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/Streams.h"
24 #include "llvm/ADT/DepthFirstIterator.h"
25 #include "llvm/ADT/SmallPtrSet.h"
29 char LoopInfo::ID = 0;
30 static RegisterPass<LoopInfo>
31 X("loops", "Natural Loop Information", true, true);
33 //===----------------------------------------------------------------------===//
34 // Loop implementation
37 /// isLoopInvariant - Return true if the specified value is loop invariant
39 bool Loop::isLoopInvariant(Value *V) const {
40 if (Instruction *I = dyn_cast<Instruction>(V))
41 return !contains(I->getParent());
42 return true; // All non-instructions are loop invariant
45 /// getCanonicalInductionVariable - Check to see if the loop has a canonical
46 /// induction variable: an integer recurrence that starts at 0 and increments
47 /// by one each time through the loop. If so, return the phi node that
48 /// corresponds to it.
50 /// The IndVarSimplify pass transforms loops to have a canonical induction
53 PHINode *Loop::getCanonicalInductionVariable() const {
54 BasicBlock *H = getHeader();
56 BasicBlock *Incoming = 0, *Backedge = 0;
57 typedef GraphTraits<Inverse<BasicBlock*> > InvBlockTraits;
58 InvBlockTraits::ChildIteratorType PI = InvBlockTraits::child_begin(H);
59 assert(PI != InvBlockTraits::child_end(H) &&
60 "Loop must have at least one backedge!");
62 if (PI == InvBlockTraits::child_end(H)) return 0; // dead loop
64 if (PI != InvBlockTraits::child_end(H)) return 0; // multiple backedges?
66 if (contains(Incoming)) {
67 if (contains(Backedge))
69 std::swap(Incoming, Backedge);
70 } else if (!contains(Backedge))
73 // Loop over all of the PHI nodes, looking for a canonical indvar.
74 for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
75 PHINode *PN = cast<PHINode>(I);
77 dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
78 if (CI->isNullValue())
79 if (Instruction *Inc =
80 dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
81 if (Inc->getOpcode() == Instruction::Add &&
82 Inc->getOperand(0) == PN)
83 if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
90 /// getCanonicalInductionVariableIncrement - Return the LLVM value that holds
91 /// the canonical induction variable value for the "next" iteration of the
92 /// loop. This always succeeds if getCanonicalInductionVariable succeeds.
94 Instruction *Loop::getCanonicalInductionVariableIncrement() const {
95 if (PHINode *PN = getCanonicalInductionVariable()) {
96 bool P1InLoop = contains(PN->getIncomingBlock(1));
97 return cast<Instruction>(PN->getIncomingValue(P1InLoop));
102 /// getTripCount - Return a loop-invariant LLVM value indicating the number of
103 /// times the loop will be executed. Note that this means that the backedge
104 /// of the loop executes N-1 times. If the trip-count cannot be determined,
105 /// this returns null.
107 /// The IndVarSimplify pass transforms loops to have a form that this
108 /// function easily understands.
110 Value *Loop::getTripCount() const {
111 // Canonical loops will end with a 'cmp ne I, V', where I is the incremented
112 // canonical induction variable and V is the trip count of the loop.
113 Instruction *Inc = getCanonicalInductionVariableIncrement();
114 if (Inc == 0) return 0;
115 PHINode *IV = cast<PHINode>(Inc->getOperand(0));
117 BasicBlock *BackedgeBlock =
118 IV->getIncomingBlock(contains(IV->getIncomingBlock(1)));
120 if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator()))
121 if (BI->isConditional()) {
122 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
123 if (ICI->getOperand(0) == Inc) {
124 if (BI->getSuccessor(0) == getHeader()) {
125 if (ICI->getPredicate() == ICmpInst::ICMP_NE)
126 return ICI->getOperand(1);
127 } else if (ICI->getPredicate() == ICmpInst::ICMP_EQ) {
128 return ICI->getOperand(1);
137 /// getSmallConstantTripCount - Returns the trip count of this loop as a
138 /// normal unsigned value, if possible. Returns 0 if the trip count is unknown
139 /// of not constant. Will also return 0 if the trip count is very large
141 unsigned Loop::getSmallConstantTripCount() const {
142 Value* TripCount = this->getTripCount();
144 if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCount)) {
145 // Guard against huge trip counts.
146 if (TripCountC->getValue().getActiveBits() <= 32) {
147 return (unsigned)TripCountC->getZExtValue();
154 /// getSmallConstantTripMultiple - Returns the largest constant divisor of the
155 /// trip count of this loop as a normal unsigned value, if possible. This
156 /// means that the actual trip count is always a multiple of the returned
157 /// value (don't forget the trip count could very well be zero as well!).
159 /// Returns 1 if the trip count is unknown or not guaranteed to be the
160 /// multiple of a constant (which is also the case if the trip count is simply
161 /// constant, use getSmallConstantTripCount for that case), Will also return 1
162 /// if the trip count is very large (>= 2^32).
163 unsigned Loop::getSmallConstantTripMultiple() const {
164 Value* TripCount = this->getTripCount();
165 // This will hold the ConstantInt result, if any
166 ConstantInt *Result = NULL;
168 // See if the trip count is constant itself
169 Result = dyn_cast<ConstantInt>(TripCount);
170 // if not, see if it is a multiplication
172 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCount)) {
173 switch (BO->getOpcode()) {
174 case BinaryOperator::Mul:
175 Result = dyn_cast<ConstantInt>(BO->getOperand(1));
182 // Guard against huge trip counts.
183 if (Result && Result->getValue().getActiveBits() <= 32) {
184 return (unsigned)Result->getZExtValue();
190 /// isLCSSAForm - Return true if the Loop is in LCSSA form
191 bool Loop::isLCSSAForm() const {
192 // Sort the blocks vector so that we can use binary search to do quick
194 SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
196 for (block_iterator BI = block_begin(), E = block_end(); BI != E; ++BI) {
197 BasicBlock *BB = *BI;
198 for (BasicBlock ::iterator I = BB->begin(), E = BB->end(); I != E;++I)
199 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
201 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
202 if (PHINode *P = dyn_cast<PHINode>(*UI)) {
203 UserBB = P->getIncomingBlock(UI);
206 // Check the current block, as a fast-path. Most values are used in
207 // the same block they are defined in.
208 if (UserBB != BB && !LoopBBs.count(UserBB))
215 //===----------------------------------------------------------------------===//
216 // LoopInfo implementation
218 bool LoopInfo::runOnFunction(Function &) {
220 LI.Calculate(getAnalysis<DominatorTree>().getBase()); // Update
224 void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
225 AU.setPreservesAll();
226 AU.addRequired<DominatorTree>();