LoopDeletion.cpp

//===- LoopDeletion.cpp - Dead Loop Deletion 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 Deletion Pass. This pass is responsible
// for eliminating loops with non-infinite computable trip counts that have no
// side effects or volatile instructions, and do not contribute to the
// computation of the function's return value.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "loop-delete"

#include "llvm/Transforms/Scalar.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/SmallVector.h"

using namespace llvm;

STATISTIC(NumDeleted, "Number of loops deleted");

namespace {
  class VISIBILITY_HIDDEN LoopDeletion : public LoopPass {
  public:
    static char ID; // Pass ID, replacement for typeid
    LoopDeletion() : LoopPass(&ID) {}
    
    // Possibly eliminate loop L if it is dead.
    bool runOnLoop(Loop* L, LPPassManager& LPM);
    
    bool SingleDominatingExit(Loop* L,
                              SmallVector<BasicBlock*, 4>& exitingBlocks);
    bool IsLoopDead(Loop* L, SmallVector<BasicBlock*, 4>& exitingBlocks,
                    SmallVector<BasicBlock*, 4>& exitBlocks);
    bool IsLoopInvariantInst(Instruction *I, Loop* L);
    
    virtual void getAnalysisUsage(AnalysisUsage& AU) const {
      AU.addRequired<ScalarEvolution>();
      AU.addRequired<DominatorTree>();
      AU.addRequired<LoopInfo>();
      AU.addRequiredID(LoopSimplifyID);
      AU.addRequiredID(LCSSAID);
      
      AU.addPreserved<ScalarEvolution>();
      AU.addPreserved<DominatorTree>();
      AU.addPreserved<LoopInfo>();
      AU.addPreservedID(LoopSimplifyID);
      AU.addPreservedID(LCSSAID);
    }
  };
}
  
char LoopDeletion::ID = 0;
static RegisterPass<LoopDeletion> X("loop-deletion", "Delete dead loops");

LoopPass* llvm::createLoopDeletionPass() {
  return new LoopDeletion();
}

/// SingleDominatingExit - Checks that there is only a single blocks that 
/// branches out of the loop, and that it also g the latch block.  Loops
/// with multiple or non-latch-dominating exiting blocks could be dead, but we'd
/// have to do more extensive analysis to make sure, for instance, that the 
/// control flow logic involved was or could be made loop-invariant.
bool LoopDeletion::SingleDominatingExit(Loop* L,
                                   SmallVector<BasicBlock*, 4>& exitingBlocks) {
  
  if (exitingBlocks.size() != 1)
    return false;
  
  BasicBlock* latch = L->getLoopLatch();
  if (!latch)
    return false;
  
  DominatorTree& DT = getAnalysis<DominatorTree>();
  return DT.dominates(exitingBlocks[0], latch);
}

/// IsLoopInvariantInst - Checks if an instruction is invariant with respect to
/// a loop, which is defined as being true if all of its operands are defined
/// outside of the loop.  These instructions can be hoisted out of the loop
/// if their results are needed.  This could be made more aggressive by
/// recursively checking the operands for invariance, but it's not clear that
/// it's worth it.
bool LoopDeletion::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;
}

/// IsLoopDead - Determined if a loop is dead.  This assumes that we've already
/// checked for unique exit and exiting blocks, and that the code is in LCSSA
/// form.
bool LoopDeletion::IsLoopDead(Loop* L,
                              SmallVector<BasicBlock*, 4>& exitingBlocks,
                              SmallVector<BasicBlock*, 4>& exitBlocks) {
  BasicBlock* exitingBlock = exitingBlocks[0];
  BasicBlock* exitBlock = exitBlocks[0];
  
  // Make sure that all PHI entries coming from the loop are loop invariant.
  // Because the code is in LCSSA form, any values used outside of the loop
  // must pass through a PHI in the exit block, meaning that this check is
  // sufficient to guarantee that no loop-variant values are used outside
  // of the loop.
  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.
  // This includes instructions that could write to memory, and loads that are
  // marked volatile.  This could be made more aggressive by using aliasing
  // information to identify readonly and readnone calls.
  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;
      else if (LoadInst* L = dyn_cast<LoadInst>(BI))
        if (L->isVolatile())
          return false;
    }
  }
  
  return true;
}

/// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
/// observable behavior of the program other than finite running time.  Note 
/// we do ensure that this never remove a loop that might be infinite, as doing
/// so could change the halting/non-halting nature of a program.
/// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
/// in order to make various safety checks work.
bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
  // 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;
  
  SmallVector<BasicBlock*, 4> exitingBlocks;
  L->getExitingBlocks(exitingBlocks);
  
  SmallVector<BasicBlock*, 4> exitBlocks;
  L->getUniqueExitBlocks(exitBlocks);
  
  // We require that the loop only have a single exit block.  Otherwise, we'd
  // be in the situation of needing to be able to solve statically which exit
  // block will be branched to, or trying to preserve the branching logic in
  // a loop invariant manner.
  if (exitBlocks.size() != 1)
    return false;
  
  // Loops with multiple exits or exits that don't dominate the latch
  // are too complicated to handle correctly.
  if (!SingleDominatingExit(L, exitingBlocks))
    return false;
  
  // Finally, we have to check that the loop really is dead.
  if (!IsLoopDead(L, exitingBlocks, exitBlocks))
    return false;
  
  // 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.
  ScalarEvolution& SE = getAnalysis<ScalarEvolution>();
  SCEVHandle S = SE.getIterationCount(L);
  if (isa<SCEVCouldNotCompute>(S))
    return false;
  
  // Now that we know the removal is safe, remove the loop by changing the
  // branch from the preheader to go to the single exit block.  
  BasicBlock* exitBlock = exitBlocks[0];
  BasicBlock* exitingBlock = exitingBlocks[0];
  
  // Because we're deleting a large chunk of code at once, the sequence in which
  // we remove things is very important to avoid invalidation issues.  Don't
  // mess with this unless you have good reason and know what you're doing.
  
  // Move simple loop-invariant expressions out of the loop, since they
  // might be needed by the exit phis.
  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->use_empty() && IsLoopInvariantInst(I, L))
        I->moveBefore(preheader->getTerminator());
    }
  
  // Connect the preheader directly to the exit block.
  TerminatorInst* TI = preheader->getTerminator();
  TI->replaceUsesOfWith(L->getHeader(), exitBlock);

  // Rewrite phis in the exit block to get their inputs from
  // the preheader instead of the exiting block.
  BasicBlock::iterator BI = exitBlock->begin();
  while (PHINode* P = dyn_cast<PHINode>(BI)) {
    P->replaceUsesOfWith(exitingBlock, preheader);
    BI++;
  }
  
  // Update the dominator tree and remove the instructions and blocks that will
  // be deleted from the reference counting scheme.
  DominatorTree& DT = getAnalysis<DominatorTree>();
  SmallPtrSet<DomTreeNode*, 8> ChildNodes;
  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
       LI != LE; ++LI) {
    // Move all of the block's children to be children of the preheader, which
    // allows us to remove the domtree entry for the block.
    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]);
    
    ChildNodes.clear();
    DT.eraseNode(*LI);
    
    // Remove instructions that we're deleting from ScalarEvolution.
    for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
         BI != BE; ++BI)
      SE.deleteValueFromRecords(BI);
    
    SE.deleteValueFromRecords(*LI);
    
    // Remove the block from the reference counting scheme, so that we can
    // delete it freely later.
    (*LI)->dropAllReferences();
  }
  
  // Erase the instructions and the blocks without having to worry
  // about ordering because we already dropped the references.
  // NOTE: This iteration is safe because erasing the block does not remove its
  // entry from the loop's block list.  We do that in the next section.
  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
       LI != LE; ++LI)
    (*LI)->eraseFromParent();
  
  // Finally, the blocks from loopinfo.  This has to happen late because
  // otherwise our loop iterators won't work.
  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);
  
  // The last step is to inform the loop pass manager that we've
  // eliminated this loop.
  LPM.deleteLoopFromQueue(L);
  
  NumDeleted++;
  
  return true;
}