1 //===- LoopDeletion.cpp - Dead Loop Deletion Pass ---------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements the Dead Loop Deletion Pass. This pass is responsible
11 // for eliminating loops with non-infinite computable trip counts that have no
12 // side effects or volatile instructions, and do not contribute to the
13 // computation of the function's return value.
15 //===----------------------------------------------------------------------===//
17 #define DEBUG_TYPE "loop-delete"
18 #include "llvm/Transforms/Scalar.h"
19 #include "llvm/Analysis/LoopPass.h"
20 #include "llvm/Analysis/ScalarEvolution.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/ADT/SmallVector.h"
25 STATISTIC(NumDeleted, "Number of loops deleted");
28 class LoopDeletion : public LoopPass {
30 static char ID; // Pass ID, replacement for typeid
31 LoopDeletion() : LoopPass(ID) {}
33 // Possibly eliminate loop L if it is dead.
34 bool runOnLoop(Loop* L, LPPassManager& LPM);
36 bool IsLoopDead(Loop* L, SmallVector<BasicBlock*, 4>& exitingBlocks,
37 SmallVector<BasicBlock*, 4>& exitBlocks,
38 bool &Changed, BasicBlock *Preheader);
40 virtual void getAnalysisUsage(AnalysisUsage& AU) const {
41 AU.addRequired<DominatorTree>();
42 AU.addRequired<LoopInfo>();
43 AU.addRequired<ScalarEvolution>();
44 AU.addRequiredID(LoopSimplifyID);
45 AU.addRequiredID(LCSSAID);
47 AU.addPreserved<ScalarEvolution>();
48 AU.addPreserved<DominatorTree>();
49 AU.addPreserved<LoopInfo>();
50 AU.addPreservedID(LoopSimplifyID);
51 AU.addPreservedID(LCSSAID);
52 AU.addPreserved<DominanceFrontier>();
57 char LoopDeletion::ID = 0;
58 INITIALIZE_PASS(LoopDeletion, "loop-deletion",
59 "Delete dead loops", false, false);
61 Pass* llvm::createLoopDeletionPass() {
62 return new LoopDeletion();
65 /// IsLoopDead - Determined if a loop is dead. This assumes that we've already
66 /// checked for unique exit and exiting blocks, and that the code is in LCSSA
68 bool LoopDeletion::IsLoopDead(Loop* L,
69 SmallVector<BasicBlock*, 4>& exitingBlocks,
70 SmallVector<BasicBlock*, 4>& exitBlocks,
71 bool &Changed, BasicBlock *Preheader) {
72 BasicBlock* exitingBlock = exitingBlocks[0];
73 BasicBlock* exitBlock = exitBlocks[0];
75 // Make sure that all PHI entries coming from the loop are loop invariant.
76 // Because the code is in LCSSA form, any values used outside of the loop
77 // must pass through a PHI in the exit block, meaning that this check is
78 // sufficient to guarantee that no loop-variant values are used outside
80 BasicBlock::iterator BI = exitBlock->begin();
81 while (PHINode* P = dyn_cast<PHINode>(BI)) {
82 Value* incoming = P->getIncomingValueForBlock(exitingBlock);
83 if (Instruction* I = dyn_cast<Instruction>(incoming))
84 if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator()))
90 // Make sure that no instructions in the block have potential side-effects.
91 // This includes instructions that could write to memory, and loads that are
92 // marked volatile. This could be made more aggressive by using aliasing
93 // information to identify readonly and readnone calls.
94 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
96 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
98 if (BI->mayHaveSideEffects())
106 /// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
107 /// observable behavior of the program other than finite running time. Note
108 /// we do ensure that this never remove a loop that might be infinite, as doing
109 /// so could change the halting/non-halting nature of a program.
110 /// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
111 /// in order to make various safety checks work.
112 bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
113 // We can only remove the loop if there is a preheader that we can
114 // branch from after removing it.
115 BasicBlock* preheader = L->getLoopPreheader();
119 // If LoopSimplify form is not available, stay out of trouble.
120 if (!L->hasDedicatedExits())
123 // We can't remove loops that contain subloops. If the subloops were dead,
124 // they would already have been removed in earlier executions of this pass.
125 if (L->begin() != L->end())
128 SmallVector<BasicBlock*, 4> exitingBlocks;
129 L->getExitingBlocks(exitingBlocks);
131 SmallVector<BasicBlock*, 4> exitBlocks;
132 L->getUniqueExitBlocks(exitBlocks);
134 // We require that the loop only have a single exit block. Otherwise, we'd
135 // be in the situation of needing to be able to solve statically which exit
136 // block will be branched to, or trying to preserve the branching logic in
137 // a loop invariant manner.
138 if (exitBlocks.size() != 1)
141 // Loops with multiple exits are too complicated to handle correctly.
142 if (exitingBlocks.size() != 1)
145 // Finally, we have to check that the loop really is dead.
146 bool Changed = false;
147 if (!IsLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader))
150 // Don't remove loops for which we can't solve the trip count.
151 // They could be infinite, in which case we'd be changing program behavior.
152 ScalarEvolution& SE = getAnalysis<ScalarEvolution>();
153 const SCEV *S = SE.getMaxBackedgeTakenCount(L);
154 if (isa<SCEVCouldNotCompute>(S))
157 // Now that we know the removal is safe, remove the loop by changing the
158 // branch from the preheader to go to the single exit block.
159 BasicBlock* exitBlock = exitBlocks[0];
160 BasicBlock* exitingBlock = exitingBlocks[0];
162 // Because we're deleting a large chunk of code at once, the sequence in which
163 // we remove things is very important to avoid invalidation issues. Don't
164 // mess with this unless you have good reason and know what you're doing.
166 // Tell ScalarEvolution that the loop is deleted. Do this before
167 // deleting the loop so that ScalarEvolution can look at the loop
168 // to determine what it needs to clean up.
171 // Connect the preheader directly to the exit block.
172 TerminatorInst* TI = preheader->getTerminator();
173 TI->replaceUsesOfWith(L->getHeader(), exitBlock);
175 // Rewrite phis in the exit block to get their inputs from
176 // the preheader instead of the exiting block.
177 BasicBlock::iterator BI = exitBlock->begin();
178 while (PHINode* P = dyn_cast<PHINode>(BI)) {
179 P->replaceUsesOfWith(exitingBlock, preheader);
183 // Update the dominator tree and remove the instructions and blocks that will
184 // be deleted from the reference counting scheme.
185 DominatorTree& DT = getAnalysis<DominatorTree>();
186 DominanceFrontier* DF = getAnalysisIfAvailable<DominanceFrontier>();
187 SmallPtrSet<DomTreeNode*, 8> ChildNodes;
188 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
190 // Move all of the block's children to be children of the preheader, which
191 // allows us to remove the domtree entry for the block.
192 ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end());
193 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(),
194 DE = ChildNodes.end(); DI != DE; ++DI) {
195 DT.changeImmediateDominator(*DI, DT[preheader]);
196 if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT);
201 if (DF) DF->removeBlock(*LI);
203 // Remove the block from the reference counting scheme, so that we can
204 // delete it freely later.
205 (*LI)->dropAllReferences();
208 // Erase the instructions and the blocks without having to worry
209 // about ordering because we already dropped the references.
210 // NOTE: This iteration is safe because erasing the block does not remove its
211 // entry from the loop's block list. We do that in the next section.
212 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
214 (*LI)->eraseFromParent();
216 // Finally, the blocks from loopinfo. This has to happen late because
217 // otherwise our loop iterators won't work.
218 LoopInfo& loopInfo = getAnalysis<LoopInfo>();
219 SmallPtrSet<BasicBlock*, 8> blocks;
220 blocks.insert(L->block_begin(), L->block_end());
221 for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(),
222 E = blocks.end(); I != E; ++I)
223 loopInfo.removeBlock(*I);
225 // The last step is to inform the loop pass manager that we've
226 // eliminated this loop.
227 LPM.deleteLoopFromQueue(L);