Add dead loop elimination, which removes dead loops for which we can compute

the trip count.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@50382 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/LinkAllPasses.h

@@ -114,6 +114,7 @@ namespace {
       (void) llvm::createCodeGenPreparePass();
       (void) llvm::createGVNPass();
       (void) llvm::createMemCpyOptPass();
+      (void) llvm::createDeadLoopEliminationPass();
 
       (void)new llvm::IntervalPartition();
       (void)new llvm::FindUsedTypes();

include/llvm/Transforms/Scalar.h

@@ -317,6 +317,13 @@ FunctionPass *createGVNPass();
 //
 FunctionPass *createMemCpyOptPass();
 
+//===----------------------------------------------------------------------===//
+//
+// DeadLoopElimination - This pass performs DCE of non-infinite loops that it
+// can prove are dead.
+//
+LoopPass *createDeadLoopEliminationPass();
+
 //===----------------------------------------------------------------------===//
 //
 // CodeGenPrepare - This pass prepares a function for instruction selection.

lib/Transforms/Scalar/DeadLoopElimination.cpp

@@ -0,0 +1,239 @@
+//===- DeadLoopElimination.cpp - Dead Loop Elimination Pass ---------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the Dead Loop Elimination Pass.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "dead-loop"
+
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Instruction.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/SmallVector.h"
+
+using namespace llvm;
+
+STATISTIC(NumDeleted, "Number of loops deleted");
+
+namespace {
+  class VISIBILITY_HIDDEN DeadLoopElimination : public LoopPass {
+  public:
+    static char ID; // Pass ID, replacement for typeid
+    DeadLoopElimination() : LoopPass((intptr_t)&ID) { }
+    
+    // Possibly eliminate loop L if it is dead.
+    bool runOnLoop(Loop* L, LPPassManager& LPM);
+    
+    bool SingleDominatingExit(Loop* L);
+    bool IsLoopDead(Loop* L);
+    bool IsLoopInvariantInst(Instruction *I, Loop* L);
+    
+    virtual void getAnalysisUsage(AnalysisUsage& AU) const {
+      AU.addRequired<DominatorTree>();
+      AU.addRequired<LoopInfo>();
+      AU.addRequiredID(LoopSimplifyID);
+      AU.addRequiredID(LCSSAID);
+      
+      AU.addPreserved<DominatorTree>();
+      AU.addPreserved<LoopInfo>();
+      AU.addPreservedID(LoopSimplifyID);
+      AU.addPreservedID(LCSSAID);
+    }
+  };
+  
+  char DeadLoopElimination::ID = 0;
+  RegisterPass<DeadLoopElimination> X ("dead-loop", "Eliminate dead loops");
+}
+
+LoopPass* llvm::createDeadLoopEliminationPass() {
+  return new DeadLoopElimination();
+}
+
+bool DeadLoopElimination::SingleDominatingExit(Loop* L) {
+  SmallVector<BasicBlock*, 4> exitingBlocks;
+  L->getExitingBlocks(exitingBlocks);
+  
+  if (exitingBlocks.size() != 1)
+    return 0;
+  
+  BasicBlock* latch = L->getLoopLatch();
+  if (!latch)
+    return 0;
+  
+  DominatorTree& DT = getAnalysis<DominatorTree>();
+  if (DT.dominates(exitingBlocks[0], latch))
+    return exitingBlocks[0];
+  else
+    return 0;
+}
+
+bool DeadLoopElimination::IsLoopInvariantInst(Instruction *I, Loop* L)  {
+  // PHI nodes are not loop invariant if defined in  the loop.
+  if (isa<PHINode>(I) && L->contains(I->getParent()))
+    return false;
+    
+  // The instruction is loop invariant if all of its operands are loop-invariant
+  for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
+    if (!L->isLoopInvariant(I->getOperand(i)))
+      return false;
+
+  // If we got this far, the instruction is loop invariant!
+  return true;
+}
+
+bool DeadLoopElimination::IsLoopDead(Loop* L) {
+  SmallVector<BasicBlock*, 1> exitingBlocks;
+  L->getExitingBlocks(exitingBlocks);
+  BasicBlock* exitingBlock = exitingBlocks[0];
+    
+  // Get the set of out-of-loop blocks that the exiting block branches to.
+  SmallVector<BasicBlock*, 8> exitBlocks;
+  L->getUniqueExitBlocks(exitBlocks);
+  if (exitBlocks.size() > 1)
+    return false;
+  BasicBlock* exitBlock = exitBlocks[0];
+  
+  // Make sure that all PHI entries coming from the loop are loop invariant.
+  BasicBlock::iterator BI = exitBlock->begin();
+  while (PHINode* P = dyn_cast<PHINode>(BI)) {
+    Value* incoming = P->getIncomingValueForBlock(exitingBlock);
+    if (Instruction* I = dyn_cast<Instruction>(incoming))
+      if (!IsLoopInvariantInst(I, L))
+        return false;
+      
+    BI++;
+  }
+  
+  // Make sure that no instructions in the block have potential side-effects.
+  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
+       LI != LE; ++LI) {
+    for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
+         BI != BE; ++BI) {
+      if (BI->mayWriteToMemory())
+        return false;
+    }
+  }
+  
+  return true;
+}
+
+bool DeadLoopElimination::runOnLoop(Loop* L, LPPassManager& LPM) {
+  // Don't remove loops for which we can't solve the trip count.
+  // They could be infinite, in which case we'd be changing program behavior.
+  if (L->getTripCount())
+    return false;
+  
+  // We can only remove the loop if there is a preheader that we can 
+  // branch from after removing it.
+  BasicBlock* preheader = L->getLoopPreheader();
+  if (!preheader)
+    return false;
+  
+  // We can't remove loops that contain subloops.  If the subloops were dead,
+  // they would already have been removed in earlier executions of this pass.
+  if (L->begin() != L->end())
+    return false;
+  
+  // Loops with multiple exits or exits that don't dominate the latch
+  // are too complicated to handle correctly.
+  if (!SingleDominatingExit(L))
+    return false;
+  
+  // Finally, we have to check that the loop really is dead.
+  if (!IsLoopDead(L))
+    return false;
+  
+  // Now that we know the removal is safe, change the branch from the preheader
+  // to go to the single exiting block.
+  SmallVector<BasicBlock*, 1> exitingBlocks;
+  L->getExitingBlocks(exitingBlocks);
+  BasicBlock* exitingBlock = exitingBlocks[0];
+  
+  SmallVector<BasicBlock*, 1> exitBlocks;
+  L->getUniqueExitBlocks(exitBlocks);
+  BasicBlock* exitBlock = exitBlocks[0];
+  
+  Function* F = L->getLoopLatch()->getParent();
+  
+  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
+       LI != LE; ++LI)
+    for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
+         BI != BE; ) {
+      Instruction* I = BI++;
+      if (I->getNumUses() > 0 && IsLoopInvariantInst(I, L))
+        I->moveBefore(preheader->getTerminator());
+    }
+  
+  TerminatorInst* TI = preheader->getTerminator();
+  if (BranchInst* BI = dyn_cast<BranchInst>(TI)) {
+    if (BI->isUnconditional())
+      BI->setSuccessor(0, exitBlock);
+    else if (L->contains(BI->getSuccessor(0)))
+      BI->setSuccessor(0, exitBlock);
+    else
+      BI->setSuccessor(1, exitBlock);
+  } else {
+    return false;
+  }
+  
+  BasicBlock::iterator BI = exitBlock->begin();
+  while (PHINode* P = dyn_cast<PHINode>(BI)) {
+    unsigned i = P->getBasicBlockIndex(exitingBlock);
+    P->setIncomingBlock(i, preheader);
+    BI++;
+  }
+  
+  DominatorTree& DT = getAnalysis<DominatorTree>();
+  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
+       LI != LE; ++LI) {
+    SmallPtrSet<DomTreeNode*, 8> childNodes;
+    childNodes.insert(DT[*LI]->begin(), DT[*LI]->end());
+    for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = childNodes.begin(),
+         DE = childNodes.end(); DI != DE; ++DI)
+      DT.changeImmediateDominator(*DI, DT[preheader]);
+    
+    DT.eraseNode(*LI);
+    
+    for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
+         BI != BE; ++BI) {
+      BI->dropAllReferences();
+    }
+    
+    (*LI)->dropAllReferences();
+  }
+  
+  unsigned bar = 0;
+  
+  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
+       LI != LE; ++LI) {
+    for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
+         BI != BE; ) {
+      Instruction* I = BI++;
+      I->eraseFromParent();
+    }
+    
+    (*LI)->eraseFromParent();
+  }
+  
+  LoopInfo& loopInfo = getAnalysis<LoopInfo>();
+  SmallPtrSet<BasicBlock*, 8> blocks;
+  blocks.insert(L->block_begin(), L->block_end());
+  for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(),
+       E = blocks.end(); I != E; ++I)
+    loopInfo.removeBlock(*I);
+  
+  LPM.deleteLoopFromQueue(L);
+  
+  NumDeleted++;
+  
+  return true;
+}