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<ScalarEvolution>();
42 AU.addRequired<DominatorTree>();
43 AU.addRequired<LoopInfo>();
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 static RegisterPass<LoopDeletion> X("loop-deletion", "Delete dead loops");
60 Pass* llvm::createLoopDeletionPass() {
61 return new LoopDeletion();
64 /// IsLoopDead - Determined if a loop is dead. This assumes that we've already
65 /// checked for unique exit and exiting blocks, and that the code is in LCSSA
67 bool LoopDeletion::IsLoopDead(Loop* L,
68 SmallVector<BasicBlock*, 4>& exitingBlocks,
69 SmallVector<BasicBlock*, 4>& exitBlocks,
70 bool &Changed, BasicBlock *Preheader) {
71 BasicBlock* exitingBlock = exitingBlocks[0];
72 BasicBlock* exitBlock = exitBlocks[0];
74 // Make sure that all PHI entries coming from the loop are loop invariant.
75 // Because the code is in LCSSA form, any values used outside of the loop
76 // must pass through a PHI in the exit block, meaning that this check is
77 // sufficient to guarantee that no loop-variant values are used outside
79 BasicBlock::iterator BI = exitBlock->begin();
80 while (PHINode* P = dyn_cast<PHINode>(BI)) {
81 Value* incoming = P->getIncomingValueForBlock(exitingBlock);
82 if (Instruction* I = dyn_cast<Instruction>(incoming))
83 if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator()))
89 // Make sure that no instructions in the block have potential side-effects.
90 // This includes instructions that could write to memory, and loads that are
91 // marked volatile. This could be made more aggressive by using aliasing
92 // information to identify readonly and readnone calls.
93 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
95 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
97 if (BI->mayHaveSideEffects())
105 /// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
106 /// observable behavior of the program other than finite running time. Note
107 /// we do ensure that this never remove a loop that might be infinite, as doing
108 /// so could change the halting/non-halting nature of a program.
109 /// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
110 /// in order to make various safety checks work.
111 bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
112 // We can only remove the loop if there is a preheader that we can
113 // branch from after removing it.
114 BasicBlock* preheader = L->getLoopPreheader();
118 // We can't remove loops that contain subloops. If the subloops were dead,
119 // they would already have been removed in earlier executions of this pass.
120 if (L->begin() != L->end())
123 SmallVector<BasicBlock*, 4> exitingBlocks;
124 L->getExitingBlocks(exitingBlocks);
126 SmallVector<BasicBlock*, 4> exitBlocks;
127 L->getUniqueExitBlocks(exitBlocks);
129 // We require that the loop only have a single exit block. Otherwise, we'd
130 // be in the situation of needing to be able to solve statically which exit
131 // block will be branched to, or trying to preserve the branching logic in
132 // a loop invariant manner.
133 if (exitBlocks.size() != 1)
136 // Loops with multiple exits are too complicated to handle correctly.
137 if (exitingBlocks.size() != 1)
140 // Finally, we have to check that the loop really is dead.
141 bool Changed = false;
142 if (!IsLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader))
145 // Don't remove loops for which we can't solve the trip count.
146 // They could be infinite, in which case we'd be changing program behavior.
147 ScalarEvolution& SE = getAnalysis<ScalarEvolution>();
148 const SCEV *S = SE.getMaxBackedgeTakenCount(L);
149 if (isa<SCEVCouldNotCompute>(S))
152 // Now that we know the removal is safe, remove the loop by changing the
153 // branch from the preheader to go to the single exit block.
154 BasicBlock* exitBlock = exitBlocks[0];
155 BasicBlock* exitingBlock = exitingBlocks[0];
157 // Because we're deleting a large chunk of code at once, the sequence in which
158 // we remove things is very important to avoid invalidation issues. Don't
159 // mess with this unless you have good reason and know what you're doing.
161 // Tell ScalarEvolution that the loop is deleted. Do this before
162 // deleting the loop so that ScalarEvolution can look at the loop
163 // to determine what it needs to clean up.
164 SE.forgetLoopBackedgeTakenCount(L);
166 // Connect the preheader directly to the exit block.
167 TerminatorInst* TI = preheader->getTerminator();
168 TI->replaceUsesOfWith(L->getHeader(), exitBlock);
170 // Rewrite phis in the exit block to get their inputs from
171 // the preheader instead of the exiting block.
172 BasicBlock::iterator BI = exitBlock->begin();
173 while (PHINode* P = dyn_cast<PHINode>(BI)) {
174 P->replaceUsesOfWith(exitingBlock, preheader);
178 // Update the dominator tree and remove the instructions and blocks that will
179 // be deleted from the reference counting scheme.
180 DominatorTree& DT = getAnalysis<DominatorTree>();
181 DominanceFrontier* DF = getAnalysisIfAvailable<DominanceFrontier>();
182 SmallPtrSet<DomTreeNode*, 8> ChildNodes;
183 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
185 // Move all of the block's children to be children of the preheader, which
186 // allows us to remove the domtree entry for the block.
187 ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end());
188 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(),
189 DE = ChildNodes.end(); DI != DE; ++DI) {
190 DT.changeImmediateDominator(*DI, DT[preheader]);
191 if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT);
196 if (DF) DF->removeBlock(*LI);
198 // Remove the block from the reference counting scheme, so that we can
199 // delete it freely later.
200 (*LI)->dropAllReferences();
203 // Erase the instructions and the blocks without having to worry
204 // about ordering because we already dropped the references.
205 // NOTE: This iteration is safe because erasing the block does not remove its
206 // entry from the loop's block list. We do that in the next section.
207 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
209 (*LI)->eraseFromParent();
211 // Finally, the blocks from loopinfo. This has to happen late because
212 // otherwise our loop iterators won't work.
213 LoopInfo& loopInfo = getAnalysis<LoopInfo>();
214 SmallPtrSet<BasicBlock*, 8> blocks;
215 blocks.insert(L->block_begin(), L->block_end());
216 for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(),
217 E = blocks.end(); I != E; ++I)
218 loopInfo.removeBlock(*I);
220 // The last step is to inform the loop pass manager that we've
221 // eliminated this loop.
222 LPM.deleteLoopFromQueue(L);