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"
19 #include "llvm/Transforms/Scalar.h"
20 #include "llvm/Analysis/LoopPass.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/ADT/SmallVector.h"
26 STATISTIC(NumDeleted, "Number of loops deleted");
29 class VISIBILITY_HIDDEN LoopDeletion : public LoopPass {
31 static char ID; // Pass ID, replacement for typeid
32 LoopDeletion() : LoopPass((intptr_t)&ID) { }
34 // Possibly eliminate loop L if it is dead.
35 bool runOnLoop(Loop* L, LPPassManager& LPM);
37 bool SingleDominatingExit(Loop* L,
38 SmallVector<BasicBlock*, 4>& exitingBlocks);
39 bool IsLoopDead(Loop* L, SmallVector<BasicBlock*, 4>& exitingBlocks,
40 SmallVector<BasicBlock*, 4>& exitBlocks);
41 bool IsLoopInvariantInst(Instruction *I, Loop* L);
43 virtual void getAnalysisUsage(AnalysisUsage& AU) const {
44 AU.addRequired<DominatorTree>();
45 AU.addRequired<LoopInfo>();
46 AU.addRequiredID(LoopSimplifyID);
47 AU.addRequiredID(LCSSAID);
49 AU.addPreserved<DominatorTree>();
50 AU.addPreserved<LoopInfo>();
51 AU.addPreservedID(LoopSimplifyID);
52 AU.addPreservedID(LCSSAID);
57 char LoopDeletion::ID = 0;
58 static RegisterPass<LoopDeletion> X("loop-deletion", "Delete dead loops");
60 LoopPass* llvm::createLoopDeletionPass() {
61 return new LoopDeletion();
64 /// SingleDominatingExit - Checks that there is only a single blocks that
65 /// branches out of the loop, and that it also dominates the latch block. Loops
66 /// with multiple or non-latch-dominating exiting blocks could be dead, but we'd
67 /// have to do more extensive analysis to make sure, for instance, that the
68 /// control flow logic involves was or could be made loop-invariant.
69 bool LoopDeletion::SingleDominatingExit(Loop* L,
70 SmallVector<BasicBlock*, 4>& exitingBlocks) {
72 if (exitingBlocks.size() != 1)
75 BasicBlock* latch = L->getLoopLatch();
79 DominatorTree& DT = getAnalysis<DominatorTree>();
80 if (DT.dominates(exitingBlocks[0], latch))
86 /// IsLoopInvariantInst - Checks if an instruction is invariant with respect to
87 /// a loop, which is defined as being true if all of its operands are defined
88 /// outside of the loop. These instructions can be hoisted out of the loop
89 /// if their results are needed. This could be made more aggressive by
90 /// recursively checking the operands for invariance, but it's not clear that
92 bool LoopDeletion::IsLoopInvariantInst(Instruction *I, Loop* L) {
93 // PHI nodes are not loop invariant if defined in the loop.
94 if (isa<PHINode>(I) && L->contains(I->getParent()))
97 // The instruction is loop invariant if all of its operands are loop-invariant
98 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
99 if (!L->isLoopInvariant(I->getOperand(i)))
102 // If we got this far, the instruction is loop invariant!
106 /// IsLoopDead - Determined if a loop is dead. This assumes that we've already
107 /// checked for unique exit and exiting blocks, and that the code is in LCSSA
109 bool LoopDeletion::IsLoopDead(Loop* L,
110 SmallVector<BasicBlock*, 4>& exitingBlocks,
111 SmallVector<BasicBlock*, 4>& exitBlocks) {
112 BasicBlock* exitingBlock = exitingBlocks[0];
113 BasicBlock* exitBlock = exitBlocks[0];
115 // Make sure that all PHI entries coming from the loop are loop invariant.
116 // Because the code is in LCSSA form, any values used outside of the loop
117 // must pass through a PHI in the exit block, meaning that this check is
118 // sufficient to guarantee that no loop-variant values are used outside
120 BasicBlock::iterator BI = exitBlock->begin();
121 while (PHINode* P = dyn_cast<PHINode>(BI)) {
122 Value* incoming = P->getIncomingValueForBlock(exitingBlock);
123 if (Instruction* I = dyn_cast<Instruction>(incoming))
124 if (!IsLoopInvariantInst(I, L))
130 // Make sure that no instructions in the block have potential side-effects.
131 // This includes instructions that could write to memory, and loads that are
132 // marked volatile. This could be made more aggressive by using aliasing
133 // information to identify readonly and readnone calls.
134 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
136 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
138 if (BI->mayWriteToMemory())
140 else if (LoadInst* L = dyn_cast<LoadInst>(BI))
149 /// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
150 /// observable behavior of the program other than finite running time. Note
151 /// we do ensure that this never remove a loop that might be infinite, as doing
152 /// so could change the halting/non-halting nature of a program.
153 /// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
154 /// in order to make various safety checks work.
155 bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
156 SmallVector<BasicBlock*, 4> exitingBlocks;
157 L->getExitingBlocks(exitingBlocks);
159 SmallVector<BasicBlock*, 4> exitBlocks;
160 L->getUniqueExitBlocks(exitBlocks);
162 // We require that the loop only have a single exit block. Otherwise, we'd
163 // be in the situation of needing to be able to solve statically which exit
164 // block will be branced to, or trying to preserve the branching logic in
165 // a loop invariant manner.
166 if (exitBlocks.size() != 1)
169 // We can only remove the loop if there is a preheader that we can
170 // branch from after removing it.
171 BasicBlock* preheader = L->getLoopPreheader();
175 // We can't remove loops that contain subloops. If the subloops were dead,
176 // they would already have been removed in earlier executions of this pass.
177 if (L->begin() != L->end())
180 // Don't remove loops for which we can't solve the trip count.
181 // They could be infinite, in which case we'd be changing program behavior.
182 if (!L->getTripCount())
185 // Loops with multiple exits or exits that don't dominate the latch
186 // are too complicated to handle correctly.
187 if (!SingleDominatingExit(L, exitingBlocks))
190 // Finally, we have to check that the loop really is dead.
191 if (!IsLoopDead(L, exitingBlocks, exitBlocks))
194 // Now that we know the removal is safe, remove the loop by changing the
195 // branch from the preheader to go to the single exit block.
196 BasicBlock* exitBlock = exitBlocks[0];
197 BasicBlock* exitingBlock = exitingBlocks[0];
199 // Because we're deleting a large chunk of code at once, the sequence in which
200 // we remove things is very important to avoid invalidation issues. Don't
201 // mess with this unless you have good reason and know what you're doing.
203 // Move simple loop-invariant expressions out of the loop, since they
204 // might be needed by the exit phis.
205 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
207 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
209 Instruction* I = BI++;
210 if (I->getNumUses() > 0 && IsLoopInvariantInst(I, L))
211 I->moveBefore(preheader->getTerminator());
214 // Connect the preheader directly to the exit block.
215 TerminatorInst* TI = preheader->getTerminator();
216 TI->replaceUsesOfWith(L->getHeader(), exitBlock);
218 // Rewrite phis in the exit block to get their inputs from
219 // the preheader instead of the exiting block.
220 BasicBlock::iterator BI = exitBlock->begin();
221 while (PHINode* P = dyn_cast<PHINode>(BI)) {
222 P->replaceUsesOfWith(exitingBlock, preheader);
226 // Update the dominator tree and remove the instructions and blocks that will
227 // be deleted from the reference counting scheme.
228 DominatorTree& DT = getAnalysis<DominatorTree>();
229 SmallPtrSet<DomTreeNode*, 8> ChildNodes;
230 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
232 // Move all of the block's children to be children of the preheader, which
233 // allows us to remove the domtree entry for the block.
234 ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end());
235 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(),
236 DE = ChildNodes.end(); DI != DE; ++DI)
237 DT.changeImmediateDominator(*DI, DT[preheader]);
242 // Drop all references between the instructions and the block so
243 // that we don't have reference counting problems later.
244 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
246 BI->dropAllReferences();
249 (*LI)->dropAllReferences();
252 // Erase the instructions and the blocks without having to worry
253 // about ordering because we already dropped the references.
254 // NOTE: This iteration is safe because erasing the block does not remove its
255 // entry from the loop's block list. We do that in the next section.
256 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
258 for (Value::use_iterator UI = (*LI)->use_begin(), UE = (*LI)->use_end();
261 (*LI)->eraseFromParent();
264 // Finally, the blocks from loopinfo. This has to happen late because
265 // otherwise our loop iterators won't work.
266 LoopInfo& loopInfo = getAnalysis<LoopInfo>();
267 SmallPtrSet<BasicBlock*, 8> blocks;
268 blocks.insert(L->block_begin(), L->block_end());
269 for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(),
270 E = blocks.end(); I != E; ++I)
271 loopInfo.removeBlock(*I);
273 // The last step is to inform the loop pass manager that we've
274 // eliminated this loop.
275 LPM.deleteLoopFromQueue(L);