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/DominanceFrontier.h"
21 #include "llvm/Analysis/ScalarEvolution.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/ADT/SmallVector.h"
26 STATISTIC(NumDeleted, "Number of loops deleted");
29 class LoopDeletion : public LoopPass {
31 static char ID; // Pass ID, replacement for typeid
32 LoopDeletion() : LoopPass(ID) {
33 initializeLoopDeletionPass(*PassRegistry::getPassRegistry());
36 // Possibly eliminate loop L if it is dead.
37 bool runOnLoop(Loop* L, LPPassManager& LPM);
39 bool IsLoopDead(Loop* L, SmallVector<BasicBlock*, 4>& exitingBlocks,
40 SmallVector<BasicBlock*, 4>& exitBlocks,
41 bool &Changed, BasicBlock *Preheader);
43 virtual void getAnalysisUsage(AnalysisUsage& AU) const {
44 AU.addRequired<DominatorTree>();
45 AU.addRequired<LoopInfo>();
46 AU.addRequired<ScalarEvolution>();
47 AU.addRequiredID(LoopSimplifyID);
48 AU.addRequiredID(LCSSAID);
50 AU.addPreserved<ScalarEvolution>();
51 AU.addPreserved<DominatorTree>();
52 AU.addPreserved<LoopInfo>();
53 AU.addPreservedID(LoopSimplifyID);
54 AU.addPreservedID(LCSSAID);
55 AU.addPreserved<DominanceFrontier>();
60 char LoopDeletion::ID = 0;
61 INITIALIZE_PASS_BEGIN(LoopDeletion, "loop-deletion",
62 "Delete dead loops", false, false)
63 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
64 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
65 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
66 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
67 INITIALIZE_PASS_DEPENDENCY(LCSSA)
68 INITIALIZE_PASS_END(LoopDeletion, "loop-deletion",
69 "Delete dead loops", false, false)
71 Pass* llvm::createLoopDeletionPass() {
72 return new LoopDeletion();
75 /// IsLoopDead - Determined if a loop is dead. This assumes that we've already
76 /// checked for unique exit and exiting blocks, and that the code is in LCSSA
78 bool LoopDeletion::IsLoopDead(Loop* L,
79 SmallVector<BasicBlock*, 4>& exitingBlocks,
80 SmallVector<BasicBlock*, 4>& exitBlocks,
81 bool &Changed, BasicBlock *Preheader) {
82 BasicBlock* exitingBlock = exitingBlocks[0];
83 BasicBlock* exitBlock = exitBlocks[0];
85 // Make sure that all PHI entries coming from the loop are loop invariant.
86 // Because the code is in LCSSA form, any values used outside of the loop
87 // must pass through a PHI in the exit block, meaning that this check is
88 // sufficient to guarantee that no loop-variant values are used outside
90 BasicBlock::iterator BI = exitBlock->begin();
91 while (PHINode* P = dyn_cast<PHINode>(BI)) {
92 Value* incoming = P->getIncomingValueForBlock(exitingBlock);
93 if (Instruction* I = dyn_cast<Instruction>(incoming))
94 if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator()))
100 // Make sure that no instructions in the block have potential side-effects.
101 // This includes instructions that could write to memory, and loads that are
102 // marked volatile. This could be made more aggressive by using aliasing
103 // information to identify readonly and readnone calls.
104 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
106 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
108 if (BI->mayHaveSideEffects())
116 /// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
117 /// observable behavior of the program other than finite running time. Note
118 /// we do ensure that this never remove a loop that might be infinite, as doing
119 /// so could change the halting/non-halting nature of a program.
120 /// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
121 /// in order to make various safety checks work.
122 bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
123 // We can only remove the loop if there is a preheader that we can
124 // branch from after removing it.
125 BasicBlock* preheader = L->getLoopPreheader();
129 // If LoopSimplify form is not available, stay out of trouble.
130 if (!L->hasDedicatedExits())
133 // We can't remove loops that contain subloops. If the subloops were dead,
134 // they would already have been removed in earlier executions of this pass.
135 if (L->begin() != L->end())
138 SmallVector<BasicBlock*, 4> exitingBlocks;
139 L->getExitingBlocks(exitingBlocks);
141 SmallVector<BasicBlock*, 4> exitBlocks;
142 L->getUniqueExitBlocks(exitBlocks);
144 // We require that the loop only have a single exit block. Otherwise, we'd
145 // be in the situation of needing to be able to solve statically which exit
146 // block will be branched to, or trying to preserve the branching logic in
147 // a loop invariant manner.
148 if (exitBlocks.size() != 1)
151 // Loops with multiple exits are too complicated to handle correctly.
152 if (exitingBlocks.size() != 1)
155 // Finally, we have to check that the loop really is dead.
156 bool Changed = false;
157 if (!IsLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader))
160 // Don't remove loops for which we can't solve the trip count.
161 // They could be infinite, in which case we'd be changing program behavior.
162 ScalarEvolution& SE = getAnalysis<ScalarEvolution>();
163 const SCEV *S = SE.getMaxBackedgeTakenCount(L);
164 if (isa<SCEVCouldNotCompute>(S))
167 // Now that we know the removal is safe, remove the loop by changing the
168 // branch from the preheader to go to the single exit block.
169 BasicBlock* exitBlock = exitBlocks[0];
170 BasicBlock* exitingBlock = exitingBlocks[0];
172 // Because we're deleting a large chunk of code at once, the sequence in which
173 // we remove things is very important to avoid invalidation issues. Don't
174 // mess with this unless you have good reason and know what you're doing.
176 // Tell ScalarEvolution that the loop is deleted. Do this before
177 // deleting the loop so that ScalarEvolution can look at the loop
178 // to determine what it needs to clean up.
181 // Connect the preheader directly to the exit block.
182 TerminatorInst* TI = preheader->getTerminator();
183 TI->replaceUsesOfWith(L->getHeader(), exitBlock);
185 // Rewrite phis in the exit block to get their inputs from
186 // the preheader instead of the exiting block.
187 BasicBlock::iterator BI = exitBlock->begin();
188 while (PHINode* P = dyn_cast<PHINode>(BI)) {
189 P->replaceUsesOfWith(exitingBlock, preheader);
193 // Update the dominator tree and remove the instructions and blocks that will
194 // be deleted from the reference counting scheme.
195 DominatorTree& DT = getAnalysis<DominatorTree>();
196 DominanceFrontier* DF = getAnalysisIfAvailable<DominanceFrontier>();
197 SmallVector<DomTreeNode*, 8> ChildNodes;
198 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
200 // Move all of the block's children to be children of the preheader, which
201 // allows us to remove the domtree entry for the block.
202 ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
203 for (SmallVector<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(),
204 DE = ChildNodes.end(); DI != DE; ++DI) {
205 DT.changeImmediateDominator(*DI, DT[preheader]);
206 if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT);
211 if (DF) DF->removeBlock(*LI);
213 // Remove the block from the reference counting scheme, so that we can
214 // delete it freely later.
215 (*LI)->dropAllReferences();
218 // Erase the instructions and the blocks without having to worry
219 // about ordering because we already dropped the references.
220 // NOTE: This iteration is safe because erasing the block does not remove its
221 // entry from the loop's block list. We do that in the next section.
222 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
224 (*LI)->eraseFromParent();
226 // Finally, the blocks from loopinfo. This has to happen late because
227 // otherwise our loop iterators won't work.
228 LoopInfo& loopInfo = getAnalysis<LoopInfo>();
229 SmallPtrSet<BasicBlock*, 8> blocks;
230 blocks.insert(L->block_begin(), L->block_end());
231 for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(),
232 E = blocks.end(); I != E; ++I)
233 loopInfo.removeBlock(*I);
235 // The last step is to inform the loop pass manager that we've
236 // eliminated this loop.
237 LPM.deleteLoopFromQueue(L);