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 #include "llvm/Transforms/Scalar.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/LoopPass.h"
21 #include "llvm/Analysis/ScalarEvolution.h"
22 #include "llvm/IR/Dominators.h"
25 #define DEBUG_TYPE "loop-delete"
27 STATISTIC(NumDeleted, "Number of loops deleted");
30 class LoopDeletion : public LoopPass {
32 static char ID; // Pass ID, replacement for typeid
33 LoopDeletion() : LoopPass(ID) {
34 initializeLoopDeletionPass(*PassRegistry::getPassRegistry());
37 // Possibly eliminate loop L if it is dead.
38 bool runOnLoop(Loop *L, LPPassManager &LPM) override;
40 void getAnalysisUsage(AnalysisUsage &AU) const override {
41 AU.addRequired<DominatorTreeWrapperPass>();
42 AU.addRequired<LoopInfo>();
43 AU.addRequired<ScalarEvolution>();
44 AU.addRequiredID(LoopSimplifyID);
45 AU.addRequiredID(LCSSAID);
47 AU.addPreserved<ScalarEvolution>();
48 AU.addPreserved<DominatorTreeWrapperPass>();
49 AU.addPreserved<LoopInfo>();
50 AU.addPreservedID(LoopSimplifyID);
51 AU.addPreservedID(LCSSAID);
55 bool isLoopDead(Loop *L, SmallVectorImpl<BasicBlock *> &exitingBlocks,
56 SmallVectorImpl<BasicBlock *> &exitBlocks,
57 bool &Changed, BasicBlock *Preheader);
62 char LoopDeletion::ID = 0;
63 INITIALIZE_PASS_BEGIN(LoopDeletion, "loop-deletion",
64 "Delete dead loops", false, false)
65 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
66 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
67 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
68 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
69 INITIALIZE_PASS_DEPENDENCY(LCSSA)
70 INITIALIZE_PASS_END(LoopDeletion, "loop-deletion",
71 "Delete dead loops", false, false)
73 Pass *llvm::createLoopDeletionPass() {
74 return new LoopDeletion();
77 /// isLoopDead - Determined if a loop is dead. This assumes that we've already
78 /// checked for unique exit and exiting blocks, and that the code is in LCSSA
80 bool LoopDeletion::isLoopDead(Loop *L,
81 SmallVectorImpl<BasicBlock *> &exitingBlocks,
82 SmallVectorImpl<BasicBlock *> &exitBlocks,
83 bool &Changed, BasicBlock *Preheader) {
84 BasicBlock *exitBlock = exitBlocks[0];
86 // Make sure that all PHI entries coming from the loop are loop invariant.
87 // Because the code is in LCSSA form, any values used outside of the loop
88 // must pass through a PHI in the exit block, meaning that this check is
89 // sufficient to guarantee that no loop-variant values are used outside
91 BasicBlock::iterator BI = exitBlock->begin();
92 while (PHINode *P = dyn_cast<PHINode>(BI)) {
93 Value *incoming = P->getIncomingValueForBlock(exitingBlocks[0]);
95 // Make sure all exiting blocks produce the same incoming value for the exit
96 // block. If there are different incoming values for different exiting
97 // blocks, then it is impossible to statically determine which value should
99 for (unsigned i = 1, e = exitingBlocks.size(); i < e; ++i) {
100 if (incoming != P->getIncomingValueForBlock(exitingBlocks[i]))
104 if (Instruction *I = dyn_cast<Instruction>(incoming))
105 if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator()))
111 // Make sure that no instructions in the block have potential side-effects.
112 // This includes instructions that could write to memory, and loads that are
113 // marked volatile. This could be made more aggressive by using aliasing
114 // information to identify readonly and readnone calls.
115 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
117 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
119 if (BI->mayHaveSideEffects())
127 /// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
128 /// observable behavior of the program other than finite running time. Note
129 /// we do ensure that this never remove a loop that might be infinite, as doing
130 /// so could change the halting/non-halting nature of a program.
131 /// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
132 /// in order to make various safety checks work.
133 bool LoopDeletion::runOnLoop(Loop *L, LPPassManager &LPM) {
134 if (skipOptnoneFunction(L))
137 // We can only remove the loop if there is a preheader that we can
138 // branch from after removing it.
139 BasicBlock *preheader = L->getLoopPreheader();
143 // If LoopSimplify form is not available, stay out of trouble.
144 if (!L->hasDedicatedExits())
147 // We can't remove loops that contain subloops. If the subloops were dead,
148 // they would already have been removed in earlier executions of this pass.
149 if (L->begin() != L->end())
152 SmallVector<BasicBlock*, 4> exitingBlocks;
153 L->getExitingBlocks(exitingBlocks);
155 SmallVector<BasicBlock*, 4> exitBlocks;
156 L->getUniqueExitBlocks(exitBlocks);
158 // We require that the loop only have a single exit block. Otherwise, we'd
159 // be in the situation of needing to be able to solve statically which exit
160 // block will be branched to, or trying to preserve the branching logic in
161 // a loop invariant manner.
162 if (exitBlocks.size() != 1)
165 // Finally, we have to check that the loop really is dead.
166 bool Changed = false;
167 if (!isLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader))
170 // Don't remove loops for which we can't solve the trip count.
171 // They could be infinite, in which case we'd be changing program behavior.
172 ScalarEvolution &SE = getAnalysis<ScalarEvolution>();
173 const SCEV *S = SE.getMaxBackedgeTakenCount(L);
174 if (isa<SCEVCouldNotCompute>(S))
177 // Now that we know the removal is safe, remove the loop by changing the
178 // branch from the preheader to go to the single exit block.
179 BasicBlock *exitBlock = exitBlocks[0];
181 // Because we're deleting a large chunk of code at once, the sequence in which
182 // we remove things is very important to avoid invalidation issues. Don't
183 // mess with this unless you have good reason and know what you're doing.
185 // Tell ScalarEvolution that the loop is deleted. Do this before
186 // deleting the loop so that ScalarEvolution can look at the loop
187 // to determine what it needs to clean up.
190 // Connect the preheader directly to the exit block.
191 TerminatorInst *TI = preheader->getTerminator();
192 TI->replaceUsesOfWith(L->getHeader(), exitBlock);
194 // Rewrite phis in the exit block to get their inputs from
195 // the preheader instead of the exiting block.
196 BasicBlock *exitingBlock = exitingBlocks[0];
197 BasicBlock::iterator BI = exitBlock->begin();
198 while (PHINode *P = dyn_cast<PHINode>(BI)) {
199 int j = P->getBasicBlockIndex(exitingBlock);
200 assert(j >= 0 && "Can't find exiting block in exit block's phi node!");
201 P->setIncomingBlock(j, preheader);
202 for (unsigned i = 1; i < exitingBlocks.size(); ++i)
203 P->removeIncomingValue(exitingBlocks[i]);
207 // Update the dominator tree and remove the instructions and blocks that will
208 // be deleted from the reference counting scheme.
209 DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
210 SmallVector<DomTreeNode*, 8> ChildNodes;
211 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
213 // Move all of the block's children to be children of the preheader, which
214 // allows us to remove the domtree entry for the block.
215 ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
216 for (SmallVectorImpl<DomTreeNode *>::iterator DI = ChildNodes.begin(),
217 DE = ChildNodes.end(); DI != DE; ++DI) {
218 DT.changeImmediateDominator(*DI, DT[preheader]);
224 // Remove the block from the reference counting scheme, so that we can
225 // delete it freely later.
226 (*LI)->dropAllReferences();
229 // Erase the instructions and the blocks without having to worry
230 // about ordering because we already dropped the references.
231 // NOTE: This iteration is safe because erasing the block does not remove its
232 // entry from the loop's block list. We do that in the next section.
233 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
235 (*LI)->eraseFromParent();
237 // Finally, the blocks from loopinfo. This has to happen late because
238 // otherwise our loop iterators won't work.
239 LoopInfo &loopInfo = getAnalysis<LoopInfo>();
240 SmallPtrSet<BasicBlock*, 8> blocks;
241 blocks.insert(L->block_begin(), L->block_end());
242 for (BasicBlock *BB : blocks)
243 loopInfo.removeBlock(BB);
245 // The last step is to inform the loop pass manager that we've
246 // eliminated this loop.
247 LPM.deleteLoopFromQueue(L);