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))
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 {
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 pred_iterator PI = pred_begin(H);
127 assert(PI != pred_end(H) &&
128 "Loop must have at least one backedge!");
130 if (PI == pred_end(H)) return 0; // dead loop
132 if (PI != pred_end(H)) return 0; // multiple backedges?
134 if (contains(Incoming)) {
135 if (contains(Backedge))
137 std::swap(Incoming, Backedge);
138 } else if (!contains(Backedge))
141 // Loop over all of the PHI nodes, looking for a canonical indvar.
142 for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
143 PHINode *PN = cast<PHINode>(I);
144 if (ConstantInt *CI =
145 dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
146 if (CI->isNullValue())
147 if (Instruction *Inc =
148 dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
149 if (Inc->getOpcode() == Instruction::Add &&
150 Inc->getOperand(0) == PN)
151 if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
152 if (CI->equalsInt(1))
158 /// getTripCount - Return a loop-invariant LLVM value indicating the number of
159 /// times the loop will be executed. Note that this means that the backedge
160 /// of the loop executes N-1 times. If the trip-count cannot be determined,
161 /// this returns null.
163 /// The IndVarSimplify pass transforms loops to have a form that this
164 /// function easily understands.
166 Value *Loop::getTripCount() const {
167 // Canonical loops will end with a 'cmp ne I, V', where I is the incremented
168 // canonical induction variable and V is the trip count of the loop.
169 PHINode *IV = getCanonicalInductionVariable();
170 if (IV == 0 || IV->getNumIncomingValues() != 2) return 0;
172 bool P0InLoop = contains(IV->getIncomingBlock(0));
173 Value *Inc = IV->getIncomingValue(!P0InLoop);
174 BasicBlock *BackedgeBlock = IV->getIncomingBlock(!P0InLoop);
176 if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator()))
177 if (BI->isConditional()) {
178 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
179 if (ICI->getOperand(0) == Inc) {
180 if (BI->getSuccessor(0) == getHeader()) {
181 if (ICI->getPredicate() == ICmpInst::ICMP_NE)
182 return ICI->getOperand(1);
183 } else if (ICI->getPredicate() == ICmpInst::ICMP_EQ) {
184 return ICI->getOperand(1);
193 /// getSmallConstantTripCount - Returns the trip count of this loop as a
194 /// normal unsigned value, if possible. Returns 0 if the trip count is unknown
195 /// of not constant. Will also return 0 if the trip count is very large
197 unsigned Loop::getSmallConstantTripCount() const {
198 Value* TripCount = this->getTripCount();
200 if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCount)) {
201 // Guard against huge trip counts.
202 if (TripCountC->getValue().getActiveBits() <= 32) {
203 return (unsigned)TripCountC->getZExtValue();
210 /// getSmallConstantTripMultiple - Returns the largest constant divisor of the
211 /// trip count of this loop as a normal unsigned value, if possible. This
212 /// means that the actual trip count is always a multiple of the returned
213 /// value (don't forget the trip count could very well be zero as well!).
215 /// Returns 1 if the trip count is unknown or not guaranteed to be the
216 /// multiple of a constant (which is also the case if the trip count is simply
217 /// constant, use getSmallConstantTripCount for that case), Will also return 1
218 /// if the trip count is very large (>= 2^32).
219 unsigned Loop::getSmallConstantTripMultiple() const {
220 Value* TripCount = this->getTripCount();
221 // This will hold the ConstantInt result, if any
222 ConstantInt *Result = NULL;
224 // See if the trip count is constant itself
225 Result = dyn_cast<ConstantInt>(TripCount);
226 // if not, see if it is a multiplication
228 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCount)) {
229 switch (BO->getOpcode()) {
230 case BinaryOperator::Mul:
231 Result = dyn_cast<ConstantInt>(BO->getOperand(1));
233 case BinaryOperator::Shl:
234 if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1)))
235 if (CI->getValue().getActiveBits() <= 5)
236 return 1u << CI->getZExtValue();
243 // Guard against huge trip counts.
244 if (Result && Result->getValue().getActiveBits() <= 32) {
245 return (unsigned)Result->getZExtValue();
251 /// isLCSSAForm - Return true if the Loop is in LCSSA form
252 bool Loop::isLCSSAForm(DominatorTree &DT) const {
253 // Sort the blocks vector so that we can use binary search to do quick
255 SmallPtrSet<BasicBlock*, 16> LoopBBs(block_begin(), block_end());
257 for (block_iterator BI = block_begin(), E = block_end(); BI != E; ++BI) {
258 BasicBlock *BB = *BI;
259 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;++I)
260 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
263 BasicBlock *UserBB = cast<Instruction>(U)->getParent();
264 if (PHINode *P = dyn_cast<PHINode>(U))
265 UserBB = P->getIncomingBlock(UI);
267 // Check the current block, as a fast-path, before checking whether
268 // the use is anywhere in the loop. Most values are used in the same
269 // block they are defined in. Also, blocks not reachable from the
270 // entry are special; uses in them don't need to go through PHIs.
272 !LoopBBs.count(UserBB) &&
273 DT.isReachableFromEntry(UserBB))
281 /// isLoopSimplifyForm - Return true if the Loop is in the form that
282 /// the LoopSimplify form transforms loops to, which is sometimes called
284 bool Loop::isLoopSimplifyForm() const {
285 // Normal-form loops have a preheader, a single backedge, and all of their
286 // exits have all their predecessors inside the loop.
287 return getLoopPreheader() && getLoopLatch() && hasDedicatedExits();
290 /// hasDedicatedExits - Return true if no exit block for the loop
291 /// has a predecessor that is outside the loop.
292 bool Loop::hasDedicatedExits() const {
293 // Sort the blocks vector so that we can use binary search to do quick
295 SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
296 // Each predecessor of each exit block of a normal loop is contained
298 SmallVector<BasicBlock *, 4> ExitBlocks;
299 getExitBlocks(ExitBlocks);
300 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
301 for (pred_iterator PI = pred_begin(ExitBlocks[i]),
302 PE = pred_end(ExitBlocks[i]); PI != PE; ++PI)
303 if (!LoopBBs.count(*PI))
305 // All the requirements are met.
309 /// getUniqueExitBlocks - Return all unique successor blocks of this loop.
310 /// These are the blocks _outside of the current loop_ which are branched to.
311 /// This assumes that loop exits are in canonical form.
314 Loop::getUniqueExitBlocks(SmallVectorImpl<BasicBlock *> &ExitBlocks) const {
315 assert(hasDedicatedExits() &&
316 "getUniqueExitBlocks assumes the loop has canonical form exits!");
318 // Sort the blocks vector so that we can use binary search to do quick
320 SmallVector<BasicBlock *, 128> LoopBBs(block_begin(), block_end());
321 std::sort(LoopBBs.begin(), LoopBBs.end());
323 SmallVector<BasicBlock *, 32> switchExitBlocks;
325 for (block_iterator BI = block_begin(), BE = block_end(); BI != BE; ++BI) {
327 BasicBlock *current = *BI;
328 switchExitBlocks.clear();
330 for (succ_iterator I = succ_begin(*BI), E = succ_end(*BI); I != E; ++I) {
331 // If block is inside the loop then it is not a exit block.
332 if (std::binary_search(LoopBBs.begin(), LoopBBs.end(), *I))
335 pred_iterator PI = pred_begin(*I);
336 BasicBlock *firstPred = *PI;
338 // If current basic block is this exit block's first predecessor
339 // then only insert exit block in to the output ExitBlocks vector.
340 // This ensures that same exit block is not inserted twice into
341 // ExitBlocks vector.
342 if (current != firstPred)
345 // If a terminator has more then two successors, for example SwitchInst,
346 // then it is possible that there are multiple edges from current block
347 // to one exit block.
348 if (std::distance(succ_begin(current), succ_end(current)) <= 2) {
349 ExitBlocks.push_back(*I);
353 // In case of multiple edges from current block to exit block, collect
354 // only one edge in ExitBlocks. Use switchExitBlocks to keep track of
356 if (std::find(switchExitBlocks.begin(), switchExitBlocks.end(), *I)
357 == switchExitBlocks.end()) {
358 switchExitBlocks.push_back(*I);
359 ExitBlocks.push_back(*I);
365 /// getUniqueExitBlock - If getUniqueExitBlocks would return exactly one
366 /// block, return that block. Otherwise return null.
367 BasicBlock *Loop::getUniqueExitBlock() const {
368 SmallVector<BasicBlock *, 8> UniqueExitBlocks;
369 getUniqueExitBlocks(UniqueExitBlocks);
370 if (UniqueExitBlocks.size() == 1)
371 return UniqueExitBlocks[0];
375 void Loop::dump() const {
379 //===----------------------------------------------------------------------===//
380 // LoopInfo implementation
382 bool LoopInfo::runOnFunction(Function &) {
384 LI.Calculate(getAnalysis<DominatorTree>().getBase()); // Update
388 void LoopInfo::verifyAnalysis() const {
389 // LoopInfo is a FunctionPass, but verifying every loop in the function
390 // each time verifyAnalysis is called is very expensive. The
391 // -verify-loop-info option can enable this. In order to perform some
392 // checking by default, LoopPass has been taught to call verifyLoop
393 // manually during loop pass sequences.
395 if (!VerifyLoopInfo) return;
397 for (iterator I = begin(), E = end(); I != E; ++I) {
398 assert(!(*I)->getParentLoop() && "Top-level loop has a parent!");
399 (*I)->verifyLoopNest();
402 // TODO: check BBMap consistency.
405 void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
406 AU.setPreservesAll();
407 AU.addRequired<DominatorTree>();
410 void LoopInfo::print(raw_ostream &OS, const Module*) const {