(I->getOperand(i), Changed, InsertPt))
113 I->moveBefore(InsertPt);
118 /// getCanonicalInductionVariable - Check to see if the loop has a canonical
119 /// induction variable: an integer recurrence that starts at 0 and increments
120 /// by one each time through the loop. If so, return the phi node that
121 /// corresponds to it.
123 /// The IndVarSimplify pass transforms loops to have a canonical induction
126 PHINode *Loop::getCanonicalInductionVariable() const {
127 BasicBlock *H = getHeader();
129 BasicBlock *Incoming = 0, *Backedge = 0;
130 pred_iterator PI = pred_begin(H);
131 assert(PI != pred_end(H) &&
132 "Loop must have at least one backedge!");
134 if (PI == pred_end(H)) return 0; // dead loop
136 if (PI != pred_end(H)) return 0; // multiple backedges?
138 if (contains(Incoming)) {
139 if (contains(Backedge))
141 std::swap(Incoming, Backedge);
142 } else if (!contains(Backedge))
145 // Loop over all of the PHI nodes, looking for a canonical indvar.
146 for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
147 PHINode *PN = cast<PHINode>(I);
148 if (ConstantInt *CI =
149 dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
150 if (CI->isNullValue())
151 if (Instruction *Inc =
152 dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
153 if (Inc->getOpcode() == Instruction::Add &&
154 Inc->getOperand(0) == PN)
155 if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
156 if (CI->equalsInt(1))
162 /// getTripCount - Return a loop-invariant LLVM value indicating the number of
163 /// times the loop will be executed. Note that this means that the backedge
164 /// of the loop executes N-1 times. If the trip-count cannot be determined,
165 /// this returns null.
167 /// The IndVarSimplify pass transforms loops to have a form that this
168 /// function easily understands.
170 Value *Loop::getTripCount() const {
171 // Canonical loops will end with a 'cmp ne I, V', where I is the incremented
172 // canonical induction variable and V is the trip count of the loop.
173 PHINode *IV = getCanonicalInductionVariable();
174 if (IV == 0 || IV->getNumIncomingValues() != 2) return 0;
176 bool P0InLoop = contains(IV->getIncomingBlock(0));
177 Value *Inc = IV->getIncomingValue(!P0InLoop);
178 BasicBlock *BackedgeBlock = IV->getIncomingBlock(!P0InLoop);
180 if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator()))
181 if (BI->isConditional()) {
182 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
183 if (ICI->getOperand(0) == Inc) {
184 if (BI->getSuccessor(0) == getHeader()) {
185 if (ICI->getPredicate() == ICmpInst::ICMP_NE)
186 return ICI->getOperand(1);
187 } else if (ICI->getPredicate() == ICmpInst::ICMP_EQ) {
188 return ICI->getOperand(1);
197 /// getSmallConstantTripCount - Returns the trip count of this loop as a
198 /// normal unsigned value, if possible. Returns 0 if the trip count is unknown
199 /// of not constant. Will also return 0 if the trip count is very large
201 unsigned Loop::getSmallConstantTripCount() const {
202 Value* TripCount = this->getTripCount();
204 if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCount)) {
205 // Guard against huge trip counts.
206 if (TripCountC->getValue().getActiveBits() <= 32) {
207 return (unsigned)TripCountC->getZExtValue();
214 /// getSmallConstantTripMultiple - Returns the largest constant divisor of the
215 /// trip count of this loop as a normal unsigned value, if possible. This
216 /// means that the actual trip count is always a multiple of the returned
217 /// value (don't forget the trip count could very well be zero as well!).
219 /// Returns 1 if the trip count is unknown or not guaranteed to be the
220 /// multiple of a constant (which is also the case if the trip count is simply
221 /// constant, use getSmallConstantTripCount for that case), Will also return 1
222 /// if the trip count is very large (>= 2^32).
223 unsigned Loop::getSmallConstantTripMultiple() const {
224 Value* TripCount = this->getTripCount();
225 // This will hold the ConstantInt result, if any
226 ConstantInt *Result = NULL;
228 // See if the trip count is constant itself
229 Result = dyn_cast<ConstantInt>(TripCount);
230 // if not, see if it is a multiplication
232 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCount)) {
233 switch (BO->getOpcode()) {
234 case BinaryOperator::Mul:
235 Result = dyn_cast<ConstantInt>(BO->getOperand(1));
237 case BinaryOperator::Shl:
238 if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1)))
239 if (CI->getValue().getActiveBits() <= 5)
240 return 1u << CI->getZExtValue();
247 // Guard against huge trip counts.
248 if (Result && Result->getValue().getActiveBits() <= 32) {
249 return (unsigned)Result->getZExtValue();
255 /// isLCSSAForm - Return true if the Loop is in LCSSA form
256 bool Loop::isLCSSAForm(DominatorTree &DT) const {
257 // Sort the blocks vector so that we can use binary search to do quick
259 SmallPtrSet<BasicBlock*, 16> LoopBBs(block_begin(), block_end());
261 for (block_iterator BI = block_begin(), E = block_end(); BI != E; ++BI) {
262 BasicBlock *BB = *BI;
263 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;++I)
264 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
267 BasicBlock *UserBB = cast<Instruction>(U)->getParent();
268 if (PHINode *P = dyn_cast<PHINode>(U))
269 UserBB = P->getIncomingBlock(UI);
271 // Check the current block, as a fast-path, before checking whether
272 // the use is anywhere in the loop. Most values are used in the same
273 // block they are defined in. Also, blocks not reachable from the
274 // entry are special; uses in them don't need to go through PHIs.
276 !LoopBBs.count(UserBB) &&
277 DT.isReachableFromEntry(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 exits are in canonical form.
318 Loop::getUniqueExitBlocks(SmallVectorImpl<BasicBlock *> &ExitBlocks) const {
319 assert(hasDedicatedExits() &&
320 "getUniqueExitBlocks assumes the loop has canonical form exits!");
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 for (succ_iterator I = succ_begin(*BI), E = succ_end(*BI); I != E; ++I) {
335 // If block is inside the loop then it is not a exit block.
336 if (std::binary_search(LoopBBs.begin(), LoopBBs.end(), *I))
339 pred_iterator PI = pred_begin(*I);
340 BasicBlock *firstPred = *PI;
342 // If current basic block is this exit block's first predecessor
343 // then only insert exit block in to the output ExitBlocks vector.
344 // This ensures that same exit block is not inserted twice into
345 // ExitBlocks vector.
346 if (current != firstPred)
349 // If a terminator has more then two successors, for example SwitchInst,
350 // then it is possible that there are multiple edges from current block
351 // to one exit block.
352 if (std::distance(succ_begin(current), succ_end(current)) <= 2) {
353 ExitBlocks.push_back(*I);
357 // In case of multiple edges from current block to exit block, collect
358 // only one edge in ExitBlocks. Use switchExitBlocks to keep track of
360 if (std::find(switchExitBlocks.begin(), switchExitBlocks.end(), *I)
361 == switchExitBlocks.end()) {
362 switchExitBlocks.push_back(*I);
363 ExitBlocks.push_back(*I);
369 /// getUniqueExitBlock - If getUniqueExitBlocks would return exactly one
370 /// block, return that block. Otherwise return null.
371 BasicBlock *Loop::getUniqueExitBlock() const {
372 SmallVector<BasicBlock *, 8> UniqueExitBlocks;
373 getUniqueExitBlocks(UniqueExitBlocks);
374 if (UniqueExitBlocks.size() == 1)
375 return UniqueExitBlocks[0];
379 void Loop::dump() const {
383 //===----------------------------------------------------------------------===//
384 // LoopInfo implementation
386 bool LoopInfo::runOnFunction(Function &) {
388 LI.Calculate(getAnalysis<DominatorTree>().getBase()); // Update
392 void LoopInfo::verifyAnalysis() const {
393 // LoopInfo is a FunctionPass, but verifying every loop in the function
394 // each time verifyAnalysis is called is very expensive. The
395 // -verify-loop-info option can enable this. In order to perform some
396 // checking by default, LoopPass has been taught to call verifyLoop
397 // manually during loop pass sequences.
399 if (!VerifyLoopInfo) return;
401 for (iterator I = begin(), E = end(); I != E; ++I) {
402 assert(!(*I)->getParentLoop() && "Top-level loop has a parent!");
403 (*I)->verifyLoopNest();
406 // TODO: check BBMap consistency.
409 void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
410 AU.setPreservesAll();
411 AU.addRequired<DominatorTree>();
414 void LoopInfo::print(raw_ostream &OS, const Module*) const {