llvm-6502/lib/Transforms/Scalar/LoopDeletion.cpp
2008-04-29 19:58:07 +00:00

261 lines
8.7 KiB
C++

//===- 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;
}
/// 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.
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];
// 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());
}
// Connect the preheader directly to the exit block.
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 {
// FIXME: Support switches
return false;
}
// 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)) {
unsigned i = P->getBasicBlockIndex(exitingBlock);
P->setIncomingBlock(i, preheader);
BI++;
}
// Update lots of internal structures...
DominatorTree& DT = getAnalysis<DominatorTree>();
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.
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);
// 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();
}
(*LI)->dropAllReferences();
}
// Erase the instructions and the blocks without having to worry
// about ordering because we already dropped the references.
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();
}
// 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;
}