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, bool &Changed,
62 Instruction *InsertPt) const {
63 if (Instruction *I = dyn_cast<Instruction>(V))
64 return makeLoopInvariant(I, Changed, InsertPt);
65 return true; // All non-instructions are loop-invariant.
68 /// makeLoopInvariant - If the given instruction is inside of the
69 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
70 /// Return true if the instruction after any hoisting is loop invariant. This
71 /// function can be used as a slightly more aggressive replacement for
74 /// If InsertPt is specified, it is the point to hoist instructions to.
75 /// If null, the terminator of the loop preheader is used.
77 bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
78 Instruction *InsertPt) const {
79 // Test if the value is already loop-invariant.
80 if (isLoopInvariant(I))
82 // Don't hoist instructions with side-effects.
85 // Don't hoist PHI nodes.
88 // Don't hoist allocation instructions.
89 if (isa<AllocationInst>(I))
91 // Determine the insertion point, unless one was given.
93 BasicBlock *Preheader = getLoopPreheader();
94 // Without a preheader, hoisting is not feasible.
97 InsertPt = Preheader->getTerminator();
99 // Don't hoist instructions with loop-variant operands.
100 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
101 if (!makeLoopInvariant(I->getOperand(i), Changed, InsertPt))
104 I->moveBefore(InsertPt);
109 /// getCanonicalInductionVariable - Check to see if the loop has a canonical
110 /// induction variable: an integer recurrence that starts at 0 and increments
111 /// by one each time through the loop. If so, return the phi node that
112 /// corresponds to it.
114 /// The IndVarSimplify pass transforms loops to have a canonical induction
117 PHINode *Loop::getCanonicalInductionVariable() const {
118 BasicBlock *H = getHeader();
120 BasicBlock *Incoming = 0, *Backedge = 0;
121 typedef GraphTraits<Inverse<BasicBlock*> > InvBlockTraits;
122 InvBlockTraits::ChildIteratorType PI = InvBlockTraits::child_begin(H);
123 assert(PI != InvBlockTraits::child_end(H) &&
124 "Loop must have at least one backedge!");
126 if (PI == InvBlockTraits::child_end(H)) return 0; // dead loop
128 if (PI != InvBlockTraits::child_end(H)) return 0; // multiple backedges?
130 if (contains(Incoming)) {
131 if (contains(Backedge))
133 std::swap(Incoming, Backedge);
134 } else if (!contains(Backedge))
137 // Loop over all of the PHI nodes, looking for a canonical indvar.
138 for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
139 PHINode *PN = cast<PHINode>(I);
140 if (ConstantInt *CI =
141 dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
142 if (CI->isNullValue())
143 if (Instruction *Inc =
144 dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
145 if (Inc->getOpcode() == Instruction::Add &&
146 Inc->getOperand(0) == PN)
147 if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
148 if (CI->equalsInt(1))
154 /// getCanonicalInductionVariableIncrement - Return the LLVM value that holds
155 /// the canonical induction variable value for the "next" iteration of the
156 /// loop. This always succeeds if getCanonicalInductionVariable succeeds.
158 Instruction *Loop::getCanonicalInductionVariableIncrement() const {
159 if (PHINode *PN = getCanonicalInductionVariable()) {
160 bool P1InLoop = contains(PN->getIncomingBlock(1));
161 return cast<Instruction>(PN->getIncomingValue(P1InLoop));
166 /// getTripCount - Return a loop-invariant LLVM value indicating the number of
167 /// times the loop will be executed. Note that this means that the backedge
168 /// of the loop executes N-1 times. If the trip-count cannot be determined,
169 /// this returns null.
171 /// The IndVarSimplify pass transforms loops to have a form that this
172 /// function easily understands.
174 Value *Loop::getTripCount() const {
175 // Canonical loops will end with a 'cmp ne I, V', where I is the incremented
176 // canonical induction variable and V is the trip count of the loop.
177 Instruction *Inc = getCanonicalInductionVariableIncrement();
178 if (Inc == 0) return 0;
179 PHINode *IV = cast<PHINode>(Inc->getOperand(0));
181 BasicBlock *BackedgeBlock =
182 IV->getIncomingBlock(contains(IV->getIncomingBlock(1)));
184 if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator()))
185 if (BI->isConditional()) {
186 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
187 if (ICI->getOperand(0) == Inc) {
188 if (BI->getSuccessor(0) == getHeader()) {
189 if (ICI->getPredicate() == ICmpInst::ICMP_NE)
190 return ICI->getOperand(1);
191 } else if (ICI->getPredicate() == ICmpInst::ICMP_EQ) {
192 return ICI->getOperand(1);
201 /// getSmallConstantTripCount - Returns the trip count of this loop as a
202 /// normal unsigned value, if possible. Returns 0 if the trip count is unknown
203 /// of not constant. Will also return 0 if the trip count is very large
205 unsigned Loop::getSmallConstantTripCount() const {
206 Value* TripCount = this->getTripCount();
208 if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCount)) {
209 // Guard against huge trip counts.
210 if (TripCountC->getValue().getActiveBits() <= 32) {
211 return (unsigned)TripCountC->getZExtValue();
218 /// getSmallConstantTripMultiple - Returns the largest constant divisor of the
219 /// trip count of this loop as a normal unsigned value, if possible. This
220 /// means that the actual trip count is always a multiple of the returned
221 /// value (don't forget the trip count could very well be zero as well!).
223 /// Returns 1 if the trip count is unknown or not guaranteed to be the
224 /// multiple of a constant (which is also the case if the trip count is simply
225 /// constant, use getSmallConstantTripCount for that case), Will also return 1
226 /// if the trip count is very large (>= 2^32).
227 unsigned Loop::getSmallConstantTripMultiple() const {
228 Value* TripCount = this->getTripCount();
229 // This will hold the ConstantInt result, if any
230 ConstantInt *Result = NULL;
232 // See if the trip count is constant itself
233 Result = dyn_cast<ConstantInt>(TripCount);
234 // if not, see if it is a multiplication
236 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCount)) {
237 switch (BO->getOpcode()) {
238 case BinaryOperator::Mul:
239 Result = dyn_cast<ConstantInt>(BO->getOperand(1));
246 // Guard against huge trip counts.
247 if (Result && Result->getValue().getActiveBits() <= 32) {
248 return (unsigned)Result->getZExtValue();
254 /// isLCSSAForm - Return true if the Loop is in LCSSA form
255 bool Loop::isLCSSAForm() const {
256 // Sort the blocks vector so that we can use binary search to do quick
258 SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
260 for (block_iterator BI = block_begin(), E = block_end(); BI != E; ++BI) {
261 BasicBlock *BB = *BI;
262 for (BasicBlock ::iterator I = BB->begin(), E = BB->end(); I != E;++I)
263 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
265 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
266 if (PHINode *P = dyn_cast<PHINode>(*UI)) {
267 UserBB = P->getIncomingBlock(UI);
270 // Check the current block, as a fast-path. Most values are used in
271 // the same block they are defined in.
272 if (UserBB != BB && !LoopBBs.count(UserBB))
280 /// isLoopSimplifyForm - Return true if the Loop is in the form that
281 /// the LoopSimplify form transforms loops to, which is sometimes called
283 bool Loop::isLoopSimplifyForm() const {
284 // Normal-form loops have a preheader.
285 if (!getLoopPreheader())
287 // Normal-form loops have a single backedge.
290 // Each predecessor of each exit block of a normal loop is contained
292 SmallVector<BasicBlock *, 4> ExitBlocks;
293 getExitBlocks(ExitBlocks);
294 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
295 for (pred_iterator PI = pred_begin(ExitBlocks[i]),
296 PE = pred_end(ExitBlocks[i]); PI != PE; ++PI)
299 // All the requirements are met.
303 //===----------------------------------------------------------------------===//
304 // LoopInfo implementation
306 bool LoopInfo::runOnFunction(Function &) {
308 LI.Calculate(getAnalysis<DominatorTree>().getBase()); // Update
312 void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
313 AU.setPreservesAll();
314 AU.addRequired<DominatorTree>();