//
//===----------------------------------------------------------------------===//
//
-// This file implements the Dead Loop Elimination 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.
+// 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 {
+ class LoopDeletion : public LoopPass {
public:
static char ID; // Pass ID, replacement for typeid
- LoopDeletion() : LoopPass((intptr_t)&ID) { }
+ 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);
-
+ SmallVector<BasicBlock*, 4>& exitBlocks,
+ bool &Changed, BasicBlock *Preheader);
+
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;
- 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 dominates 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 involves 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>();
- if (DT.dominates(exitingBlocks[0], latch))
- return true;
- else
- return false;
-}
-
-/// 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;
+char LoopDeletion::ID = 0;
+INITIALIZE_PASS(LoopDeletion, "loop-deletion",
+ "Delete dead loops", false, false);
- // If we got this far, the instruction is loop invariant!
- return true;
+Pass* llvm::createLoopDeletionPass() {
+ return new LoopDeletion();
}
/// IsLoopDead - Determined if a loop is dead. This assumes that we've already
/// form.
bool LoopDeletion::IsLoopDead(Loop* L,
SmallVector<BasicBlock*, 4>& exitingBlocks,
- SmallVector<BasicBlock*, 4>& exitBlocks) {
+ SmallVector<BasicBlock*, 4>& exitBlocks,
+ bool &Changed, BasicBlock *Preheader) {
BasicBlock* exitingBlock = exitingBlocks[0];
BasicBlock* exitBlock = exitBlocks[0];
while (PHINode* P = dyn_cast<PHINode>(BI)) {
Value* incoming = P->getIncomingValueForBlock(exitingBlock);
if (Instruction* I = dyn_cast<Instruction>(incoming))
- if (!IsLoopInvariantInst(I, L))
+ if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator()))
return false;
- BI++;
+ ++BI;
}
// Make sure that no instructions in the block have potential side-effects.
LI != LE; ++LI) {
for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
BI != BE; ++BI) {
- if (BI->mayWriteToMemory())
+ if (BI->mayHaveSideEffects())
return false;
- else if (LoadInst* L = dyn_cast<LoadInst>(BI))
- if (L->isVolatile())
- return false;
}
}
/// 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;
+
+ // If LoopSimplify form is not available, stay out of trouble.
+ if (!L->hasDedicatedExits())
+ 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);
// 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 branced to, or trying to preserve the branching logic in
+ // block will be branched to, or trying to preserve the branching logic in
// a loop invariant manner.
if (exitBlocks.size() != 1)
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)
+ // Loops with multiple exits are too complicated to handle correctly.
+ if (exitingBlocks.size() != 1)
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;
+ // Finally, we have to check that the loop really is dead.
+ bool Changed = false;
+ if (!IsLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader))
+ return Changed;
// 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;
-
- // 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;
+ ScalarEvolution& SE = getAnalysis<ScalarEvolution>();
+ const SCEV *S = SE.getMaxBackedgeTakenCount(L);
+ if (isa<SCEVCouldNotCompute>(S))
+ return Changed;
// 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.
// 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->getNumUses() > 0 && IsLoopInvariantInst(I, L))
- I->moveBefore(preheader->getTerminator());
- }
-
+
+ // 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.forgetLoop(L);
+
// Connect the preheader directly to the exit block.
TerminatorInst* TI = preheader->getTerminator();
TI->replaceUsesOfWith(L->getHeader(), exitBlock);
BasicBlock::iterator BI = exitBlock->begin();
while (PHINode* P = dyn_cast<PHINode>(BI)) {
P->replaceUsesOfWith(exitingBlock, preheader);
- BI++;
+ ++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) {
// 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)
+ DE = ChildNodes.end(); DI != DE; ++DI) {
DT.changeImmediateDominator(*DI, DT[preheader]);
+ if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT);
+ }
ChildNodes.clear();
DT.eraseNode(*LI);
-
- // Drop all references between the instructions and the block so
- // that we don't have reference counting problems later.
- for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
- BI != BE; ++BI) {
- BI->dropAllReferences();
- }
-
+ if (DF) DF->removeBlock(*LI);
+
+ // Remove the block from the reference counting scheme, so that we can
+ // delete it freely later.
(*LI)->dropAllReferences();
}
// 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) {
- for (Value::use_iterator UI = (*LI)->use_begin(), UE = (*LI)->use_end();
- UI != UE; ++UI)
- (*UI)->dump();
+ 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>();
// The last step is to inform the loop pass manager that we've
// eliminated this loop.
LPM.deleteLoopFromQueue(L);
+ Changed = true;
- NumDeleted++;
+ ++NumDeleted;
- return true;
+ return Changed;
}
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