1 //===- LoopInstSimplify.cpp - Loop Instruction Simplification Pass --------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This pass performs lightweight instruction simplification on loop bodies. 10 // 11 //===----------------------------------------------------------------------===// 12 13 #include "llvm/Transforms/Scalar/LoopInstSimplify.h" 14 #include "llvm/ADT/PointerIntPair.h" 15 #include "llvm/ADT/STLExtras.h" 16 #include "llvm/ADT/SmallPtrSet.h" 17 #include "llvm/ADT/SmallVector.h" 18 #include "llvm/ADT/Statistic.h" 19 #include "llvm/Analysis/AssumptionCache.h" 20 #include "llvm/Analysis/InstructionSimplify.h" 21 #include "llvm/Analysis/LoopInfo.h" 22 #include "llvm/Analysis/LoopIterator.h" 23 #include "llvm/Analysis/LoopPass.h" 24 #include "llvm/Analysis/MemorySSA.h" 25 #include "llvm/Analysis/MemorySSAUpdater.h" 26 #include "llvm/Analysis/TargetLibraryInfo.h" 27 #include "llvm/IR/BasicBlock.h" 28 #include "llvm/IR/CFG.h" 29 #include "llvm/IR/DataLayout.h" 30 #include "llvm/IR/Dominators.h" 31 #include "llvm/IR/Instruction.h" 32 #include "llvm/IR/Instructions.h" 33 #include "llvm/IR/Module.h" 34 #include "llvm/IR/PassManager.h" 35 #include "llvm/IR/User.h" 36 #include "llvm/Pass.h" 37 #include "llvm/Support/Casting.h" 38 #include "llvm/Transforms/Scalar.h" 39 #include "llvm/Transforms/Utils/Local.h" 40 #include "llvm/Transforms/Utils/LoopUtils.h" 41 #include <algorithm> 42 #include <utility> 43 44 using namespace llvm; 45 46 #define DEBUG_TYPE "loop-instsimplify" 47 48 STATISTIC(NumSimplified, "Number of redundant instructions simplified"); 49 50 static bool simplifyLoopInst(Loop &L, DominatorTree &DT, LoopInfo &LI, 51 AssumptionCache &AC, const TargetLibraryInfo &TLI, 52 MemorySSAUpdater *MSSAU) { 53 const DataLayout &DL = L.getHeader()->getModule()->getDataLayout(); 54 SimplifyQuery SQ(DL, &TLI, &DT, &AC); 55 56 // On the first pass over the loop body we try to simplify every instruction. 57 // On subsequent passes, we can restrict this to only simplifying instructions 58 // where the inputs have been updated. We end up needing two sets: one 59 // containing the instructions we are simplifying in *this* pass, and one for 60 // the instructions we will want to simplify in the *next* pass. We use 61 // pointers so we can swap between two stably allocated sets. 62 SmallPtrSet<const Instruction *, 8> S1, S2, *ToSimplify = &S1, *Next = &S2; 63 64 // Track the PHI nodes that have already been visited during each iteration so 65 // that we can identify when it is necessary to iterate. 66 SmallPtrSet<PHINode *, 4> VisitedPHIs; 67 68 // While simplifying we may discover dead code or cause code to become dead. 69 // Keep track of all such instructions and we will delete them at the end. 70 SmallVector<Instruction *, 8> DeadInsts; 71 72 // First we want to create an RPO traversal of the loop body. By processing in 73 // RPO we can ensure that definitions are processed prior to uses (for non PHI 74 // uses) in all cases. This ensures we maximize the simplifications in each 75 // iteration over the loop and minimizes the possible causes for continuing to 76 // iterate. 77 LoopBlocksRPO RPOT(&L); 78 RPOT.perform(&LI); 79 MemorySSA *MSSA = MSSAU ? MSSAU->getMemorySSA() : nullptr; 80 81 bool Changed = false; 82 for (;;) { 83 if (MSSAU && VerifyMemorySSA) 84 MSSA->verifyMemorySSA(); 85 for (BasicBlock *BB : RPOT) { 86 for (Instruction &I : *BB) { 87 if (auto *PI = dyn_cast<PHINode>(&I)) 88 VisitedPHIs.insert(PI); 89 90 if (I.use_empty()) { 91 if (isInstructionTriviallyDead(&I, &TLI)) 92 DeadInsts.push_back(&I); 93 continue; 94 } 95 96 // We special case the first iteration which we can detect due to the 97 // empty `ToSimplify` set. 98 bool IsFirstIteration = ToSimplify->empty(); 99 100 if (!IsFirstIteration && !ToSimplify->count(&I)) 101 continue; 102 103 Value *V = SimplifyInstruction(&I, SQ.getWithInstruction(&I)); 104 if (!V || !LI.replacementPreservesLCSSAForm(&I, V)) 105 continue; 106 107 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); 108 UI != UE;) { 109 Use &U = *UI++; 110 auto *UserI = cast<Instruction>(U.getUser()); 111 U.set(V); 112 113 // If the instruction is used by a PHI node we have already processed 114 // we'll need to iterate on the loop body to converge, so add it to 115 // the next set. 116 if (auto *UserPI = dyn_cast<PHINode>(UserI)) 117 if (VisitedPHIs.count(UserPI)) { 118 Next->insert(UserPI); 119 continue; 120 } 121 122 // If we are only simplifying targeted instructions and the user is an 123 // instruction in the loop body, add it to our set of targeted 124 // instructions. Because we process defs before uses (outside of PHIs) 125 // we won't have visited it yet. 126 // 127 // We also skip any uses outside of the loop being simplified. Those 128 // should always be PHI nodes due to LCSSA form, and we don't want to 129 // try to simplify those away. 130 assert((L.contains(UserI) || isa<PHINode>(UserI)) && 131 "Uses outside the loop should be PHI nodes due to LCSSA!"); 132 if (!IsFirstIteration && L.contains(UserI)) 133 ToSimplify->insert(UserI); 134 } 135 136 if (MSSAU) 137 if (Instruction *SimpleI = dyn_cast_or_null<Instruction>(V)) 138 if (MemoryAccess *MA = MSSA->getMemoryAccess(&I)) 139 if (MemoryAccess *ReplacementMA = MSSA->getMemoryAccess(SimpleI)) 140 MA->replaceAllUsesWith(ReplacementMA); 141 142 assert(I.use_empty() && "Should always have replaced all uses!"); 143 if (isInstructionTriviallyDead(&I, &TLI)) 144 DeadInsts.push_back(&I); 145 ++NumSimplified; 146 Changed = true; 147 } 148 } 149 150 // Delete any dead instructions found thus far now that we've finished an 151 // iteration over all instructions in all the loop blocks. 152 if (!DeadInsts.empty()) { 153 Changed = true; 154 RecursivelyDeleteTriviallyDeadInstructions(DeadInsts, &TLI, MSSAU); 155 } 156 157 if (MSSAU && VerifyMemorySSA) 158 MSSA->verifyMemorySSA(); 159 160 // If we never found a PHI that needs to be simplified in the next 161 // iteration, we're done. 162 if (Next->empty()) 163 break; 164 165 // Otherwise, put the next set in place for the next iteration and reset it 166 // and the visited PHIs for that iteration. 167 std::swap(Next, ToSimplify); 168 Next->clear(); 169 VisitedPHIs.clear(); 170 DeadInsts.clear(); 171 } 172 173 return Changed; 174 } 175 176 namespace { 177 178 class LoopInstSimplifyLegacyPass : public LoopPass { 179 public: 180 static char ID; // Pass ID, replacement for typeid 181 182 LoopInstSimplifyLegacyPass() : LoopPass(ID) { 183 initializeLoopInstSimplifyLegacyPassPass(*PassRegistry::getPassRegistry()); 184 } 185 186 bool runOnLoop(Loop *L, LPPassManager &LPM) override { 187 if (skipLoop(L)) 188 return false; 189 DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 190 LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); 191 AssumptionCache &AC = 192 getAnalysis<AssumptionCacheTracker>().getAssumptionCache( 193 *L->getHeader()->getParent()); 194 const TargetLibraryInfo &TLI = 195 getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(); 196 MemorySSA *MSSA = nullptr; 197 Optional<MemorySSAUpdater> MSSAU; 198 if (EnableMSSALoopDependency) { 199 MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA(); 200 MSSAU = MemorySSAUpdater(MSSA); 201 } 202 203 return simplifyLoopInst(*L, DT, LI, AC, TLI, 204 MSSAU.hasValue() ? MSSAU.getPointer() : nullptr); 205 } 206 207 void getAnalysisUsage(AnalysisUsage &AU) const override { 208 AU.addRequired<AssumptionCacheTracker>(); 209 AU.addRequired<DominatorTreeWrapperPass>(); 210 AU.addRequired<TargetLibraryInfoWrapperPass>(); 211 AU.setPreservesCFG(); 212 if (EnableMSSALoopDependency) { 213 AU.addRequired<MemorySSAWrapperPass>(); 214 AU.addPreserved<MemorySSAWrapperPass>(); 215 } 216 getLoopAnalysisUsage(AU); 217 } 218 }; 219 220 } // end anonymous namespace 221 222 PreservedAnalyses LoopInstSimplifyPass::run(Loop &L, LoopAnalysisManager &AM, 223 LoopStandardAnalysisResults &AR, 224 LPMUpdater &) { 225 Optional<MemorySSAUpdater> MSSAU; 226 if (AR.MSSA) { 227 MSSAU = MemorySSAUpdater(AR.MSSA); 228 AR.MSSA->verifyMemorySSA(); 229 } 230 if (!simplifyLoopInst(L, AR.DT, AR.LI, AR.AC, AR.TLI, 231 MSSAU.hasValue() ? MSSAU.getPointer() : nullptr)) 232 return PreservedAnalyses::all(); 233 234 auto PA = getLoopPassPreservedAnalyses(); 235 PA.preserveSet<CFGAnalyses>(); 236 if (EnableMSSALoopDependency) 237 PA.preserve<MemorySSAAnalysis>(); 238 return PA; 239 } 240 241 char LoopInstSimplifyLegacyPass::ID = 0; 242 243 INITIALIZE_PASS_BEGIN(LoopInstSimplifyLegacyPass, "loop-instsimplify", 244 "Simplify instructions in loops", false, false) 245 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) 246 INITIALIZE_PASS_DEPENDENCY(LoopPass) 247 INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass) 248 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass) 249 INITIALIZE_PASS_END(LoopInstSimplifyLegacyPass, "loop-instsimplify", 250 "Simplify instructions in loops", false, false) 251 252 Pass *llvm::createLoopInstSimplifyPass() { 253 return new LoopInstSimplifyLegacyPass(); 254 } 255