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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.
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/IR/Dominators.h"
using namespace llvm;
#define DEBUG_TYPE "loop-delete"
STATISTIC(NumDeleted, "Number of loops deleted");
namespace {
class LoopDeletion : public LoopPass {
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) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool isLoopDead(Loop *L, SmallVectorImpl<BasicBlock *> &exitingBlocks,
SmallVectorImpl<BasicBlock *> &exitBlocks,
bool &Changed, BasicBlock *Preheader);
char LoopDeletion::ID = 0;
INITIALIZE_PASS_BEGIN(LoopDeletion, "loop-deletion",
"Delete dead loops", false, false)
INITIALIZE_PASS_END(LoopDeletion, "loop-deletion",
"Delete dead loops", false, false)
Pass *llvm::createLoopDeletionPass() {
return new LoopDeletion();
/// 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,
SmallVectorImpl<BasicBlock *> &exitingBlocks,
SmallVectorImpl<BasicBlock *> &exitBlocks,
bool &Changed, BasicBlock *Preheader) {
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(exitingBlocks[0]);
// Make sure all exiting blocks produce the same incoming value for the exit
// block. If there are different incoming values for different exiting
// blocks, then it is impossible to statically determine which value should
// be used.
for (unsigned i = 1, e = exitingBlocks.size(); i < e; ++i) {
if (incoming != P->getIncomingValueForBlock(exitingBlocks[i]))
return false;
if (Instruction *I = dyn_cast<Instruction>(incoming))
if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator()))
return false;
// 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) {
if (skipOptnoneFunction(L))
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;
// 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;
SmallVector<BasicBlock*, 4> 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;
// 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.
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.
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.
// 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.
// 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 *exitingBlock = exitingBlocks[0];
BasicBlock::iterator BI = exitBlock->begin();
while (PHINode *P = dyn_cast<PHINode>(BI)) {
int j = P->getBasicBlockIndex(exitingBlock);
assert(j >= 0 && "Can't find exiting block in exit block's phi node!");
P->setIncomingBlock(j, preheader);
for (unsigned i = 1; i < exitingBlocks.size(); ++i)
// Update the dominator tree and remove the instructions and blocks that will
// be deleted from the reference counting scheme.
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
SmallVector<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(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
for (SmallVectorImpl<DomTreeNode *>::iterator DI = ChildNodes.begin(),
DE = ChildNodes.end(); DI != DE; ++DI) {
DT.changeImmediateDominator(*DI, DT[preheader]);
// Remove the block from the reference counting scheme, so that we can
// delete it freely later.
// 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)
// Finally, the blocks from loopinfo. This has to happen late because
// otherwise our loop iterators won't work.
LoopInfo &loopInfo = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
SmallPtrSet<BasicBlock*, 8> blocks;
blocks.insert(L->block_begin(), L->block_end());
for (BasicBlock *BB : blocks)
// The last step is to inform the loop pass manager that we've
// eliminated this loop.
Changed = true;
return Changed;