1 //===-- LoopSink.cpp - Loop Sink 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 does the inverse transformation of what LICM does. 10 // It traverses all of the instructions in the loop's preheader and sinks 11 // them to the loop body where frequency is lower than the loop's preheader. 12 // This pass is a reverse-transformation of LICM. It differs from the Sink 13 // pass in the following ways: 14 // 15 // * It only handles sinking of instructions from the loop's preheader to the 16 // loop's body 17 // * It uses alias set tracker to get more accurate alias info 18 // * It uses block frequency info to find the optimal sinking locations 19 // 20 // Overall algorithm: 21 // 22 // For I in Preheader: 23 // InsertBBs = BBs that uses I 24 // For BB in sorted(LoopBBs): 25 // DomBBs = BBs in InsertBBs that are dominated by BB 26 // if freq(DomBBs) > freq(BB) 27 // InsertBBs = UseBBs - DomBBs + BB 28 // For BB in InsertBBs: 29 // Insert I at BB's beginning 30 // 31 //===----------------------------------------------------------------------===// 32 33 #include "llvm/Transforms/Scalar/LoopSink.h" 34 #include "llvm/ADT/Statistic.h" 35 #include "llvm/Analysis/AliasAnalysis.h" 36 #include "llvm/Analysis/AliasSetTracker.h" 37 #include "llvm/Analysis/BasicAliasAnalysis.h" 38 #include "llvm/Analysis/BlockFrequencyInfo.h" 39 #include "llvm/Analysis/Loads.h" 40 #include "llvm/Analysis/LoopInfo.h" 41 #include "llvm/Analysis/LoopPass.h" 42 #include "llvm/Analysis/ScalarEvolution.h" 43 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h" 44 #include "llvm/IR/Dominators.h" 45 #include "llvm/IR/Instructions.h" 46 #include "llvm/IR/LLVMContext.h" 47 #include "llvm/IR/Metadata.h" 48 #include "llvm/InitializePasses.h" 49 #include "llvm/Support/CommandLine.h" 50 #include "llvm/Transforms/Scalar.h" 51 #include "llvm/Transforms/Scalar/LoopPassManager.h" 52 #include "llvm/Transforms/Utils/Local.h" 53 #include "llvm/Transforms/Utils/LoopUtils.h" 54 using namespace llvm; 55 56 #define DEBUG_TYPE "loopsink" 57 58 STATISTIC(NumLoopSunk, "Number of instructions sunk into loop"); 59 STATISTIC(NumLoopSunkCloned, "Number of cloned instructions sunk into loop"); 60 61 static cl::opt<unsigned> SinkFrequencyPercentThreshold( 62 "sink-freq-percent-threshold", cl::Hidden, cl::init(90), 63 cl::desc("Do not sink instructions that require cloning unless they " 64 "execute less than this percent of the time.")); 65 66 static cl::opt<unsigned> MaxNumberOfUseBBsForSinking( 67 "max-uses-for-sinking", cl::Hidden, cl::init(30), 68 cl::desc("Do not sink instructions that have too many uses.")); 69 70 /// Return adjusted total frequency of \p BBs. 71 /// 72 /// * If there is only one BB, sinking instruction will not introduce code 73 /// size increase. Thus there is no need to adjust the frequency. 74 /// * If there are more than one BB, sinking would lead to code size increase. 75 /// In this case, we add some "tax" to the total frequency to make it harder 76 /// to sink. E.g. 77 /// Freq(Preheader) = 100 78 /// Freq(BBs) = sum(50, 49) = 99 79 /// Even if Freq(BBs) < Freq(Preheader), we will not sink from Preheade to 80 /// BBs as the difference is too small to justify the code size increase. 81 /// To model this, The adjusted Freq(BBs) will be: 82 /// AdjustedFreq(BBs) = 99 / SinkFrequencyPercentThreshold% 83 static BlockFrequency adjustedSumFreq(SmallPtrSetImpl<BasicBlock *> &BBs, 84 BlockFrequencyInfo &BFI) { 85 BlockFrequency T = 0; 86 for (BasicBlock *B : BBs) 87 T += BFI.getBlockFreq(B); 88 if (BBs.size() > 1) 89 T /= BranchProbability(SinkFrequencyPercentThreshold, 100); 90 return T; 91 } 92 93 /// Return a set of basic blocks to insert sinked instructions. 94 /// 95 /// The returned set of basic blocks (BBsToSinkInto) should satisfy: 96 /// 97 /// * Inside the loop \p L 98 /// * For each UseBB in \p UseBBs, there is at least one BB in BBsToSinkInto 99 /// that domintates the UseBB 100 /// * Has minimum total frequency that is no greater than preheader frequency 101 /// 102 /// The purpose of the function is to find the optimal sinking points to 103 /// minimize execution cost, which is defined as "sum of frequency of 104 /// BBsToSinkInto". 105 /// As a result, the returned BBsToSinkInto needs to have minimum total 106 /// frequency. 107 /// Additionally, if the total frequency of BBsToSinkInto exceeds preheader 108 /// frequency, the optimal solution is not sinking (return empty set). 109 /// 110 /// \p ColdLoopBBs is used to help find the optimal sinking locations. 111 /// It stores a list of BBs that is: 112 /// 113 /// * Inside the loop \p L 114 /// * Has a frequency no larger than the loop's preheader 115 /// * Sorted by BB frequency 116 /// 117 /// The complexity of the function is O(UseBBs.size() * ColdLoopBBs.size()). 118 /// To avoid expensive computation, we cap the maximum UseBBs.size() in its 119 /// caller. 120 static SmallPtrSet<BasicBlock *, 2> 121 findBBsToSinkInto(const Loop &L, const SmallPtrSetImpl<BasicBlock *> &UseBBs, 122 const SmallVectorImpl<BasicBlock *> &ColdLoopBBs, 123 DominatorTree &DT, BlockFrequencyInfo &BFI) { 124 SmallPtrSet<BasicBlock *, 2> BBsToSinkInto; 125 if (UseBBs.size() == 0) 126 return BBsToSinkInto; 127 128 BBsToSinkInto.insert(UseBBs.begin(), UseBBs.end()); 129 SmallPtrSet<BasicBlock *, 2> BBsDominatedByColdestBB; 130 131 // For every iteration: 132 // * Pick the ColdestBB from ColdLoopBBs 133 // * Find the set BBsDominatedByColdestBB that satisfy: 134 // - BBsDominatedByColdestBB is a subset of BBsToSinkInto 135 // - Every BB in BBsDominatedByColdestBB is dominated by ColdestBB 136 // * If Freq(ColdestBB) < Freq(BBsDominatedByColdestBB), remove 137 // BBsDominatedByColdestBB from BBsToSinkInto, add ColdestBB to 138 // BBsToSinkInto 139 for (BasicBlock *ColdestBB : ColdLoopBBs) { 140 BBsDominatedByColdestBB.clear(); 141 for (BasicBlock *SinkedBB : BBsToSinkInto) 142 if (DT.dominates(ColdestBB, SinkedBB)) 143 BBsDominatedByColdestBB.insert(SinkedBB); 144 if (BBsDominatedByColdestBB.size() == 0) 145 continue; 146 if (adjustedSumFreq(BBsDominatedByColdestBB, BFI) > 147 BFI.getBlockFreq(ColdestBB)) { 148 for (BasicBlock *DominatedBB : BBsDominatedByColdestBB) { 149 BBsToSinkInto.erase(DominatedBB); 150 } 151 BBsToSinkInto.insert(ColdestBB); 152 } 153 } 154 155 // Can't sink into blocks that have no valid insertion point. 156 for (BasicBlock *BB : BBsToSinkInto) { 157 if (BB->getFirstInsertionPt() == BB->end()) { 158 BBsToSinkInto.clear(); 159 break; 160 } 161 } 162 163 // If the total frequency of BBsToSinkInto is larger than preheader frequency, 164 // do not sink. 165 if (adjustedSumFreq(BBsToSinkInto, BFI) > 166 BFI.getBlockFreq(L.getLoopPreheader())) 167 BBsToSinkInto.clear(); 168 return BBsToSinkInto; 169 } 170 171 // Sinks \p I from the loop \p L's preheader to its uses. Returns true if 172 // sinking is successful. 173 // \p LoopBlockNumber is used to sort the insertion blocks to ensure 174 // determinism. 175 static bool sinkInstruction(Loop &L, Instruction &I, 176 const SmallVectorImpl<BasicBlock *> &ColdLoopBBs, 177 const SmallDenseMap<BasicBlock *, int, 16> &LoopBlockNumber, 178 LoopInfo &LI, DominatorTree &DT, 179 BlockFrequencyInfo &BFI) { 180 // Compute the set of blocks in loop L which contain a use of I. 181 SmallPtrSet<BasicBlock *, 2> BBs; 182 for (auto &U : I.uses()) { 183 Instruction *UI = cast<Instruction>(U.getUser()); 184 // We cannot sink I to PHI-uses. 185 if (dyn_cast<PHINode>(UI)) 186 return false; 187 // We cannot sink I if it has uses outside of the loop. 188 if (!L.contains(LI.getLoopFor(UI->getParent()))) 189 return false; 190 BBs.insert(UI->getParent()); 191 } 192 193 // findBBsToSinkInto is O(BBs.size() * ColdLoopBBs.size()). We cap the max 194 // BBs.size() to avoid expensive computation. 195 // FIXME: Handle code size growth for min_size and opt_size. 196 if (BBs.size() > MaxNumberOfUseBBsForSinking) 197 return false; 198 199 // Find the set of BBs that we should insert a copy of I. 200 SmallPtrSet<BasicBlock *, 2> BBsToSinkInto = 201 findBBsToSinkInto(L, BBs, ColdLoopBBs, DT, BFI); 202 if (BBsToSinkInto.empty()) 203 return false; 204 205 // Return if any of the candidate blocks to sink into is non-cold. 206 if (BBsToSinkInto.size() > 1) { 207 for (auto *BB : BBsToSinkInto) 208 if (!LoopBlockNumber.count(BB)) 209 return false; 210 } 211 212 // Copy the final BBs into a vector and sort them using the total ordering 213 // of the loop block numbers as iterating the set doesn't give a useful 214 // order. No need to stable sort as the block numbers are a total ordering. 215 SmallVector<BasicBlock *, 2> SortedBBsToSinkInto; 216 SortedBBsToSinkInto.insert(SortedBBsToSinkInto.begin(), BBsToSinkInto.begin(), 217 BBsToSinkInto.end()); 218 llvm::sort(SortedBBsToSinkInto, [&](BasicBlock *A, BasicBlock *B) { 219 return LoopBlockNumber.find(A)->second < LoopBlockNumber.find(B)->second; 220 }); 221 222 BasicBlock *MoveBB = *SortedBBsToSinkInto.begin(); 223 // FIXME: Optimize the efficiency for cloned value replacement. The current 224 // implementation is O(SortedBBsToSinkInto.size() * I.num_uses()). 225 for (BasicBlock *N : makeArrayRef(SortedBBsToSinkInto).drop_front(1)) { 226 assert(LoopBlockNumber.find(N)->second > 227 LoopBlockNumber.find(MoveBB)->second && 228 "BBs not sorted!"); 229 // Clone I and replace its uses. 230 Instruction *IC = I.clone(); 231 IC->setName(I.getName()); 232 IC->insertBefore(&*N->getFirstInsertionPt()); 233 // Replaces uses of I with IC in N 234 I.replaceUsesWithIf(IC, [N](Use &U) { 235 return cast<Instruction>(U.getUser())->getParent() == N; 236 }); 237 // Replaces uses of I with IC in blocks dominated by N 238 replaceDominatedUsesWith(&I, IC, DT, N); 239 LLVM_DEBUG(dbgs() << "Sinking a clone of " << I << " To: " << N->getName() 240 << '\n'); 241 NumLoopSunkCloned++; 242 } 243 LLVM_DEBUG(dbgs() << "Sinking " << I << " To: " << MoveBB->getName() << '\n'); 244 NumLoopSunk++; 245 I.moveBefore(&*MoveBB->getFirstInsertionPt()); 246 247 return true; 248 } 249 250 /// Sinks instructions from loop's preheader to the loop body if the 251 /// sum frequency of inserted copy is smaller than preheader's frequency. 252 static bool sinkLoopInvariantInstructions(Loop &L, AAResults &AA, LoopInfo &LI, 253 DominatorTree &DT, 254 BlockFrequencyInfo &BFI, 255 ScalarEvolution *SE) { 256 BasicBlock *Preheader = L.getLoopPreheader(); 257 if (!Preheader) 258 return false; 259 260 // Enable LoopSink only when runtime profile is available. 261 // With static profile, the sinking decision may be sub-optimal. 262 if (!Preheader->getParent()->hasProfileData()) 263 return false; 264 265 const BlockFrequency PreheaderFreq = BFI.getBlockFreq(Preheader); 266 // If there are no basic blocks with lower frequency than the preheader then 267 // we can avoid the detailed analysis as we will never find profitable sinking 268 // opportunities. 269 if (all_of(L.blocks(), [&](const BasicBlock *BB) { 270 return BFI.getBlockFreq(BB) > PreheaderFreq; 271 })) 272 return false; 273 274 bool Changed = false; 275 AliasSetTracker CurAST(AA); 276 277 // Compute alias set. 278 for (BasicBlock *BB : L.blocks()) 279 CurAST.add(*BB); 280 CurAST.add(*Preheader); 281 282 // Sort loop's basic blocks by frequency 283 SmallVector<BasicBlock *, 10> ColdLoopBBs; 284 SmallDenseMap<BasicBlock *, int, 16> LoopBlockNumber; 285 int i = 0; 286 for (BasicBlock *B : L.blocks()) 287 if (BFI.getBlockFreq(B) < BFI.getBlockFreq(L.getLoopPreheader())) { 288 ColdLoopBBs.push_back(B); 289 LoopBlockNumber[B] = ++i; 290 } 291 llvm::stable_sort(ColdLoopBBs, [&](BasicBlock *A, BasicBlock *B) { 292 return BFI.getBlockFreq(A) < BFI.getBlockFreq(B); 293 }); 294 295 // Traverse preheader's instructions in reverse order becaue if A depends 296 // on B (A appears after B), A needs to be sinked first before B can be 297 // sinked. 298 for (auto II = Preheader->rbegin(), E = Preheader->rend(); II != E;) { 299 Instruction *I = &*II++; 300 // No need to check for instruction's operands are loop invariant. 301 assert(L.hasLoopInvariantOperands(I) && 302 "Insts in a loop's preheader should have loop invariant operands!"); 303 if (!canSinkOrHoistInst(*I, &AA, &DT, &L, &CurAST, nullptr, false)) 304 continue; 305 if (sinkInstruction(L, *I, ColdLoopBBs, LoopBlockNumber, LI, DT, BFI)) 306 Changed = true; 307 } 308 309 if (Changed && SE) 310 SE->forgetLoopDispositions(&L); 311 return Changed; 312 } 313 314 PreservedAnalyses LoopSinkPass::run(Function &F, FunctionAnalysisManager &FAM) { 315 LoopInfo &LI = FAM.getResult<LoopAnalysis>(F); 316 // Nothing to do if there are no loops. 317 if (LI.empty()) 318 return PreservedAnalyses::all(); 319 320 AAResults &AA = FAM.getResult<AAManager>(F); 321 DominatorTree &DT = FAM.getResult<DominatorTreeAnalysis>(F); 322 BlockFrequencyInfo &BFI = FAM.getResult<BlockFrequencyAnalysis>(F); 323 324 // We want to do a postorder walk over the loops. Since loops are a tree this 325 // is equivalent to a reversed preorder walk and preorder is easy to compute 326 // without recursion. Since we reverse the preorder, we will visit siblings 327 // in reverse program order. This isn't expected to matter at all but is more 328 // consistent with sinking algorithms which generally work bottom-up. 329 SmallVector<Loop *, 4> PreorderLoops = LI.getLoopsInPreorder(); 330 331 bool Changed = false; 332 do { 333 Loop &L = *PreorderLoops.pop_back_val(); 334 335 // Note that we don't pass SCEV here because it is only used to invalidate 336 // loops in SCEV and we don't preserve (or request) SCEV at all making that 337 // unnecessary. 338 Changed |= sinkLoopInvariantInstructions(L, AA, LI, DT, BFI, 339 /*ScalarEvolution*/ nullptr); 340 } while (!PreorderLoops.empty()); 341 342 if (!Changed) 343 return PreservedAnalyses::all(); 344 345 PreservedAnalyses PA; 346 PA.preserveSet<CFGAnalyses>(); 347 return PA; 348 } 349 350 namespace { 351 struct LegacyLoopSinkPass : public LoopPass { 352 static char ID; 353 LegacyLoopSinkPass() : LoopPass(ID) { 354 initializeLegacyLoopSinkPassPass(*PassRegistry::getPassRegistry()); 355 } 356 357 bool runOnLoop(Loop *L, LPPassManager &LPM) override { 358 if (skipLoop(L)) 359 return false; 360 361 auto *SE = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>(); 362 return sinkLoopInvariantInstructions( 363 *L, getAnalysis<AAResultsWrapperPass>().getAAResults(), 364 getAnalysis<LoopInfoWrapperPass>().getLoopInfo(), 365 getAnalysis<DominatorTreeWrapperPass>().getDomTree(), 366 getAnalysis<BlockFrequencyInfoWrapperPass>().getBFI(), 367 SE ? &SE->getSE() : nullptr); 368 } 369 370 void getAnalysisUsage(AnalysisUsage &AU) const override { 371 AU.setPreservesCFG(); 372 AU.addRequired<BlockFrequencyInfoWrapperPass>(); 373 getLoopAnalysisUsage(AU); 374 } 375 }; 376 } 377 378 char LegacyLoopSinkPass::ID = 0; 379 INITIALIZE_PASS_BEGIN(LegacyLoopSinkPass, "loop-sink", "Loop Sink", false, 380 false) 381 INITIALIZE_PASS_DEPENDENCY(LoopPass) 382 INITIALIZE_PASS_DEPENDENCY(BlockFrequencyInfoWrapperPass) 383 INITIALIZE_PASS_END(LegacyLoopSinkPass, "loop-sink", "Loop Sink", false, false) 384 385 Pass *llvm::createLoopSinkPass() { return new LegacyLoopSinkPass(); } 386