LoopInfo.cpp revision 198090
1//===- LoopInfo.cpp - Natural Loop Calculator -----------------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
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.
14//
15//===----------------------------------------------------------------------===//
16
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/ADT/DepthFirstIterator.h"
25#include "llvm/ADT/SmallPtrSet.h"
26#include <algorithm>
27using namespace llvm;
28
29// Always verify loopinfo if expensive checking is enabled.
30#ifdef XDEBUG
31bool VerifyLoopInfo = true;
32#else
33bool VerifyLoopInfo = false;
34#endif
35static cl::opt<bool,true>
36VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
37                cl::desc("Verify loop info (time consuming)"));
38
39char LoopInfo::ID = 0;
40static RegisterPass<LoopInfo>
41X("loops", "Natural Loop Information", true, true);
42
43//===----------------------------------------------------------------------===//
44// Loop implementation
45//
46
47/// isLoopInvariant - Return true if the specified value is loop invariant
48///
49bool 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
53}
54
55/// isLoopInvariant - Return true if the specified instruction is
56/// loop-invariant.
57///
58bool Loop::isLoopInvariant(Instruction *I) const {
59  return !contains(I->getParent());
60}
61
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
66/// isLoopInvariant.
67///
68/// If InsertPt is specified, it is the point to hoist instructions to.
69/// If null, the terminator of the loop preheader is used.
70///
71bool 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.
76}
77
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
82/// isLoopInvariant.
83///
84/// If InsertPt is specified, it is the point to hoist instructions to.
85/// If null, the terminator of the loop preheader is used.
86///
87bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
88                             Instruction *InsertPt) const {
89  // Test if the value is already loop-invariant.
90  if (isLoopInvariant(I))
91    return true;
92  if (!I->isSafeToSpeculativelyExecute())
93    return false;
94  if (I->mayReadFromMemory())
95    return false;
96  // Determine the insertion point, unless one was given.
97  if (!InsertPt) {
98    BasicBlock *Preheader = getLoopPreheader();
99    // Without a preheader, hoisting is not feasible.
100    if (!Preheader)
101      return false;
102    InsertPt = Preheader->getTerminator();
103  }
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))
107      return false;
108  // Hoist.
109  I->moveBefore(InsertPt);
110  Changed = true;
111  return true;
112}
113
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.
118///
119/// The IndVarSimplify pass transforms loops to have a canonical induction
120/// variable.
121///
122PHINode *Loop::getCanonicalInductionVariable() const {
123  BasicBlock *H = getHeader();
124
125  BasicBlock *Incoming = 0, *Backedge = 0;
126  typedef GraphTraits<Inverse<BasicBlock*> > InvBlockTraits;
127  InvBlockTraits::ChildIteratorType PI = InvBlockTraits::child_begin(H);
128  assert(PI != InvBlockTraits::child_end(H) &&
129         "Loop must have at least one backedge!");
130  Backedge = *PI++;
131  if (PI == InvBlockTraits::child_end(H)) return 0;  // dead loop
132  Incoming = *PI++;
133  if (PI != InvBlockTraits::child_end(H)) return 0;  // multiple backedges?
134
135  if (contains(Incoming)) {
136    if (contains(Backedge))
137      return 0;
138    std::swap(Incoming, Backedge);
139  } else if (!contains(Backedge))
140    return 0;
141
142  // Loop over all of the PHI nodes, looking for a canonical indvar.
143  for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
144    PHINode *PN = cast<PHINode>(I);
145    if (ConstantInt *CI =
146        dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
147      if (CI->isNullValue())
148        if (Instruction *Inc =
149            dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
150          if (Inc->getOpcode() == Instruction::Add &&
151                Inc->getOperand(0) == PN)
152            if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
153              if (CI->equalsInt(1))
154                return PN;
155  }
156  return 0;
157}
158
159/// getCanonicalInductionVariableIncrement - Return the LLVM value that holds
160/// the canonical induction variable value for the "next" iteration of the
161/// loop.  This always succeeds if getCanonicalInductionVariable succeeds.
162///
163Instruction *Loop::getCanonicalInductionVariableIncrement() const {
164  if (PHINode *PN = getCanonicalInductionVariable()) {
165    bool P1InLoop = contains(PN->getIncomingBlock(1));
166    return cast<Instruction>(PN->getIncomingValue(P1InLoop));
167  }
168  return 0;
169}
170
171/// getTripCount - Return a loop-invariant LLVM value indicating the number of
172/// times the loop will be executed.  Note that this means that the backedge
173/// of the loop executes N-1 times.  If the trip-count cannot be determined,
174/// this returns null.
175///
176/// The IndVarSimplify pass transforms loops to have a form that this
177/// function easily understands.
178///
179Value *Loop::getTripCount() const {
180  // Canonical loops will end with a 'cmp ne I, V', where I is the incremented
181  // canonical induction variable and V is the trip count of the loop.
182  Instruction *Inc = getCanonicalInductionVariableIncrement();
183  if (Inc == 0) return 0;
184  PHINode *IV = cast<PHINode>(Inc->getOperand(0));
185
186  BasicBlock *BackedgeBlock =
187    IV->getIncomingBlock(contains(IV->getIncomingBlock(1)));
188
189  if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator()))
190    if (BI->isConditional()) {
191      if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
192        if (ICI->getOperand(0) == Inc) {
193          if (BI->getSuccessor(0) == getHeader()) {
194            if (ICI->getPredicate() == ICmpInst::ICMP_NE)
195              return ICI->getOperand(1);
196          } else if (ICI->getPredicate() == ICmpInst::ICMP_EQ) {
197            return ICI->getOperand(1);
198          }
199        }
200      }
201    }
202
203  return 0;
204}
205
206/// getSmallConstantTripCount - Returns the trip count of this loop as a
207/// normal unsigned value, if possible. Returns 0 if the trip count is unknown
208/// of not constant. Will also return 0 if the trip count is very large
209/// (>= 2^32)
210unsigned Loop::getSmallConstantTripCount() const {
211  Value* TripCount = this->getTripCount();
212  if (TripCount) {
213    if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCount)) {
214      // Guard against huge trip counts.
215      if (TripCountC->getValue().getActiveBits() <= 32) {
216        return (unsigned)TripCountC->getZExtValue();
217      }
218    }
219  }
220  return 0;
221}
222
223/// getSmallConstantTripMultiple - Returns the largest constant divisor of the
224/// trip count of this loop as a normal unsigned value, if possible. This
225/// means that the actual trip count is always a multiple of the returned
226/// value (don't forget the trip count could very well be zero as well!).
227///
228/// Returns 1 if the trip count is unknown or not guaranteed to be the
229/// multiple of a constant (which is also the case if the trip count is simply
230/// constant, use getSmallConstantTripCount for that case), Will also return 1
231/// if the trip count is very large (>= 2^32).
232unsigned Loop::getSmallConstantTripMultiple() const {
233  Value* TripCount = this->getTripCount();
234  // This will hold the ConstantInt result, if any
235  ConstantInt *Result = NULL;
236  if (TripCount) {
237    // See if the trip count is constant itself
238    Result = dyn_cast<ConstantInt>(TripCount);
239    // if not, see if it is a multiplication
240    if (!Result)
241      if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCount)) {
242        switch (BO->getOpcode()) {
243        case BinaryOperator::Mul:
244          Result = dyn_cast<ConstantInt>(BO->getOperand(1));
245          break;
246        default:
247          break;
248        }
249      }
250  }
251  // Guard against huge trip counts.
252  if (Result && Result->getValue().getActiveBits() <= 32) {
253    return (unsigned)Result->getZExtValue();
254  } else {
255    return 1;
256  }
257}
258
259/// isLCSSAForm - Return true if the Loop is in LCSSA form
260bool Loop::isLCSSAForm() const {
261  // Sort the blocks vector so that we can use binary search to do quick
262  // lookups.
263  SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
264
265  for (block_iterator BI = block_begin(), E = block_end(); BI != E; ++BI) {
266    BasicBlock  *BB = *BI;
267    for (BasicBlock ::iterator I = BB->begin(), E = BB->end(); I != E;++I)
268      for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
269           ++UI) {
270        BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
271        if (PHINode *P = dyn_cast<PHINode>(*UI)) {
272          UserBB = P->getIncomingBlock(UI);
273        }
274
275        // Check the current block, as a fast-path.  Most values are used in
276        // the same block they are defined in.
277        if (UserBB != BB && !LoopBBs.count(UserBB))
278          return false;
279      }
280  }
281
282  return true;
283}
284
285/// isLoopSimplifyForm - Return true if the Loop is in the form that
286/// the LoopSimplify form transforms loops to, which is sometimes called
287/// normal form.
288bool Loop::isLoopSimplifyForm() const {
289  // Normal-form loops have a preheader.
290  if (!getLoopPreheader())
291    return false;
292  // Normal-form loops have a single backedge.
293  if (!getLoopLatch())
294    return false;
295  // Each predecessor of each exit block of a normal loop is contained
296  // within the loop.
297  SmallVector<BasicBlock *, 4> ExitBlocks;
298  getExitBlocks(ExitBlocks);
299  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
300    for (pred_iterator PI = pred_begin(ExitBlocks[i]),
301         PE = pred_end(ExitBlocks[i]); PI != PE; ++PI)
302      if (!contains(*PI))
303        return false;
304  // All the requirements are met.
305  return true;
306}
307
308/// getUniqueExitBlocks - Return all unique successor blocks of this loop.
309/// These are the blocks _outside of the current loop_ which are branched to.
310/// This assumes that loop is in canonical form.
311///
312void
313Loop::getUniqueExitBlocks(SmallVectorImpl<BasicBlock *> &ExitBlocks) const {
314  assert(isLoopSimplifyForm() &&
315         "getUniqueExitBlocks assumes the loop is in canonical form!");
316
317  // Sort the blocks vector so that we can use binary search to do quick
318  // lookups.
319  SmallVector<BasicBlock *, 128> LoopBBs(block_begin(), block_end());
320  std::sort(LoopBBs.begin(), LoopBBs.end());
321
322  SmallVector<BasicBlock *, 32> switchExitBlocks;
323
324  for (block_iterator BI = block_begin(), BE = block_end(); BI != BE; ++BI) {
325
326    BasicBlock *current = *BI;
327    switchExitBlocks.clear();
328
329    typedef GraphTraits<BasicBlock *> BlockTraits;
330    typedef GraphTraits<Inverse<BasicBlock *> > InvBlockTraits;
331    for (BlockTraits::ChildIteratorType I =
332         BlockTraits::child_begin(*BI), E = BlockTraits::child_end(*BI);
333         I != E; ++I) {
334      // If block is inside the loop then it is not a exit block.
335      if (std::binary_search(LoopBBs.begin(), LoopBBs.end(), *I))
336        continue;
337
338      InvBlockTraits::ChildIteratorType PI = InvBlockTraits::child_begin(*I);
339      BasicBlock *firstPred = *PI;
340
341      // If current basic block is this exit block's first predecessor
342      // then only insert exit block in to the output ExitBlocks vector.
343      // This ensures that same exit block is not inserted twice into
344      // ExitBlocks vector.
345      if (current != firstPred)
346        continue;
347
348      // If a terminator has more then two successors, for example SwitchInst,
349      // then it is possible that there are multiple edges from current block
350      // to one exit block.
351      if (std::distance(BlockTraits::child_begin(current),
352                        BlockTraits::child_end(current)) <= 2) {
353        ExitBlocks.push_back(*I);
354        continue;
355      }
356
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
359      // duplicate edges.
360      if (std::find(switchExitBlocks.begin(), switchExitBlocks.end(), *I)
361          == switchExitBlocks.end()) {
362        switchExitBlocks.push_back(*I);
363        ExitBlocks.push_back(*I);
364      }
365    }
366  }
367}
368
369/// getUniqueExitBlock - If getUniqueExitBlocks would return exactly one
370/// block, return that block. Otherwise return null.
371BasicBlock *Loop::getUniqueExitBlock() const {
372  SmallVector<BasicBlock *, 8> UniqueExitBlocks;
373  getUniqueExitBlocks(UniqueExitBlocks);
374  if (UniqueExitBlocks.size() == 1)
375    return UniqueExitBlocks[0];
376  return 0;
377}
378
379//===----------------------------------------------------------------------===//
380// LoopInfo implementation
381//
382bool LoopInfo::runOnFunction(Function &) {
383  releaseMemory();
384  LI.Calculate(getAnalysis<DominatorTree>().getBase());    // Update
385  return false;
386}
387
388void 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.
394
395  if (!VerifyLoopInfo) return;
396
397  for (iterator I = begin(), E = end(); I != E; ++I) {
398    assert(!(*I)->getParentLoop() && "Top-level loop has a parent!");
399    (*I)->verifyLoopNest();
400  }
401
402  // TODO: check BBMap consistency.
403}
404
405void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
406  AU.setPreservesAll();
407  AU.addRequired<DominatorTree>();
408}
409
410void LoopInfo::print(raw_ostream &OS, const Module*) const {
411  LI.print(OS);
412}
413
414