1 //===- LoopUnrollAndJam.cpp - Loop unroll and jam 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 implements an unroll and jam pass. Most of the work is done by 10 // Utils/UnrollLoopAndJam.cpp. 11 //===----------------------------------------------------------------------===// 12 13 #include "llvm/Transforms/Scalar/LoopUnrollAndJamPass.h" 14 #include "llvm/ADT/ArrayRef.h" 15 #include "llvm/ADT/PriorityWorklist.h" 16 #include "llvm/ADT/SmallPtrSet.h" 17 #include "llvm/ADT/StringRef.h" 18 #include "llvm/Analysis/AssumptionCache.h" 19 #include "llvm/Analysis/CodeMetrics.h" 20 #include "llvm/Analysis/DependenceAnalysis.h" 21 #include "llvm/Analysis/LoopAnalysisManager.h" 22 #include "llvm/Analysis/LoopInfo.h" 23 #include "llvm/Analysis/LoopNestAnalysis.h" 24 #include "llvm/Analysis/LoopPass.h" 25 #include "llvm/Analysis/OptimizationRemarkEmitter.h" 26 #include "llvm/Analysis/ScalarEvolution.h" 27 #include "llvm/Analysis/TargetTransformInfo.h" 28 #include "llvm/IR/BasicBlock.h" 29 #include "llvm/IR/Constants.h" 30 #include "llvm/IR/Dominators.h" 31 #include "llvm/IR/Function.h" 32 #include "llvm/IR/Instructions.h" 33 #include "llvm/IR/Metadata.h" 34 #include "llvm/IR/PassManager.h" 35 #include "llvm/Support/Casting.h" 36 #include "llvm/Support/CommandLine.h" 37 #include "llvm/Support/Compiler.h" 38 #include "llvm/Support/Debug.h" 39 #include "llvm/Support/raw_ostream.h" 40 #include "llvm/Transforms/Scalar/LoopPassManager.h" 41 #include "llvm/Transforms/Utils/LoopPeel.h" 42 #include "llvm/Transforms/Utils/LoopUtils.h" 43 #include "llvm/Transforms/Utils/UnrollLoop.h" 44 #include <cassert> 45 #include <cstdint> 46 47 namespace llvm { 48 class Instruction; 49 class Value; 50 } // namespace llvm 51 52 using namespace llvm; 53 54 #define DEBUG_TYPE "loop-unroll-and-jam" 55 56 /// @{ 57 /// Metadata attribute names 58 static const char *const LLVMLoopUnrollAndJamFollowupAll = 59 "llvm.loop.unroll_and_jam.followup_all"; 60 static const char *const LLVMLoopUnrollAndJamFollowupInner = 61 "llvm.loop.unroll_and_jam.followup_inner"; 62 static const char *const LLVMLoopUnrollAndJamFollowupOuter = 63 "llvm.loop.unroll_and_jam.followup_outer"; 64 static const char *const LLVMLoopUnrollAndJamFollowupRemainderInner = 65 "llvm.loop.unroll_and_jam.followup_remainder_inner"; 66 static const char *const LLVMLoopUnrollAndJamFollowupRemainderOuter = 67 "llvm.loop.unroll_and_jam.followup_remainder_outer"; 68 /// @} 69 70 static cl::opt<bool> 71 AllowUnrollAndJam("allow-unroll-and-jam", cl::Hidden, 72 cl::desc("Allows loops to be unroll-and-jammed.")); 73 74 static cl::opt<unsigned> UnrollAndJamCount( 75 "unroll-and-jam-count", cl::Hidden, 76 cl::desc("Use this unroll count for all loops including those with " 77 "unroll_and_jam_count pragma values, for testing purposes")); 78 79 static cl::opt<unsigned> UnrollAndJamThreshold( 80 "unroll-and-jam-threshold", cl::init(60), cl::Hidden, 81 cl::desc("Threshold to use for inner loop when doing unroll and jam.")); 82 83 static cl::opt<unsigned> PragmaUnrollAndJamThreshold( 84 "pragma-unroll-and-jam-threshold", cl::init(1024), cl::Hidden, 85 cl::desc("Unrolled size limit for loops with an unroll_and_jam(full) or " 86 "unroll_count pragma.")); 87 88 // Returns the loop hint metadata node with the given name (for example, 89 // "llvm.loop.unroll.count"). If no such metadata node exists, then nullptr is 90 // returned. 91 static MDNode *getUnrollMetadataForLoop(const Loop *L, StringRef Name) { 92 if (MDNode *LoopID = L->getLoopID()) 93 return GetUnrollMetadata(LoopID, Name); 94 return nullptr; 95 } 96 97 // Returns true if the loop has any metadata starting with Prefix. For example a 98 // Prefix of "llvm.loop.unroll." returns true if we have any unroll metadata. 99 static bool hasAnyUnrollPragma(const Loop *L, StringRef Prefix) { 100 if (MDNode *LoopID = L->getLoopID()) { 101 // First operand should refer to the loop id itself. 102 assert(LoopID->getNumOperands() > 0 && "requires at least one operand"); 103 assert(LoopID->getOperand(0) == LoopID && "invalid loop id"); 104 105 for (unsigned I = 1, E = LoopID->getNumOperands(); I < E; ++I) { 106 MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(I)); 107 if (!MD) 108 continue; 109 110 MDString *S = dyn_cast<MDString>(MD->getOperand(0)); 111 if (!S) 112 continue; 113 114 if (S->getString().starts_with(Prefix)) 115 return true; 116 } 117 } 118 return false; 119 } 120 121 // Returns true if the loop has an unroll_and_jam(enable) pragma. 122 static bool hasUnrollAndJamEnablePragma(const Loop *L) { 123 return getUnrollMetadataForLoop(L, "llvm.loop.unroll_and_jam.enable"); 124 } 125 126 // If loop has an unroll_and_jam_count pragma return the (necessarily 127 // positive) value from the pragma. Otherwise return 0. 128 static unsigned unrollAndJamCountPragmaValue(const Loop *L) { 129 MDNode *MD = getUnrollMetadataForLoop(L, "llvm.loop.unroll_and_jam.count"); 130 if (MD) { 131 assert(MD->getNumOperands() == 2 && 132 "Unroll count hint metadata should have two operands."); 133 unsigned Count = 134 mdconst::extract<ConstantInt>(MD->getOperand(1))->getZExtValue(); 135 assert(Count >= 1 && "Unroll count must be positive."); 136 return Count; 137 } 138 return 0; 139 } 140 141 // Returns loop size estimation for unrolled loop. 142 static uint64_t 143 getUnrollAndJammedLoopSize(unsigned LoopSize, 144 TargetTransformInfo::UnrollingPreferences &UP) { 145 assert(LoopSize >= UP.BEInsns && "LoopSize should not be less than BEInsns!"); 146 return static_cast<uint64_t>(LoopSize - UP.BEInsns) * UP.Count + UP.BEInsns; 147 } 148 149 // Calculates unroll and jam count and writes it to UP.Count. Returns true if 150 // unroll count was set explicitly. 151 static bool computeUnrollAndJamCount( 152 Loop *L, Loop *SubLoop, const TargetTransformInfo &TTI, DominatorTree &DT, 153 LoopInfo *LI, AssumptionCache *AC, ScalarEvolution &SE, 154 const SmallPtrSetImpl<const Value *> &EphValues, 155 OptimizationRemarkEmitter *ORE, unsigned OuterTripCount, 156 unsigned OuterTripMultiple, const UnrollCostEstimator &OuterUCE, 157 unsigned InnerTripCount, unsigned InnerLoopSize, 158 TargetTransformInfo::UnrollingPreferences &UP, 159 TargetTransformInfo::PeelingPreferences &PP) { 160 unsigned OuterLoopSize = OuterUCE.getRolledLoopSize(); 161 // First up use computeUnrollCount from the loop unroller to get a count 162 // for unrolling the outer loop, plus any loops requiring explicit 163 // unrolling we leave to the unroller. This uses UP.Threshold / 164 // UP.PartialThreshold / UP.MaxCount to come up with sensible loop values. 165 // We have already checked that the loop has no unroll.* pragmas. 166 unsigned MaxTripCount = 0; 167 bool UseUpperBound = false; 168 bool ExplicitUnroll = computeUnrollCount( 169 L, TTI, DT, LI, AC, SE, EphValues, ORE, OuterTripCount, MaxTripCount, 170 /*MaxOrZero*/ false, OuterTripMultiple, OuterUCE, UP, PP, 171 UseUpperBound); 172 if (ExplicitUnroll || UseUpperBound) { 173 // If the user explicitly set the loop as unrolled, dont UnJ it. Leave it 174 // for the unroller instead. 175 LLVM_DEBUG(dbgs() << "Won't unroll-and-jam; explicit count set by " 176 "computeUnrollCount\n"); 177 UP.Count = 0; 178 return false; 179 } 180 181 // Override with any explicit Count from the "unroll-and-jam-count" option. 182 bool UserUnrollCount = UnrollAndJamCount.getNumOccurrences() > 0; 183 if (UserUnrollCount) { 184 UP.Count = UnrollAndJamCount; 185 UP.Force = true; 186 if (UP.AllowRemainder && 187 getUnrollAndJammedLoopSize(OuterLoopSize, UP) < UP.Threshold && 188 getUnrollAndJammedLoopSize(InnerLoopSize, UP) < 189 UP.UnrollAndJamInnerLoopThreshold) 190 return true; 191 } 192 193 // Check for unroll_and_jam pragmas 194 unsigned PragmaCount = unrollAndJamCountPragmaValue(L); 195 if (PragmaCount > 0) { 196 UP.Count = PragmaCount; 197 UP.Runtime = true; 198 UP.Force = true; 199 if ((UP.AllowRemainder || (OuterTripMultiple % PragmaCount == 0)) && 200 getUnrollAndJammedLoopSize(OuterLoopSize, UP) < UP.Threshold && 201 getUnrollAndJammedLoopSize(InnerLoopSize, UP) < 202 UP.UnrollAndJamInnerLoopThreshold) 203 return true; 204 } 205 206 bool PragmaEnableUnroll = hasUnrollAndJamEnablePragma(L); 207 bool ExplicitUnrollAndJamCount = PragmaCount > 0 || UserUnrollCount; 208 bool ExplicitUnrollAndJam = PragmaEnableUnroll || ExplicitUnrollAndJamCount; 209 210 // If the loop has an unrolling pragma, we want to be more aggressive with 211 // unrolling limits. 212 if (ExplicitUnrollAndJam) 213 UP.UnrollAndJamInnerLoopThreshold = PragmaUnrollAndJamThreshold; 214 215 if (!UP.AllowRemainder && getUnrollAndJammedLoopSize(InnerLoopSize, UP) >= 216 UP.UnrollAndJamInnerLoopThreshold) { 217 LLVM_DEBUG(dbgs() << "Won't unroll-and-jam; can't create remainder and " 218 "inner loop too large\n"); 219 UP.Count = 0; 220 return false; 221 } 222 223 // We have a sensible limit for the outer loop, now adjust it for the inner 224 // loop and UP.UnrollAndJamInnerLoopThreshold. If the outer limit was set 225 // explicitly, we want to stick to it. 226 if (!ExplicitUnrollAndJamCount && UP.AllowRemainder) { 227 while (UP.Count != 0 && getUnrollAndJammedLoopSize(InnerLoopSize, UP) >= 228 UP.UnrollAndJamInnerLoopThreshold) 229 UP.Count--; 230 } 231 232 // If we are explicitly unroll and jamming, we are done. Otherwise there are a 233 // number of extra performance heuristics to check. 234 if (ExplicitUnrollAndJam) 235 return true; 236 237 // If the inner loop count is known and small, leave the entire loop nest to 238 // be the unroller 239 if (InnerTripCount && InnerLoopSize * InnerTripCount < UP.Threshold) { 240 LLVM_DEBUG(dbgs() << "Won't unroll-and-jam; small inner loop count is " 241 "being left for the unroller\n"); 242 UP.Count = 0; 243 return false; 244 } 245 246 // Check for situations where UnJ is likely to be unprofitable. Including 247 // subloops with more than 1 block. 248 if (SubLoop->getBlocks().size() != 1) { 249 LLVM_DEBUG( 250 dbgs() << "Won't unroll-and-jam; More than one inner loop block\n"); 251 UP.Count = 0; 252 return false; 253 } 254 255 // Limit to loops where there is something to gain from unrolling and 256 // jamming the loop. In this case, look for loads that are invariant in the 257 // outer loop and can become shared. 258 unsigned NumInvariant = 0; 259 for (BasicBlock *BB : SubLoop->getBlocks()) { 260 for (Instruction &I : *BB) { 261 if (auto *Ld = dyn_cast<LoadInst>(&I)) { 262 Value *V = Ld->getPointerOperand(); 263 const SCEV *LSCEV = SE.getSCEVAtScope(V, L); 264 if (SE.isLoopInvariant(LSCEV, L)) 265 NumInvariant++; 266 } 267 } 268 } 269 if (NumInvariant == 0) { 270 LLVM_DEBUG(dbgs() << "Won't unroll-and-jam; No loop invariant loads\n"); 271 UP.Count = 0; 272 return false; 273 } 274 275 return false; 276 } 277 278 static LoopUnrollResult 279 tryToUnrollAndJamLoop(Loop *L, DominatorTree &DT, LoopInfo *LI, 280 ScalarEvolution &SE, const TargetTransformInfo &TTI, 281 AssumptionCache &AC, DependenceInfo &DI, 282 OptimizationRemarkEmitter &ORE, int OptLevel) { 283 TargetTransformInfo::UnrollingPreferences UP = gatherUnrollingPreferences( 284 L, SE, TTI, nullptr, nullptr, ORE, OptLevel, std::nullopt, std::nullopt, 285 std::nullopt, std::nullopt, std::nullopt, std::nullopt); 286 TargetTransformInfo::PeelingPreferences PP = 287 gatherPeelingPreferences(L, SE, TTI, std::nullopt, std::nullopt); 288 289 TransformationMode EnableMode = hasUnrollAndJamTransformation(L); 290 if (EnableMode & TM_Disable) 291 return LoopUnrollResult::Unmodified; 292 if (EnableMode & TM_ForcedByUser) 293 UP.UnrollAndJam = true; 294 295 if (AllowUnrollAndJam.getNumOccurrences() > 0) 296 UP.UnrollAndJam = AllowUnrollAndJam; 297 if (UnrollAndJamThreshold.getNumOccurrences() > 0) 298 UP.UnrollAndJamInnerLoopThreshold = UnrollAndJamThreshold; 299 // Exit early if unrolling is disabled. 300 if (!UP.UnrollAndJam || UP.UnrollAndJamInnerLoopThreshold == 0) 301 return LoopUnrollResult::Unmodified; 302 303 LLVM_DEBUG(dbgs() << "Loop Unroll and Jam: F[" 304 << L->getHeader()->getParent()->getName() << "] Loop %" 305 << L->getHeader()->getName() << "\n"); 306 307 // A loop with any unroll pragma (enabling/disabling/count/etc) is left for 308 // the unroller, so long as it does not explicitly have unroll_and_jam 309 // metadata. This means #pragma nounroll will disable unroll and jam as well 310 // as unrolling 311 if (hasAnyUnrollPragma(L, "llvm.loop.unroll.") && 312 !hasAnyUnrollPragma(L, "llvm.loop.unroll_and_jam.")) { 313 LLVM_DEBUG(dbgs() << " Disabled due to pragma.\n"); 314 return LoopUnrollResult::Unmodified; 315 } 316 317 if (!isSafeToUnrollAndJam(L, SE, DT, DI, *LI)) { 318 LLVM_DEBUG(dbgs() << " Disabled due to not being safe.\n"); 319 return LoopUnrollResult::Unmodified; 320 } 321 322 // Approximate the loop size and collect useful info 323 SmallPtrSet<const Value *, 32> EphValues; 324 CodeMetrics::collectEphemeralValues(L, &AC, EphValues); 325 Loop *SubLoop = L->getSubLoops()[0]; 326 UnrollCostEstimator InnerUCE(SubLoop, TTI, EphValues, UP.BEInsns); 327 UnrollCostEstimator OuterUCE(L, TTI, EphValues, UP.BEInsns); 328 329 if (!InnerUCE.canUnroll() || !OuterUCE.canUnroll()) { 330 LLVM_DEBUG(dbgs() << " Loop not considered unrollable\n"); 331 return LoopUnrollResult::Unmodified; 332 } 333 334 unsigned InnerLoopSize = InnerUCE.getRolledLoopSize(); 335 LLVM_DEBUG(dbgs() << " Outer Loop Size: " << OuterUCE.getRolledLoopSize() 336 << "\n"); 337 LLVM_DEBUG(dbgs() << " Inner Loop Size: " << InnerLoopSize << "\n"); 338 339 if (InnerUCE.NumInlineCandidates != 0 || OuterUCE.NumInlineCandidates != 0) { 340 LLVM_DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n"); 341 return LoopUnrollResult::Unmodified; 342 } 343 // FIXME: The call to canUnroll() allows some controlled convergent 344 // operations, but we block them here for future changes. 345 if (InnerUCE.Convergence != ConvergenceKind::None || 346 OuterUCE.Convergence != ConvergenceKind::None) { 347 LLVM_DEBUG( 348 dbgs() << " Not unrolling loop with convergent instructions.\n"); 349 return LoopUnrollResult::Unmodified; 350 } 351 352 // Save original loop IDs for after the transformation. 353 MDNode *OrigOuterLoopID = L->getLoopID(); 354 MDNode *OrigSubLoopID = SubLoop->getLoopID(); 355 356 // To assign the loop id of the epilogue, assign it before unrolling it so it 357 // is applied to every inner loop of the epilogue. We later apply the loop ID 358 // for the jammed inner loop. 359 std::optional<MDNode *> NewInnerEpilogueLoopID = makeFollowupLoopID( 360 OrigOuterLoopID, {LLVMLoopUnrollAndJamFollowupAll, 361 LLVMLoopUnrollAndJamFollowupRemainderInner}); 362 if (NewInnerEpilogueLoopID) 363 SubLoop->setLoopID(*NewInnerEpilogueLoopID); 364 365 // Find trip count and trip multiple 366 BasicBlock *Latch = L->getLoopLatch(); 367 BasicBlock *SubLoopLatch = SubLoop->getLoopLatch(); 368 unsigned OuterTripCount = SE.getSmallConstantTripCount(L, Latch); 369 unsigned OuterTripMultiple = SE.getSmallConstantTripMultiple(L, Latch); 370 unsigned InnerTripCount = SE.getSmallConstantTripCount(SubLoop, SubLoopLatch); 371 372 // Decide if, and by how much, to unroll 373 bool IsCountSetExplicitly = computeUnrollAndJamCount( 374 L, SubLoop, TTI, DT, LI, &AC, SE, EphValues, &ORE, OuterTripCount, 375 OuterTripMultiple, OuterUCE, InnerTripCount, InnerLoopSize, UP, PP); 376 if (UP.Count <= 1) 377 return LoopUnrollResult::Unmodified; 378 // Unroll factor (Count) must be less or equal to TripCount. 379 if (OuterTripCount && UP.Count > OuterTripCount) 380 UP.Count = OuterTripCount; 381 382 Loop *EpilogueOuterLoop = nullptr; 383 LoopUnrollResult UnrollResult = UnrollAndJamLoop( 384 L, UP.Count, OuterTripCount, OuterTripMultiple, UP.UnrollRemainder, LI, 385 &SE, &DT, &AC, &TTI, &ORE, &EpilogueOuterLoop); 386 387 // Assign new loop attributes. 388 if (EpilogueOuterLoop) { 389 std::optional<MDNode *> NewOuterEpilogueLoopID = makeFollowupLoopID( 390 OrigOuterLoopID, {LLVMLoopUnrollAndJamFollowupAll, 391 LLVMLoopUnrollAndJamFollowupRemainderOuter}); 392 if (NewOuterEpilogueLoopID) 393 EpilogueOuterLoop->setLoopID(*NewOuterEpilogueLoopID); 394 } 395 396 std::optional<MDNode *> NewInnerLoopID = 397 makeFollowupLoopID(OrigOuterLoopID, {LLVMLoopUnrollAndJamFollowupAll, 398 LLVMLoopUnrollAndJamFollowupInner}); 399 if (NewInnerLoopID) 400 SubLoop->setLoopID(*NewInnerLoopID); 401 else 402 SubLoop->setLoopID(OrigSubLoopID); 403 404 if (UnrollResult == LoopUnrollResult::PartiallyUnrolled) { 405 std::optional<MDNode *> NewOuterLoopID = makeFollowupLoopID( 406 OrigOuterLoopID, 407 {LLVMLoopUnrollAndJamFollowupAll, LLVMLoopUnrollAndJamFollowupOuter}); 408 if (NewOuterLoopID) { 409 L->setLoopID(*NewOuterLoopID); 410 411 // Do not setLoopAlreadyUnrolled if a followup was given. 412 return UnrollResult; 413 } 414 } 415 416 // If loop has an unroll count pragma or unrolled by explicitly set count 417 // mark loop as unrolled to prevent unrolling beyond that requested. 418 if (UnrollResult != LoopUnrollResult::FullyUnrolled && IsCountSetExplicitly) 419 L->setLoopAlreadyUnrolled(); 420 421 return UnrollResult; 422 } 423 424 static bool tryToUnrollAndJamLoop(LoopNest &LN, DominatorTree &DT, LoopInfo &LI, 425 ScalarEvolution &SE, 426 const TargetTransformInfo &TTI, 427 AssumptionCache &AC, DependenceInfo &DI, 428 OptimizationRemarkEmitter &ORE, int OptLevel, 429 LPMUpdater &U) { 430 bool DidSomething = false; 431 ArrayRef<Loop *> Loops = LN.getLoops(); 432 Loop *OutmostLoop = &LN.getOutermostLoop(); 433 434 // Add the loop nests in the reverse order of LN. See method 435 // declaration. 436 SmallPriorityWorklist<Loop *, 4> Worklist; 437 appendLoopsToWorklist(Loops, Worklist); 438 while (!Worklist.empty()) { 439 Loop *L = Worklist.pop_back_val(); 440 std::string LoopName = std::string(L->getName()); 441 LoopUnrollResult Result = 442 tryToUnrollAndJamLoop(L, DT, &LI, SE, TTI, AC, DI, ORE, OptLevel); 443 if (Result != LoopUnrollResult::Unmodified) 444 DidSomething = true; 445 if (L == OutmostLoop && Result == LoopUnrollResult::FullyUnrolled) 446 U.markLoopAsDeleted(*L, LoopName); 447 } 448 449 return DidSomething; 450 } 451 452 PreservedAnalyses LoopUnrollAndJamPass::run(LoopNest &LN, 453 LoopAnalysisManager &AM, 454 LoopStandardAnalysisResults &AR, 455 LPMUpdater &U) { 456 Function &F = *LN.getParent(); 457 458 DependenceInfo DI(&F, &AR.AA, &AR.SE, &AR.LI); 459 OptimizationRemarkEmitter ORE(&F); 460 461 if (!tryToUnrollAndJamLoop(LN, AR.DT, AR.LI, AR.SE, AR.TTI, AR.AC, DI, ORE, 462 OptLevel, U)) 463 return PreservedAnalyses::all(); 464 465 auto PA = getLoopPassPreservedAnalyses(); 466 PA.preserve<LoopNestAnalysis>(); 467 return PA; 468 } 469