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/InitializePasses.h" 37 #include "llvm/Pass.h" 38 #include "llvm/Support/Casting.h" 39 #include "llvm/Transforms/Scalar.h" 40 #include "llvm/Transforms/Utils/Local.h" 41 #include "llvm/Transforms/Utils/LoopUtils.h" 42 #include <algorithm> 43 #include <utility> 44 45 using namespace llvm; 46 47 #define DEBUG_TYPE "loop-instsimplify" 48 49 STATISTIC(NumSimplified, "Number of redundant instructions simplified"); 50 51 static bool simplifyLoopInst(Loop &L, DominatorTree &DT, LoopInfo &LI, 52 AssumptionCache &AC, const TargetLibraryInfo &TLI, 53 MemorySSAUpdater *MSSAU) { 54 const DataLayout &DL = L.getHeader()->getModule()->getDataLayout(); 55 SimplifyQuery SQ(DL, &TLI, &DT, &AC); 56 57 // On the first pass over the loop body we try to simplify every instruction. 58 // On subsequent passes, we can restrict this to only simplifying instructions 59 // where the inputs have been updated. We end up needing two sets: one 60 // containing the instructions we are simplifying in *this* pass, and one for 61 // the instructions we will want to simplify in the *next* pass. We use 62 // pointers so we can swap between two stably allocated sets. 63 SmallPtrSet<const Instruction *, 8> S1, S2, *ToSimplify = &S1, *Next = &S2; 64 65 // Track the PHI nodes that have already been visited during each iteration so 66 // that we can identify when it is necessary to iterate. 67 SmallPtrSet<PHINode *, 4> VisitedPHIs; 68 69 // While simplifying we may discover dead code or cause code to become dead. 70 // Keep track of all such instructions and we will delete them at the end. 71 SmallVector<WeakTrackingVH, 8> DeadInsts; 72 73 // First we want to create an RPO traversal of the loop body. By processing in 74 // RPO we can ensure that definitions are processed prior to uses (for non PHI 75 // uses) in all cases. This ensures we maximize the simplifications in each 76 // iteration over the loop and minimizes the possible causes for continuing to 77 // iterate. 78 LoopBlocksRPO RPOT(&L); 79 RPOT.perform(&LI); 80 MemorySSA *MSSA = MSSAU ? MSSAU->getMemorySSA() : nullptr; 81 82 bool Changed = false; 83 for (;;) { 84 if (MSSAU && VerifyMemorySSA) 85 MSSA->verifyMemorySSA(); 86 for (BasicBlock *BB : RPOT) { 87 for (Instruction &I : *BB) { 88 if (auto *PI = dyn_cast<PHINode>(&I)) 89 VisitedPHIs.insert(PI); 90 91 if (I.use_empty()) { 92 if (isInstructionTriviallyDead(&I, &TLI)) 93 DeadInsts.push_back(&I); 94 continue; 95 } 96 97 // We special case the first iteration which we can detect due to the 98 // empty `ToSimplify` set. 99 bool IsFirstIteration = ToSimplify->empty(); 100 101 if (!IsFirstIteration && !ToSimplify->count(&I)) 102 continue; 103 104 Value *V = SimplifyInstruction(&I, SQ.getWithInstruction(&I)); 105 if (!V || !LI.replacementPreservesLCSSAForm(&I, V)) 106 continue; 107 108 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); 109 UI != UE;) { 110 Use &U = *UI++; 111 auto *UserI = cast<Instruction>(U.getUser()); 112 U.set(V); 113 114 // If the instruction is used by a PHI node we have already processed 115 // we'll need to iterate on the loop body to converge, so add it to 116 // the next set. 117 if (auto *UserPI = dyn_cast<PHINode>(UserI)) 118 if (VisitedPHIs.count(UserPI)) { 119 Next->insert(UserPI); 120 continue; 121 } 122 123 // If we are only simplifying targeted instructions and the user is an 124 // instruction in the loop body, add it to our set of targeted 125 // instructions. Because we process defs before uses (outside of PHIs) 126 // we won't have visited it yet. 127 // 128 // We also skip any uses outside of the loop being simplified. Those 129 // should always be PHI nodes due to LCSSA form, and we don't want to 130 // try to simplify those away. 131 assert((L.contains(UserI) || isa<PHINode>(UserI)) && 132 "Uses outside the loop should be PHI nodes due to LCSSA!"); 133 if (!IsFirstIteration && L.contains(UserI)) 134 ToSimplify->insert(UserI); 135 } 136 137 if (MSSAU) 138 if (Instruction *SimpleI = dyn_cast_or_null<Instruction>(V)) 139 if (MemoryAccess *MA = MSSA->getMemoryAccess(&I)) 140 if (MemoryAccess *ReplacementMA = MSSA->getMemoryAccess(SimpleI)) 141 MA->replaceAllUsesWith(ReplacementMA); 142 143 assert(I.use_empty() && "Should always have replaced all uses!"); 144 if (isInstructionTriviallyDead(&I, &TLI)) 145 DeadInsts.push_back(&I); 146 ++NumSimplified; 147 Changed = true; 148 } 149 } 150 151 // Delete any dead instructions found thus far now that we've finished an 152 // iteration over all instructions in all the loop blocks. 153 if (!DeadInsts.empty()) { 154 Changed = true; 155 RecursivelyDeleteTriviallyDeadInstructions(DeadInsts, &TLI, MSSAU); 156 } 157 158 if (MSSAU && VerifyMemorySSA) 159 MSSA->verifyMemorySSA(); 160 161 // If we never found a PHI that needs to be simplified in the next 162 // iteration, we're done. 163 if (Next->empty()) 164 break; 165 166 // Otherwise, put the next set in place for the next iteration and reset it 167 // and the visited PHIs for that iteration. 168 std::swap(Next, ToSimplify); 169 Next->clear(); 170 VisitedPHIs.clear(); 171 DeadInsts.clear(); 172 } 173 174 return Changed; 175 } 176 177 namespace { 178 179 class LoopInstSimplifyLegacyPass : public LoopPass { 180 public: 181 static char ID; // Pass ID, replacement for typeid 182 183 LoopInstSimplifyLegacyPass() : LoopPass(ID) { 184 initializeLoopInstSimplifyLegacyPassPass(*PassRegistry::getPassRegistry()); 185 } 186 187 bool runOnLoop(Loop *L, LPPassManager &LPM) override { 188 if (skipLoop(L)) 189 return false; 190 DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 191 LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); 192 AssumptionCache &AC = 193 getAnalysis<AssumptionCacheTracker>().getAssumptionCache( 194 *L->getHeader()->getParent()); 195 const TargetLibraryInfo &TLI = 196 getAnalysis<TargetLibraryInfoWrapperPass>().getTLI( 197 *L->getHeader()->getParent()); 198 MemorySSA *MSSA = nullptr; 199 Optional<MemorySSAUpdater> MSSAU; 200 if (EnableMSSALoopDependency) { 201 MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA(); 202 MSSAU = MemorySSAUpdater(MSSA); 203 } 204 205 return simplifyLoopInst(*L, DT, LI, AC, TLI, 206 MSSAU.hasValue() ? MSSAU.getPointer() : nullptr); 207 } 208 209 void getAnalysisUsage(AnalysisUsage &AU) const override { 210 AU.addRequired<AssumptionCacheTracker>(); 211 AU.addRequired<DominatorTreeWrapperPass>(); 212 AU.addRequired<TargetLibraryInfoWrapperPass>(); 213 AU.setPreservesCFG(); 214 if (EnableMSSALoopDependency) { 215 AU.addRequired<MemorySSAWrapperPass>(); 216 AU.addPreserved<MemorySSAWrapperPass>(); 217 } 218 getLoopAnalysisUsage(AU); 219 } 220 }; 221 222 } // end anonymous namespace 223 224 PreservedAnalyses LoopInstSimplifyPass::run(Loop &L, LoopAnalysisManager &AM, 225 LoopStandardAnalysisResults &AR, 226 LPMUpdater &) { 227 Optional<MemorySSAUpdater> MSSAU; 228 if (AR.MSSA) { 229 MSSAU = MemorySSAUpdater(AR.MSSA); 230 if (VerifyMemorySSA) 231 AR.MSSA->verifyMemorySSA(); 232 } 233 if (!simplifyLoopInst(L, AR.DT, AR.LI, AR.AC, AR.TLI, 234 MSSAU.hasValue() ? MSSAU.getPointer() : nullptr)) 235 return PreservedAnalyses::all(); 236 237 auto PA = getLoopPassPreservedAnalyses(); 238 PA.preserveSet<CFGAnalyses>(); 239 if (AR.MSSA) 240 PA.preserve<MemorySSAAnalysis>(); 241 return PA; 242 } 243 244 char LoopInstSimplifyLegacyPass::ID = 0; 245 246 INITIALIZE_PASS_BEGIN(LoopInstSimplifyLegacyPass, "loop-instsimplify", 247 "Simplify instructions in loops", false, false) 248 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) 249 INITIALIZE_PASS_DEPENDENCY(LoopPass) 250 INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass) 251 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass) 252 INITIALIZE_PASS_END(LoopInstSimplifyLegacyPass, "loop-instsimplify", 253 "Simplify instructions in loops", false, false) 254 255 Pass *llvm::createLoopInstSimplifyPass() { 256 return new LoopInstSimplifyLegacyPass(); 257 } 258