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 isLoopInvariant(I);
42 return true; // All non-instructions are loop invariant
45 /// isLoopInvariant - Return true if the specified instruction is
48 bool Loop::isLoopInvariant(Instruction *I) const {
49 return !contains(I->getParent());
52 /// makeLoopInvariant - If the given value is an instruciton inside of the
53 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
54 /// Return true if the value after any hoisting is loop invariant. This
55 /// function can be used as a slightly more aggressive replacement for
58 /// If InsertPt is specified, it is the point to hoist instructions to.
59 /// If null, the terminator of the loop preheader is used.
61 bool Loop::makeLoopInvariant(Value *V, Instruction *InsertPt) const {
62 if (Instruction *I = dyn_cast<Instruction>(V))
63 return makeLoopInvariant(I);
64 return true; // All non-instructions are loop-invariant.
67 /// makeLoopInvariant - If the given instruction is inside of the
68 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
69 /// Return true if the instruction after any hoisting is loop invariant. This
70 /// function can be used as a slightly more aggressive replacement for
73 /// If InsertPt is specified, it is the point to hoist instructions to.
74 /// If null, the terminator of the loop preheader is used.
76 bool Loop::makeLoopInvariant(Instruction *I, Instruction *InsertPt) const {
77 // Test if the value is already loop-invariant.
78 if (isLoopInvariant(I))
80 // Don't hoist instructions with side-effects.
83 // Don't hoist PHI nodes.
86 // Don't hoist allocation instructions.
87 if (isa<AllocationInst>(I))
89 // Determine the insertion point, unless one was given.
91 BasicBlock *Preheader = getLoopPreheader();
92 // Without a preheader, hoisting is not feasible.
95 InsertPt = Preheader->getTerminator();
97 // Don't hoist instructions with loop-variant operands.
98 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
99 if (!makeLoopInvariant(I->getOperand(i), InsertPt))
102 I->moveBefore(InsertPt);
106 /// getCanonicalInductionVariable - Check to see if the loop has a canonical
107 /// induction variable: an integer recurrence that starts at 0 and increments
108 /// by one each time through the loop. If so, return the phi node that
109 /// corresponds to it.
111 /// The IndVarSimplify pass transforms loops to have a canonical induction
114 PHINode *Loop::getCanonicalInductionVariable() const {
115 BasicBlock *H = getHeader();
117 BasicBlock *Incoming = 0, *Backedge = 0;
118 typedef GraphTraits<Inverse<BasicBlock*> > InvBlockTraits;
119 InvBlockTraits::ChildIteratorType PI = InvBlockTraits::child_begin(H);
120 assert(PI != InvBlockTraits::child_end(H) &&
121 "Loop must have at least one backedge!");
123 if (PI == InvBlockTraits::child_end(H)) return 0; // dead loop
125 if (PI != InvBlockTraits::child_end(H)) return 0; // multiple backedges?
127 if (contains(Incoming)) {
128 if (contains(Backedge))
130 std::swap(Incoming, Backedge);
131 } else if (!contains(Backedge))
134 // Loop over all of the PHI nodes, looking for a canonical indvar.
135 for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
136 PHINode *PN = cast<PHINode>(I);
137 if (ConstantInt *CI =
138 dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
139 if (CI->isNullValue())
140 if (Instruction *Inc =
141 dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
142 if (Inc->getOpcode() == Instruction::Add &&
143 Inc->getOperand(0) == PN)
144 if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
145 if (CI->equalsInt(1))
151 /// getCanonicalInductionVariableIncrement - Return the LLVM value that holds
152 /// the canonical induction variable value for the "next" iteration of the
153 /// loop. This always succeeds if getCanonicalInductionVariable succeeds.
155 Instruction *Loop::getCanonicalInductionVariableIncrement() const {
156 if (PHINode *PN = getCanonicalInductionVariable()) {
157 bool P1InLoop = contains(PN->getIncomingBlock(1));
158 return cast<Instruction>(PN->getIncomingValue(P1InLoop));
163 /// getTripCount - Return a loop-invariant LLVM value indicating the number of
164 /// times the loop will be executed. Note that this means that the backedge
165 /// of the loop executes N-1 times. If the trip-count cannot be determined,
166 /// this returns null.
168 /// The IndVarSimplify pass transforms loops to have a form that this
169 /// function easily understands.
171 Value *Loop::getTripCount() const {
172 // Canonical loops will end with a 'cmp ne I, V', where I is the incremented
173 // canonical induction variable and V is the trip count of the loop.
174 Instruction *Inc = getCanonicalInductionVariableIncrement();
175 if (Inc == 0) return 0;
176 PHINode *IV = cast<PHINode>(Inc->getOperand(0));
178 BasicBlock *BackedgeBlock =
179 IV->getIncomingBlock(contains(IV->getIncomingBlock(1)));
181 if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator()))
182 if (BI->isConditional()) {
183 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
184 if (ICI->getOperand(0) == Inc) {
185 if (BI->getSuccessor(0) == getHeader()) {
186 if (ICI->getPredicate() == ICmpInst::ICMP_NE)
187 return ICI->getOperand(1);
188 } else if (ICI->getPredicate() == ICmpInst::ICMP_EQ) {
189 return ICI->getOperand(1);
198 /// getSmallConstantTripCount - Returns the trip count of this loop as a
199 /// normal unsigned value, if possible. Returns 0 if the trip count is unknown
200 /// of not constant. Will also return 0 if the trip count is very large
202 unsigned Loop::getSmallConstantTripCount() const {
203 Value* TripCount = this->getTripCount();
205 if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCount)) {
206 // Guard against huge trip counts.
207 if (TripCountC->getValue().getActiveBits() <= 32) {
208 return (unsigned)TripCountC->getZExtValue();
215 /// getSmallConstantTripMultiple - Returns the largest constant divisor of the
216 /// trip count of this loop as a normal unsigned value, if possible. This
217 /// means that the actual trip count is always a multiple of the returned
218 /// value (don't forget the trip count could very well be zero as well!).
220 /// Returns 1 if the trip count is unknown or not guaranteed to be the
221 /// multiple of a constant (which is also the case if the trip count is simply
222 /// constant, use getSmallConstantTripCount for that case), Will also return 1
223 /// if the trip count is very large (>= 2^32).
224 unsigned Loop::getSmallConstantTripMultiple() const {
225 Value* TripCount = this->getTripCount();
226 // This will hold the ConstantInt result, if any
227 ConstantInt *Result = NULL;
229 // See if the trip count is constant itself
230 Result = dyn_cast<ConstantInt>(TripCount);
231 // if not, see if it is a multiplication
233 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCount)) {
234 switch (BO->getOpcode()) {
235 case BinaryOperator::Mul:
236 Result = dyn_cast<ConstantInt>(BO->getOperand(1));
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() 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;
262 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
263 if (PHINode *P = dyn_cast<PHINode>(*UI)) {
264 UserBB = P->getIncomingBlock(UI);
267 // Check the current block, as a fast-path. Most values are used in
268 // the same block they are defined in.
269 if (UserBB != BB && !LoopBBs.count(UserBB))
276 //===----------------------------------------------------------------------===//
277 // LoopInfo implementation
279 bool LoopInfo::runOnFunction(Function &) {
281 LI.Calculate(getAnalysis<DominatorTree>().getBase()); // Update
285 void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
286 AU.setPreservesAll();
287 AU.addRequired<DominatorTree>();