[llvm-commits] [llvm] r50382 - in /llvm/trunk: include/llvm/LinkAllPasses.h include/llvm/Transforms/Scalar.h lib/Transforms/Scalar/DeadLoopElimination.cpp
Owen Anderson
resistor at mac.com
Mon Apr 28 17:38:35 PDT 2008
Author: resistor
Date: Mon Apr 28 19:38:34 2008
New Revision: 50382
URL: http://llvm.org/viewvc/llvm-project?rev=50382&view=rev
Log:
Add dead loop elimination, which removes dead loops for which we can compute
the trip count.
Added:
llvm/trunk/lib/Transforms/Scalar/DeadLoopElimination.cpp
Modified:
llvm/trunk/include/llvm/LinkAllPasses.h
llvm/trunk/include/llvm/Transforms/Scalar.h
Modified: llvm/trunk/include/llvm/LinkAllPasses.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/LinkAllPasses.h?rev=50382&r1=50381&r2=50382&view=diff
==============================================================================
--- llvm/trunk/include/llvm/LinkAllPasses.h (original)
+++ llvm/trunk/include/llvm/LinkAllPasses.h Mon Apr 28 19:38:34 2008
@@ -114,6 +114,7 @@
(void) llvm::createCodeGenPreparePass();
(void) llvm::createGVNPass();
(void) llvm::createMemCpyOptPass();
+ (void) llvm::createDeadLoopEliminationPass();
(void)new llvm::IntervalPartition();
(void)new llvm::FindUsedTypes();
Modified: llvm/trunk/include/llvm/Transforms/Scalar.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Transforms/Scalar.h?rev=50382&r1=50381&r2=50382&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Transforms/Scalar.h (original)
+++ llvm/trunk/include/llvm/Transforms/Scalar.h Mon Apr 28 19:38:34 2008
@@ -319,6 +319,13 @@
//===----------------------------------------------------------------------===//
//
+// 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.
//
FunctionPass *createCodeGenPreparePass(const TargetLowering *TLI = 0);
Added: llvm/trunk/lib/Transforms/Scalar/DeadLoopElimination.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/DeadLoopElimination.cpp?rev=50382&view=auto
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/DeadLoopElimination.cpp (added)
+++ llvm/trunk/lib/Transforms/Scalar/DeadLoopElimination.cpp Mon Apr 28 19:38:34 2008
@@ -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;
+}
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