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