lib/Transforms/Scalar/LoopDeletion.cpp

   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 #define DEBUG_TYPE "loop-delete"
  18 #include "llvm/Transforms/Scalar.h"
  19 #include "llvm/Analysis/LoopPass.h"
  20 #include "llvm/Analysis/ScalarEvolution.h"
  21 #include "llvm/ADT/Statistic.h"
  22 #include "llvm/ADT/SmallVector.h"
  23 using namespace llvm;
  24
  25 STATISTIC(NumDeleted, "Number of loops deleted");
  26
  27 namespace {
  28   class LoopDeletion : public LoopPass {
  29   public:
  30     static char ID; // Pass ID, replacement for typeid
  31     LoopDeletion() : LoopPass(ID) {
  32       initializeLoopDeletionPass(*PassRegistry::getPassRegistry());
  33     }
  34
  35     // Possibly eliminate loop L if it is dead.
  36     bool runOnLoop(Loop* L, LPPassManager& LPM);
  37
  38     bool IsLoopDead(Loop* L, SmallVector<BasicBlock*, 4>& exitingBlocks,
  39                     SmallVector<BasicBlock*, 4>& exitBlocks,
  40                     bool &Changed, BasicBlock *Preheader);
  41
  42     virtual void getAnalysisUsage(AnalysisUsage& AU) const {
  43       AU.addRequired<DominatorTree>();
  44       AU.addRequired<LoopInfo>();
  45       AU.addRequired<ScalarEvolution>();
  46       AU.addRequiredID(LoopSimplifyID);
  47       AU.addRequiredID(LCSSAID);
  48
  49       AU.addPreserved<ScalarEvolution>();
  50       AU.addPreserved<DominatorTree>();
  51       AU.addPreserved<LoopInfo>();
  52       AU.addPreservedID(LoopSimplifyID);
  53       AU.addPreservedID(LCSSAID);
  54       AU.addPreserved<DominanceFrontier>();
  55     }
  56   };
  57 }
  58
  59 char LoopDeletion::ID = 0;
  60 INITIALIZE_PASS_BEGIN(LoopDeletion, "loop-deletion",
  61                 "Delete dead loops", false, false)
  62 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
  63 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  64 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
  65 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
  66 INITIALIZE_PASS_DEPENDENCY(LCSSA)
  67 INITIALIZE_PASS_END(LoopDeletion, "loop-deletion",
  68                 "Delete dead loops", false, false)
  69
  70 Pass* llvm::createLoopDeletionPass() {
  71   return new LoopDeletion();
  72 }
  73
  74 /// IsLoopDead - Determined if a loop is dead.  This assumes that we've already
  75 /// checked for unique exit and exiting blocks, and that the code is in LCSSA
  76 /// form.
  77 bool LoopDeletion::IsLoopDead(Loop* L,
  78                               SmallVector<BasicBlock*, 4>& exitingBlocks,
  79                               SmallVector<BasicBlock*, 4>& exitBlocks,
  80                               bool &Changed, BasicBlock *Preheader) {
  81   BasicBlock* exitingBlock = exitingBlocks[0];
  82   BasicBlock* exitBlock = exitBlocks[0];
  83
  84   // Make sure that all PHI entries coming from the loop are loop invariant.
  85   // Because the code is in LCSSA form, any values used outside of the loop
  86   // must pass through a PHI in the exit block, meaning that this check is
  87   // sufficient to guarantee that no loop-variant values are used outside
  88   // of the loop.
  89   BasicBlock::iterator BI = exitBlock->begin();
  90   while (PHINode* P = dyn_cast<PHINode>(BI)) {
  91     Value* incoming = P->getIncomingValueForBlock(exitingBlock);
  92     if (Instruction* I = dyn_cast<Instruction>(incoming))
  93       if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator()))
  94         return false;
  95
  96     ++BI;
  97   }
  98
  99   // Make sure that no instructions in the block have potential side-effects.
 100   // This includes instructions that could write to memory, and loads that are
 101   // marked volatile.  This could be made more aggressive by using aliasing
 102   // information to identify readonly and readnone calls.
 103   for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
 104        LI != LE; ++LI) {
 105     for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
 106          BI != BE; ++BI) {
 107       if (BI->mayHaveSideEffects())
 108         return false;
 109     }
 110   }
 111
 112   return true;
 113 }
 114
 115 /// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
 116 /// observable behavior of the program other than finite running time.  Note
 117 /// we do ensure that this never remove a loop that might be infinite, as doing
 118 /// so could change the halting/non-halting nature of a program.
 119 /// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
 120 /// in order to make various safety checks work.
 121 bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
 122   // We can only remove the loop if there is a preheader that we can
 123   // branch from after removing it.
 124   BasicBlock* preheader = L->getLoopPreheader();
 125   if (!preheader)
 126     return false;
 127
 128   // If LoopSimplify form is not available, stay out of trouble.
 129   if (!L->hasDedicatedExits())
 130     return false;
 131
 132   // We can't remove loops that contain subloops.  If the subloops were dead,
 133   // they would already have been removed in earlier executions of this pass.
 134   if (L->begin() != L->end())
 135     return false;
 136
 137   SmallVector<BasicBlock*, 4> exitingBlocks;
 138   L->getExitingBlocks(exitingBlocks);
 139
 140   SmallVector<BasicBlock*, 4> exitBlocks;
 141   L->getUniqueExitBlocks(exitBlocks);
 142
 143   // We require that the loop only have a single exit block.  Otherwise, we'd
 144   // be in the situation of needing to be able to solve statically which exit
 145   // block will be branched to, or trying to preserve the branching logic in
 146   // a loop invariant manner.
 147   if (exitBlocks.size() != 1)
 148     return false;
 149
 150   // Loops with multiple exits are too complicated to handle correctly.
 151   if (exitingBlocks.size() != 1)
 152     return false;
 153
 154   // Finally, we have to check that the loop really is dead.
 155   bool Changed = false;
 156   if (!IsLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader))
 157     return Changed;
 158
 159   // Don't remove loops for which we can't solve the trip count.
 160   // They could be infinite, in which case we'd be changing program behavior.
 161   ScalarEvolution& SE = getAnalysis<ScalarEvolution>();
 162   const SCEV *S = SE.getMaxBackedgeTakenCount(L);
 163   if (isa<SCEVCouldNotCompute>(S))
 164     return Changed;
 165
 166   // Now that we know the removal is safe, remove the loop by changing the
 167   // branch from the preheader to go to the single exit block.
 168   BasicBlock* exitBlock = exitBlocks[0];
 169   BasicBlock* exitingBlock = exitingBlocks[0];
 170
 171   // Because we're deleting a large chunk of code at once, the sequence in which
 172   // we remove things is very important to avoid invalidation issues.  Don't
 173   // mess with this unless you have good reason and know what you're doing.
 174
 175   // Tell ScalarEvolution that the loop is deleted. Do this before
 176   // deleting the loop so that ScalarEvolution can look at the loop
 177   // to determine what it needs to clean up.
 178   SE.forgetLoop(L);
 179
 180   // Connect the preheader directly to the exit block.
 181   TerminatorInst* TI = preheader->getTerminator();
 182   TI->replaceUsesOfWith(L->getHeader(), exitBlock);
 183
 184   // Rewrite phis in the exit block to get their inputs from
 185   // the preheader instead of the exiting block.
 186   BasicBlock::iterator BI = exitBlock->begin();
 187   while (PHINode* P = dyn_cast<PHINode>(BI)) {
 188     P->replaceUsesOfWith(exitingBlock, preheader);
 189     ++BI;
 190   }
 191
 192   // Update the dominator tree and remove the instructions and blocks that will
 193   // be deleted from the reference counting scheme.
 194   DominatorTree& DT = getAnalysis<DominatorTree>();
 195   DominanceFrontier* DF = getAnalysisIfAvailable<DominanceFrontier>();
 196   SmallPtrSet<DomTreeNode*, 8> ChildNodes;
 197   for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
 198        LI != LE; ++LI) {
 199     // Move all of the block's children to be children of the preheader, which
 200     // allows us to remove the domtree entry for the block.
 201     ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end());
 202     for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(),
 203          DE = ChildNodes.end(); DI != DE; ++DI) {
 204       DT.changeImmediateDominator(*DI, DT[preheader]);
 205       if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT);
 206     }
 207
 208     ChildNodes.clear();
 209     DT.eraseNode(*LI);
 210     if (DF) DF->removeBlock(*LI);
 211
 212     // Remove the block from the reference counting scheme, so that we can
 213     // delete it freely later.
 214     (*LI)->dropAllReferences();
 215   }
 216
 217   // Erase the instructions and the blocks without having to worry
 218   // about ordering because we already dropped the references.
 219   // NOTE: This iteration is safe because erasing the block does not remove its
 220   // entry from the loop's block list.  We do that in the next section.
 221   for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
 222        LI != LE; ++LI)
 223     (*LI)->eraseFromParent();
 224
 225   // Finally, the blocks from loopinfo.  This has to happen late because
 226   // otherwise our loop iterators won't work.
 227   LoopInfo& loopInfo = getAnalysis<LoopInfo>();
 228   SmallPtrSet<BasicBlock*, 8> blocks;
 229   blocks.insert(L->block_begin(), L->block_end());
 230   for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(),
 231        E = blocks.end(); I != E; ++I)
 232     loopInfo.removeBlock(*I);
 233
 234   // The last step is to inform the loop pass manager that we've
 235   // eliminated this loop.
 236   LPM.deleteLoopFromQueue(L);
 237   Changed = true;
 238
 239   ++NumDeleted;
 240
 241   return Changed;
 242 }