llvm-6502/lib/Transforms/Scalar/LoopDeletion.cpp
Dan Gohman 46bdfb0e6b Rename ScalarEvolution's getIterationCount to getBackedgeTakenCount,
to more accurately describe what it does. Expand its doxygen comment
to describe what the backedge-taken count is and how it differs
from the actual iteration count of the loop. Adjust names and
comments in associated code accordingly.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@65382 91177308-0d34-0410-b5e6-96231b3b80d8
2009-02-24 18:55:53 +00:00

289 lines
11 KiB
C++

//===- 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->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.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 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();
// Tell ScalarEvolution that the loop is deleted.
SE.forgetLoopBackedgeTakenCount(L);
// 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;
}