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Owen Anderson0396cd32008-04-29 20:06:54 +00001//===- LoopDeletion.cpp - Dead Loop Deletion Pass ---------------===//
Owen Anderson0ff77082008-04-29 00:38:34 +00002//
3// The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
Gordon Henriksena8a118b2008-05-08 17:46:35 +000010// 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.
Owen Anderson0ff77082008-04-29 00:38:34 +000014//
15//===----------------------------------------------------------------------===//
16
Owen Anderson0396cd32008-04-29 20:06:54 +000017#define DEBUG_TYPE "loop-delete"
Owen Anderson0ff77082008-04-29 00:38:34 +000018
19#include "llvm/Transforms/Scalar.h"
Owen Anderson0ff77082008-04-29 00:38:34 +000020#include "llvm/Analysis/LoopPass.h"
Owen Anderson0db198d2008-05-16 04:32:45 +000021#include "llvm/Analysis/ScalarEvolution.h"
Owen Anderson0ff77082008-04-29 00:38:34 +000022#include "llvm/ADT/Statistic.h"
23#include "llvm/ADT/SmallVector.h"
24
25using namespace llvm;
26
27STATISTIC(NumDeleted, "Number of loops deleted");
28
29namespace {
Owen Anderson0396cd32008-04-29 20:06:54 +000030 class VISIBILITY_HIDDEN LoopDeletion : public LoopPass {
Owen Anderson0ff77082008-04-29 00:38:34 +000031 public:
32 static char ID; // Pass ID, replacement for typeid
Dan Gohmanae73dc12008-09-04 17:05:41 +000033 LoopDeletion() : LoopPass(&ID) {}
Owen Anderson0ff77082008-04-29 00:38:34 +000034
35 // Possibly eliminate loop L if it is dead.
36 bool runOnLoop(Loop* L, LPPassManager& LPM);
37
Owen Anderson9862f312008-04-29 20:59:33 +000038 bool SingleDominatingExit(Loop* L,
39 SmallVector<BasicBlock*, 4>& exitingBlocks);
40 bool IsLoopDead(Loop* L, SmallVector<BasicBlock*, 4>& exitingBlocks,
41 SmallVector<BasicBlock*, 4>& exitBlocks);
Owen Anderson0ff77082008-04-29 00:38:34 +000042 bool IsLoopInvariantInst(Instruction *I, Loop* L);
43
44 virtual void getAnalysisUsage(AnalysisUsage& AU) const {
Owen Anderson0db198d2008-05-16 04:32:45 +000045 AU.addRequired<ScalarEvolution>();
Owen Anderson0ff77082008-04-29 00:38:34 +000046 AU.addRequired<DominatorTree>();
47 AU.addRequired<LoopInfo>();
48 AU.addRequiredID(LoopSimplifyID);
49 AU.addRequiredID(LCSSAID);
50
Owen Anderson0db198d2008-05-16 04:32:45 +000051 AU.addPreserved<ScalarEvolution>();
Owen Anderson0ff77082008-04-29 00:38:34 +000052 AU.addPreserved<DominatorTree>();
53 AU.addPreserved<LoopInfo>();
54 AU.addPreservedID(LoopSimplifyID);
55 AU.addPreservedID(LCSSAID);
Dan Gohmand0a90b92009-02-24 01:21:53 +000056 AU.addPreserved<DominanceFrontier>();
Owen Anderson0ff77082008-04-29 00:38:34 +000057 }
58 };
Owen Anderson0ff77082008-04-29 00:38:34 +000059}
Dan Gohman844731a2008-05-13 00:00:25 +000060
61char LoopDeletion::ID = 0;
62static RegisterPass<LoopDeletion> X("loop-deletion", "Delete dead loops");
Owen Anderson0ff77082008-04-29 00:38:34 +000063
Daniel Dunbar394f0442008-10-22 23:32:42 +000064Pass* llvm::createLoopDeletionPass() {
Owen Anderson0396cd32008-04-29 20:06:54 +000065 return new LoopDeletion();
Owen Anderson0ff77082008-04-29 00:38:34 +000066}
67
Owen Anderson9862f312008-04-29 20:59:33 +000068/// SingleDominatingExit - Checks that there is only a single blocks that
Owen Anderson0db198d2008-05-16 04:32:45 +000069/// branches out of the loop, and that it also g the latch block. Loops
Owen Anderson9862f312008-04-29 20:59:33 +000070/// with multiple or non-latch-dominating exiting blocks could be dead, but we'd
71/// have to do more extensive analysis to make sure, for instance, that the
Owen Anderson0db198d2008-05-16 04:32:45 +000072/// control flow logic involved was or could be made loop-invariant.
Owen Anderson9862f312008-04-29 20:59:33 +000073bool LoopDeletion::SingleDominatingExit(Loop* L,
74 SmallVector<BasicBlock*, 4>& exitingBlocks) {
Owen Anderson0ff77082008-04-29 00:38:34 +000075
76 if (exitingBlocks.size() != 1)
Owen Anderson9862f312008-04-29 20:59:33 +000077 return false;
Owen Anderson0ff77082008-04-29 00:38:34 +000078
79 BasicBlock* latch = L->getLoopLatch();
80 if (!latch)
Owen Anderson9862f312008-04-29 20:59:33 +000081 return false;
Owen Anderson0ff77082008-04-29 00:38:34 +000082
83 DominatorTree& DT = getAnalysis<DominatorTree>();
Owen Anderson0db198d2008-05-16 04:32:45 +000084 return DT.dominates(exitingBlocks[0], latch);
Owen Anderson0ff77082008-04-29 00:38:34 +000085}
86
Owen Anderson9862f312008-04-29 20:59:33 +000087/// IsLoopInvariantInst - Checks if an instruction is invariant with respect to
88/// a loop, which is defined as being true if all of its operands are defined
89/// outside of the loop. These instructions can be hoisted out of the loop
90/// if their results are needed. This could be made more aggressive by
91/// recursively checking the operands for invariance, but it's not clear that
92/// it's worth it.
Owen Anderson0396cd32008-04-29 20:06:54 +000093bool LoopDeletion::IsLoopInvariantInst(Instruction *I, Loop* L) {
Owen Anderson0ff77082008-04-29 00:38:34 +000094 // PHI nodes are not loop invariant if defined in the loop.
95 if (isa<PHINode>(I) && L->contains(I->getParent()))
96 return false;
97
98 // The instruction is loop invariant if all of its operands are loop-invariant
99 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
100 if (!L->isLoopInvariant(I->getOperand(i)))
101 return false;
Owen Anderson0db198d2008-05-16 04:32:45 +0000102
Owen Anderson0ff77082008-04-29 00:38:34 +0000103 // If we got this far, the instruction is loop invariant!
104 return true;
105}
106
Owen Anderson9862f312008-04-29 20:59:33 +0000107/// IsLoopDead - Determined if a loop is dead. This assumes that we've already
108/// checked for unique exit and exiting blocks, and that the code is in LCSSA
109/// form.
110bool LoopDeletion::IsLoopDead(Loop* L,
111 SmallVector<BasicBlock*, 4>& exitingBlocks,
112 SmallVector<BasicBlock*, 4>& exitBlocks) {
Owen Anderson0ff77082008-04-29 00:38:34 +0000113 BasicBlock* exitingBlock = exitingBlocks[0];
Owen Anderson0ff77082008-04-29 00:38:34 +0000114 BasicBlock* exitBlock = exitBlocks[0];
115
116 // Make sure that all PHI entries coming from the loop are loop invariant.
Owen Anderson9862f312008-04-29 20:59:33 +0000117 // Because the code is in LCSSA form, any values used outside of the loop
118 // must pass through a PHI in the exit block, meaning that this check is
119 // sufficient to guarantee that no loop-variant values are used outside
120 // of the loop.
Owen Anderson0ff77082008-04-29 00:38:34 +0000121 BasicBlock::iterator BI = exitBlock->begin();
122 while (PHINode* P = dyn_cast<PHINode>(BI)) {
123 Value* incoming = P->getIncomingValueForBlock(exitingBlock);
124 if (Instruction* I = dyn_cast<Instruction>(incoming))
125 if (!IsLoopInvariantInst(I, L))
126 return false;
127
128 BI++;
129 }
130
131 // Make sure that no instructions in the block have potential side-effects.
Owen Anderson9862f312008-04-29 20:59:33 +0000132 // This includes instructions that could write to memory, and loads that are
133 // marked volatile. This could be made more aggressive by using aliasing
134 // information to identify readonly and readnone calls.
Owen Anderson0ff77082008-04-29 00:38:34 +0000135 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
136 LI != LE; ++LI) {
137 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
138 BI != BE; ++BI) {
139 if (BI->mayWriteToMemory())
140 return false;
Owen Anderson9862f312008-04-29 20:59:33 +0000141 else if (LoadInst* L = dyn_cast<LoadInst>(BI))
142 if (L->isVolatile())
143 return false;
Owen Anderson0ff77082008-04-29 00:38:34 +0000144 }
145 }
146
147 return true;
148}
149
Owen Anderson5f8b3442008-04-29 06:34:55 +0000150/// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
151/// observable behavior of the program other than finite running time. Note
152/// we do ensure that this never remove a loop that might be infinite, as doing
153/// so could change the halting/non-halting nature of a program.
Owen Anderson9862f312008-04-29 20:59:33 +0000154/// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
155/// in order to make various safety checks work.
Owen Anderson0396cd32008-04-29 20:06:54 +0000156bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
Owen Anderson0ff77082008-04-29 00:38:34 +0000157 // We can only remove the loop if there is a preheader that we can
158 // branch from after removing it.
159 BasicBlock* preheader = L->getLoopPreheader();
160 if (!preheader)
161 return false;
162
163 // We can't remove loops that contain subloops. If the subloops were dead,
164 // they would already have been removed in earlier executions of this pass.
165 if (L->begin() != L->end())
166 return false;
167
Owen Anderson0db198d2008-05-16 04:32:45 +0000168 SmallVector<BasicBlock*, 4> exitingBlocks;
169 L->getExitingBlocks(exitingBlocks);
170
171 SmallVector<BasicBlock*, 4> exitBlocks;
172 L->getUniqueExitBlocks(exitBlocks);
173
174 // We require that the loop only have a single exit block. Otherwise, we'd
175 // be in the situation of needing to be able to solve statically which exit
176 // block will be branched to, or trying to preserve the branching logic in
177 // a loop invariant manner.
178 if (exitBlocks.size() != 1)
Owen Anderson9862f312008-04-29 20:59:33 +0000179 return false;
180
Owen Anderson0ff77082008-04-29 00:38:34 +0000181 // Loops with multiple exits or exits that don't dominate the latch
182 // are too complicated to handle correctly.
Owen Anderson9862f312008-04-29 20:59:33 +0000183 if (!SingleDominatingExit(L, exitingBlocks))
Owen Anderson0ff77082008-04-29 00:38:34 +0000184 return false;
185
186 // Finally, we have to check that the loop really is dead.
Owen Anderson9862f312008-04-29 20:59:33 +0000187 if (!IsLoopDead(L, exitingBlocks, exitBlocks))
Owen Anderson0ff77082008-04-29 00:38:34 +0000188 return false;
189
Owen Anderson0db198d2008-05-16 04:32:45 +0000190 // Don't remove loops for which we can't solve the trip count.
191 // They could be infinite, in which case we'd be changing program behavior.
192 ScalarEvolution& SE = getAnalysis<ScalarEvolution>();
193 SCEVHandle S = SE.getIterationCount(L);
194 if (isa<SCEVCouldNotCompute>(S))
195 return false;
196
Owen Anderson9862f312008-04-29 20:59:33 +0000197 // Now that we know the removal is safe, remove the loop by changing the
Owen Anderson8b23bb72008-05-06 20:55:16 +0000198 // branch from the preheader to go to the single exit block.
Owen Anderson0ff77082008-04-29 00:38:34 +0000199 BasicBlock* exitBlock = exitBlocks[0];
Owen Anderson8b23bb72008-05-06 20:55:16 +0000200 BasicBlock* exitingBlock = exitingBlocks[0];
Owen Anderson0ff77082008-04-29 00:38:34 +0000201
Owen Andersone54cfdb2008-04-29 00:45:15 +0000202 // Because we're deleting a large chunk of code at once, the sequence in which
203 // we remove things is very important to avoid invalidation issues. Don't
204 // mess with this unless you have good reason and know what you're doing.
205
206 // Move simple loop-invariant expressions out of the loop, since they
207 // might be needed by the exit phis.
Owen Anderson0ff77082008-04-29 00:38:34 +0000208 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
209 LI != LE; ++LI)
210 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
211 BI != BE; ) {
212 Instruction* I = BI++;
Dan Gohmancb41e6b2008-06-23 23:23:49 +0000213 if (!I->use_empty() && IsLoopInvariantInst(I, L))
Owen Anderson0ff77082008-04-29 00:38:34 +0000214 I->moveBefore(preheader->getTerminator());
215 }
216
Owen Andersone54cfdb2008-04-29 00:45:15 +0000217 // Connect the preheader directly to the exit block.
Owen Anderson0ff77082008-04-29 00:38:34 +0000218 TerminatorInst* TI = preheader->getTerminator();
Owen Anderson9862f312008-04-29 20:59:33 +0000219 TI->replaceUsesOfWith(L->getHeader(), exitBlock);
220
Owen Andersone54cfdb2008-04-29 00:45:15 +0000221 // Rewrite phis in the exit block to get their inputs from
222 // the preheader instead of the exiting block.
Owen Anderson0ff77082008-04-29 00:38:34 +0000223 BasicBlock::iterator BI = exitBlock->begin();
224 while (PHINode* P = dyn_cast<PHINode>(BI)) {
Owen Anderson8b23bb72008-05-06 20:55:16 +0000225 P->replaceUsesOfWith(exitingBlock, preheader);
Owen Anderson0ff77082008-04-29 00:38:34 +0000226 BI++;
227 }
228
Owen Anderson9862f312008-04-29 20:59:33 +0000229 // Update the dominator tree and remove the instructions and blocks that will
230 // be deleted from the reference counting scheme.
Owen Anderson0ff77082008-04-29 00:38:34 +0000231 DominatorTree& DT = getAnalysis<DominatorTree>();
Dan Gohmand0a90b92009-02-24 01:21:53 +0000232 DominanceFrontier* DF = getAnalysisIfAvailable<DominanceFrontier>();
Owen Anderson9862f312008-04-29 20:59:33 +0000233 SmallPtrSet<DomTreeNode*, 8> ChildNodes;
Owen Anderson0ff77082008-04-29 00:38:34 +0000234 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
235 LI != LE; ++LI) {
Owen Andersone54cfdb2008-04-29 00:45:15 +0000236 // Move all of the block's children to be children of the preheader, which
237 // allows us to remove the domtree entry for the block.
Owen Anderson9862f312008-04-29 20:59:33 +0000238 ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end());
239 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(),
Dan Gohmand0a90b92009-02-24 01:21:53 +0000240 DE = ChildNodes.end(); DI != DE; ++DI) {
Owen Anderson0ff77082008-04-29 00:38:34 +0000241 DT.changeImmediateDominator(*DI, DT[preheader]);
Dan Gohmand0a90b92009-02-24 01:21:53 +0000242 if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT);
243 }
Owen Anderson0ff77082008-04-29 00:38:34 +0000244
Owen Anderson9862f312008-04-29 20:59:33 +0000245 ChildNodes.clear();
Owen Anderson0ff77082008-04-29 00:38:34 +0000246 DT.eraseNode(*LI);
Dan Gohmand0a90b92009-02-24 01:21:53 +0000247 if (DF) DF->removeBlock(*LI);
248
Owen Anderson0db198d2008-05-16 04:32:45 +0000249 // Remove instructions that we're deleting from ScalarEvolution.
Owen Anderson0ff77082008-04-29 00:38:34 +0000250 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
Owen Anderson0db198d2008-05-16 04:32:45 +0000251 BI != BE; ++BI)
252 SE.deleteValueFromRecords(BI);
Owen Anderson0ff77082008-04-29 00:38:34 +0000253
Owen Anderson0db198d2008-05-16 04:32:45 +0000254 SE.deleteValueFromRecords(*LI);
255
256 // Remove the block from the reference counting scheme, so that we can
257 // delete it freely later.
Owen Anderson0ff77082008-04-29 00:38:34 +0000258 (*LI)->dropAllReferences();
259 }
260
Owen Andersone54cfdb2008-04-29 00:45:15 +0000261 // Erase the instructions and the blocks without having to worry
262 // about ordering because we already dropped the references.
Owen Anderson9862f312008-04-29 20:59:33 +0000263 // NOTE: This iteration is safe because erasing the block does not remove its
264 // entry from the loop's block list. We do that in the next section.
Owen Anderson0ff77082008-04-29 00:38:34 +0000265 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
Owen Andersoncd5e6dd2008-05-29 08:15:48 +0000266 LI != LE; ++LI)
Owen Anderson0ff77082008-04-29 00:38:34 +0000267 (*LI)->eraseFromParent();
Dan Gohmane2abdd32009-02-23 17:10:29 +0000268
269 // Tell ScalarEvolution that the loop is deleted.
270 SE.forgetLoopIterationCount(L);
271
Owen Andersone54cfdb2008-04-29 00:45:15 +0000272 // Finally, the blocks from loopinfo. This has to happen late because
273 // otherwise our loop iterators won't work.
Owen Anderson0ff77082008-04-29 00:38:34 +0000274 LoopInfo& loopInfo = getAnalysis<LoopInfo>();
275 SmallPtrSet<BasicBlock*, 8> blocks;
276 blocks.insert(L->block_begin(), L->block_end());
277 for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(),
278 E = blocks.end(); I != E; ++I)
279 loopInfo.removeBlock(*I);
280
Owen Andersone54cfdb2008-04-29 00:45:15 +0000281 // The last step is to inform the loop pass manager that we've
282 // eliminated this loop.
Owen Anderson0ff77082008-04-29 00:38:34 +0000283 LPM.deleteLoopFromQueue(L);
284
285 NumDeleted++;
286
287 return true;
288}