Import LLVM, at r72732.

author: ed <ed@FreeBSD.org> 2009-06-02 17:52:33 +0000
committer: ed <ed@FreeBSD.org> 2009-06-02 17:52:33 +0000
commit: 3277b69d734b9c90b44ebde4ede005717e2c3b2e (patch)
tree: 64ba909838c23261cace781ece27d106134ea451 /lib/Transforms/Scalar/LoopDeletion.cpp
download: FreeBSD-src-3277b69d734b9c90b44ebde4ede005717e2c3b2e.zip
FreeBSD-src-3277b69d734b9c90b44ebde4ede005717e2c3b2e.tar.gz
1 files changed, 280 insertions, 0 deletions
diff --git a/lib/Transforms/Scalar/LoopDeletion.cpp b/lib/Transforms/Scalar/LoopDeletion.cpp
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+//===- 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);
+      AU.addPreserved<DominanceFrontier>();
+    }
+  };
+}
+  
+char LoopDeletion::ID = 0;
+static RegisterPass<LoopDeletion> X("loop-deletion", "Delete dead loops");
+
+Pass* 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->mayHaveSideEffects())
+        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.getBackedgeTakenCount(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>();
+  DominanceFrontier* DF = getAnalysisIfAvailable<DominanceFrontier>();
+  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]);
+      if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT);
+    }
+    
+    ChildNodes.clear();
+    DT.eraseNode(*LI);
+    if (DF) DF->removeBlock(*LI);
+
+    // Remove the block from the reference counting scheme, so that we can
+    // delete it freely later.
+    (*LI)->dropAllReferences();
+  }
+  
+  // Tell ScalarEvolution that the loop is deleted. Do this before
+  // deleting the loop so that ScalarEvolution can look at the loop
+  // to determine what it needs to clean up.
+  SE.forgetLoopBackedgeTakenCount(L);
+
+  // 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;
+}
author	ed <ed@FreeBSD.org>	2009-06-02 17:52:33 +0000
committer	ed <ed@FreeBSD.org>	2009-06-02 17:52:33 +0000
commit	3277b69d734b9c90b44ebde4ede005717e2c3b2e (patch)
tree	64ba909838c23261cace781ece27d106134ea451 /lib/Transforms/Scalar/LoopDeletion.cpp
download	FreeBSD-src-3277b69d734b9c90b44ebde4ede005717e2c3b2e.zip FreeBSD-src-3277b69d734b9c90b44ebde4ede005717e2c3b2e.tar.gz