LLVM  3.7.0
LoopDeletion.cpp
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1 //===- LoopDeletion.cpp - Dead Loop Deletion Pass ---------------===//
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 implements the Dead Loop Deletion Pass. This pass is responsible
11 // for eliminating loops with non-infinite computable trip counts that have no
12 // side effects or volatile instructions, and do not contribute to the
13 // computation of the function's return value.
14 //
15 //===----------------------------------------------------------------------===//
16 
17 #include "llvm/Transforms/Scalar.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/LoopPass.h"
22 #include "llvm/IR/Dominators.h"
23 using namespace llvm;
24 
25 #define DEBUG_TYPE "loop-delete"
26 
27 STATISTIC(NumDeleted, "Number of loops deleted");
28 
29 namespace {
30  class LoopDeletion : public LoopPass {
31  public:
32  static char ID; // Pass ID, replacement for typeid
33  LoopDeletion() : LoopPass(ID) {
35  }
36 
37  // Possibly eliminate loop L if it is dead.
38  bool runOnLoop(Loop *L, LPPassManager &LPM) override;
39 
40  void getAnalysisUsage(AnalysisUsage &AU) const override {
46 
52  }
53 
54  private:
55  bool isLoopDead(Loop *L, SmallVectorImpl<BasicBlock *> &exitingBlocks,
57  bool &Changed, BasicBlock *Preheader);
58 
59  };
60 }
61 
62 char LoopDeletion::ID = 0;
63 INITIALIZE_PASS_BEGIN(LoopDeletion, "loop-deletion",
64  "Delete dead loops", false, false)
68 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
70 INITIALIZE_PASS_END(LoopDeletion, "loop-deletion",
71  "Delete dead loops", false, false)
72 
74  return new LoopDeletion();
75 }
76 
77 /// isLoopDead - Determined if a loop is dead. This assumes that we've already
78 /// checked for unique exit and exiting blocks, and that the code is in LCSSA
79 /// form.
80 bool LoopDeletion::isLoopDead(Loop *L,
81  SmallVectorImpl<BasicBlock *> &exitingBlocks,
83  bool &Changed, BasicBlock *Preheader) {
84  BasicBlock *exitBlock = exitBlocks[0];
85 
86  // Make sure that all PHI entries coming from the loop are loop invariant.
87  // Because the code is in LCSSA form, any values used outside of the loop
88  // must pass through a PHI in the exit block, meaning that this check is
89  // sufficient to guarantee that no loop-variant values are used outside
90  // of the loop.
91  BasicBlock::iterator BI = exitBlock->begin();
92  while (PHINode *P = dyn_cast<PHINode>(BI)) {
93  Value *incoming = P->getIncomingValueForBlock(exitingBlocks[0]);
94 
95  // Make sure all exiting blocks produce the same incoming value for the exit
96  // block. If there are different incoming values for different exiting
97  // blocks, then it is impossible to statically determine which value should
98  // be used.
99  for (unsigned i = 1, e = exitingBlocks.size(); i < e; ++i) {
100  if (incoming != P->getIncomingValueForBlock(exitingBlocks[i]))
101  return false;
102  }
103 
104  if (Instruction *I = dyn_cast<Instruction>(incoming))
105  if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator()))
106  return false;
107 
108  ++BI;
109  }
110 
111  // Make sure that no instructions in the block have potential side-effects.
112  // This includes instructions that could write to memory, and loads that are
113  // marked volatile. This could be made more aggressive by using aliasing
114  // information to identify readonly and readnone calls.
115  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
116  LI != LE; ++LI) {
117  for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
118  BI != BE; ++BI) {
119  if (BI->mayHaveSideEffects())
120  return false;
121  }
122  }
123 
124  return true;
125 }
126 
127 /// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
128 /// observable behavior of the program other than finite running time. Note
129 /// we do ensure that this never remove a loop that might be infinite, as doing
130 /// so could change the halting/non-halting nature of a program.
131 /// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
132 /// in order to make various safety checks work.
133 bool LoopDeletion::runOnLoop(Loop *L, LPPassManager &LPM) {
134  if (skipOptnoneFunction(L))
135  return false;
136 
137  // We can only remove the loop if there is a preheader that we can
138  // branch from after removing it.
139  BasicBlock *preheader = L->getLoopPreheader();
140  if (!preheader)
141  return false;
142 
143  // If LoopSimplify form is not available, stay out of trouble.
144  if (!L->hasDedicatedExits())
145  return false;
146 
147  // We can't remove loops that contain subloops. If the subloops were dead,
148  // they would already have been removed in earlier executions of this pass.
149  if (L->begin() != L->end())
150  return false;
151 
152  SmallVector<BasicBlock*, 4> exitingBlocks;
153  L->getExitingBlocks(exitingBlocks);
154 
155  SmallVector<BasicBlock*, 4> exitBlocks;
156  L->getUniqueExitBlocks(exitBlocks);
157 
158  // We require that the loop only have a single exit block. Otherwise, we'd
159  // be in the situation of needing to be able to solve statically which exit
160  // block will be branched to, or trying to preserve the branching logic in
161  // a loop invariant manner.
162  if (exitBlocks.size() != 1)
163  return false;
164 
165  // Finally, we have to check that the loop really is dead.
166  bool Changed = false;
167  if (!isLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader))
168  return Changed;
169 
170  // Don't remove loops for which we can't solve the trip count.
171  // They could be infinite, in which case we'd be changing program behavior.
172  ScalarEvolution &SE = getAnalysis<ScalarEvolution>();
173  const SCEV *S = SE.getMaxBackedgeTakenCount(L);
174  if (isa<SCEVCouldNotCompute>(S))
175  return Changed;
176 
177  // Now that we know the removal is safe, remove the loop by changing the
178  // branch from the preheader to go to the single exit block.
179  BasicBlock *exitBlock = exitBlocks[0];
180 
181  // Because we're deleting a large chunk of code at once, the sequence in which
182  // we remove things is very important to avoid invalidation issues. Don't
183  // mess with this unless you have good reason and know what you're doing.
184 
185  // Tell ScalarEvolution that the loop is deleted. Do this before
186  // deleting the loop so that ScalarEvolution can look at the loop
187  // to determine what it needs to clean up.
188  SE.forgetLoop(L);
189 
190  // Connect the preheader directly to the exit block.
191  TerminatorInst *TI = preheader->getTerminator();
192  TI->replaceUsesOfWith(L->getHeader(), exitBlock);
193 
194  // Rewrite phis in the exit block to get their inputs from
195  // the preheader instead of the exiting block.
196  BasicBlock *exitingBlock = exitingBlocks[0];
197  BasicBlock::iterator BI = exitBlock->begin();
198  while (PHINode *P = dyn_cast<PHINode>(BI)) {
199  int j = P->getBasicBlockIndex(exitingBlock);
200  assert(j >= 0 && "Can't find exiting block in exit block's phi node!");
201  P->setIncomingBlock(j, preheader);
202  for (unsigned i = 1; i < exitingBlocks.size(); ++i)
203  P->removeIncomingValue(exitingBlocks[i]);
204  ++BI;
205  }
206 
207  // Update the dominator tree and remove the instructions and blocks that will
208  // be deleted from the reference counting scheme.
209  DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
210  SmallVector<DomTreeNode*, 8> ChildNodes;
211  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
212  LI != LE; ++LI) {
213  // Move all of the block's children to be children of the preheader, which
214  // allows us to remove the domtree entry for the block.
215  ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
216  for (SmallVectorImpl<DomTreeNode *>::iterator DI = ChildNodes.begin(),
217  DE = ChildNodes.end(); DI != DE; ++DI) {
218  DT.changeImmediateDominator(*DI, DT[preheader]);
219  }
220 
221  ChildNodes.clear();
222  DT.eraseNode(*LI);
223 
224  // Remove the block from the reference counting scheme, so that we can
225  // delete it freely later.
226  (*LI)->dropAllReferences();
227  }
228 
229  // Erase the instructions and the blocks without having to worry
230  // about ordering because we already dropped the references.
231  // NOTE: This iteration is safe because erasing the block does not remove its
232  // entry from the loop's block list. We do that in the next section.
233  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
234  LI != LE; ++LI)
235  (*LI)->eraseFromParent();
236 
237  // Finally, the blocks from loopinfo. This has to happen late because
238  // otherwise our loop iterators won't work.
239  LoopInfo &loopInfo = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
241  blocks.insert(L->block_begin(), L->block_end());
242  for (BasicBlock *BB : blocks)
243  loopInfo.removeBlock(BB);
244 
245  // The last step is to inform the loop pass manager that we've
246  // eliminated this loop.
247  LPM.deleteLoopFromQueue(L);
248  Changed = true;
249 
250  ++NumDeleted;
251 
252  return Changed;
253 }
Pass interface - Implemented by all 'passes'.
Definition: Pass.h:82
AnalysisUsage & addPreserved()
Add the specified Pass class to the set of analyses preserved by this pass.
INITIALIZE_PASS_BEGIN(LoopDeletion,"loop-deletion","Delete dead loops", false, false) INITIALIZE_PASS_END(LoopDeletion
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
void removeBlock(BlockT *BB)
removeBlock - This method completely removes BB from all data structures, including all of the Loop o...
Definition: LoopInfo.h:605
STATISTIC(NumFunctions,"Total number of functions")
ScalarEvolution - This class is the main scalar evolution driver.
BlockT * getHeader() const
Definition: LoopInfo.h:96
iterator begin()
Instruction iterator methods.
Definition: BasicBlock.h:231
AnalysisUsage & addRequired()
#define INITIALIZE_PASS_DEPENDENCY(depName)
Definition: PassSupport.h:70
#define INITIALIZE_PASS_END(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:75
This class consists of common code factored out of the SmallVector class to reduce code duplication b...
Definition: APInt.h:33
void getExitingBlocks(SmallVectorImpl< BlockT * > &ExitingBlocks) const
getExitingBlocks - Return all blocks inside the loop that have successors outside of the loop...
Definition: LoopInfoImpl.h:35
loop Delete dead loops
AnalysisUsage & addPreservedID(const void *ID)
Concrete subclass of DominatorTreeBase that is used to compute a normal dominator tree...
Definition: Dominators.h:67
void replaceUsesOfWith(Value *From, Value *To)
Replace uses of one Value with another.
Definition: User.cpp:24
#define P(N)
iterator begin() const
Definition: LoopInfo.h:131
Subclasses of this class are all able to terminate a basic block.
Definition: InstrTypes.h:35
BlockT * getLoopPreheader() const
getLoopPreheader - If there is a preheader for this loop, return it.
Definition: LoopInfoImpl.h:108
LLVM Basic Block Representation.
Definition: BasicBlock.h:65
void eraseNode(NodeT *BB)
eraseNode - Removes a node from the dominator tree.
char & LCSSAID
Definition: LCSSA.cpp:312
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:264
void initializeLoopDeletionPass(PassRegistry &)
iterator end() const
Definition: LoopInfo.h:132
Represent the analysis usage information of a pass.
const SCEV * getMaxBackedgeTakenCount(const Loop *L)
getMaxBackedgeTakenCount - Similar to getBackedgeTakenCount, except return the least SCEV value that ...
bool hasDedicatedExits() const
hasDedicatedExits - Return true if no exit block for the loop has a predecessor that is outside the l...
Definition: LoopInfo.cpp:328
loop deletion
void getUniqueExitBlocks(SmallVectorImpl< BasicBlock * > &ExitBlocks) const
getUniqueExitBlocks - Return all unique successor blocks of this loop.
Definition: LoopInfo.cpp:347
char & LoopSimplifyID
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:299
bool makeLoopInvariant(Value *V, bool &Changed, Instruction *InsertPt=nullptr) const
makeLoopInvariant - If the given value is an instruction inside of the loop and it can be hoisted...
Definition: LoopInfo.cpp:80
AnalysisUsage & addRequiredID(const void *ID)
Definition: Pass.cpp:276
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:861
void deleteLoopFromQueue(Loop *L)
Delete loop from the loop queue and loop hierarchy (LoopInfo).
Definition: LoopPass.cpp:76
Pass * createLoopDeletionPass()
std::vector< BlockT * >::const_iterator block_iterator
Definition: LoopInfo.h:140
iterator insert(iterator I, T &&Elt)
Definition: SmallVector.h:481
block_iterator block_end() const
Definition: LoopInfo.h:142
void forgetLoop(const Loop *L)
forgetLoop - This method should be called by the client when it has changed a loop in a way that may ...
SCEV - This class represents an analyzed expression in the program.
#define I(x, y, z)
Definition: MD5.cpp:54
TerminatorInst * getTerminator()
Returns the terminator instruction if the block is well formed or null if the block is not well forme...
Definition: BasicBlock.cpp:124
void changeImmediateDominator(DomTreeNodeBase< NodeT > *N, DomTreeNodeBase< NodeT > *NewIDom)
changeImmediateDominator - This method is used to update the dominator tree information when a node's...
LLVM Value Representation.
Definition: Value.h:69
block_iterator block_begin() const
Definition: LoopInfo.h:141
The legacy pass manager's analysis pass to compute loop information.
Definition: LoopInfo.h:737
Legacy analysis pass which computes a DominatorTree.
Definition: Dominators.h:203
loop Delete dead false